ASNT NDT Handbook Volume 4 Infrared and Thermal Testing

Hile, J. “Automatic Radiography with Landauer, R.S., Jr., and E.T. Clarke. “Field
Cobalt-60.” Materials and Methods. Determination of Output and Effective
Vol. 40. New York, NY: Reinhold Size of Iridium-192 Radiographic
Publishing Corporation (1954): p 108. Sources.” Materials Evaluation. Vol. 37,
No. 12. Columbus, OH: American
Isenburger, H.R. “Exposure Charts for Society for Nondestructive Testing
Cobalt-60 Radiography.” Modern (November 1979): p 35-37.
Castings [American Foundryman].
Vol. 18. Des Plaines, IL: American Larabie, P. “Iridium-192 Production.”
Foundrymen’s Society (1950): p 48. Materials Evaluation. Vol. 50, No. 9.
Columbus, OH: American Society for
Kastner, J. “Units Used in Industrial Nondestructive Testing (September
Radiography to Describe Strength of 1992): p 1022-1023, 1025-1026.
Cobalt-60 Sources.” Nondestructive
Testing. Vol. 11, No. 1. Columbus, OH: Morrison, A. “Iridium 192 for Gamma-Ray
American Society for Nondestructive Radiography.” Nondestructive Testing.
Testing (Fall 1952): p 21-23. Vol. 10, No. 1. Columbus, OH:
American Society for Nondestructive
“Material Required to Carry Out Testing (Summer 1951): p 26-28.
Radiography with Cobalt-60 or
Radium.” Report 2008. Boucherville, Munro, J.J. “Calculation of Scattered
Canada: National Research Council Radiation Intensities of 192 Iridium
(1949). Gamma Rays from a Steel Slab.”
Materials Evaluation. Vol. 35, No. 2.
Morrison, A. “Radiography with Columbus, OH: American Society for
Cobalt-60.” Nondestructive Testing. Nondestructive (February 1977):
Vol. 9, No. 4. Columbus, OH: p 51-53.
American Society for Nondestructive
Testing (1951): p 14. Rigbey, J.V. and C.F. Baxter. “Iridium-192
in Industrial Radiography.”
Nir-El, Y. “Accurate Calibration of a Co-60 Nondestructive Testing. Vol. 11, No. 1.
Gamma Radiographic Source.” Columbus, OH: American Society for
Materials Evaluation. Vol. 54, No. 2. Nondestructive Testing (Fall 1952):
Columbus, OH: American Society for p 34-40.
Nondestructive Testing (February
1996): p 138-139. Ritz, V.H. “Broad and Narrow Beam
Attenuation of Iridium-192 Gamma
O’Conner, D.T. and J.J. Hirschfield. “Some Rays in Concrete, Steel and Lead.”
Aspects of Cobalt Radiography.” Nondestructive Testing. Vol. 16, No. 3.
Nondestructive Testing. Vol. 10, No. 1 Columbus, OH: American Society for
Columbus, OH: American Society for Nondestructive Testing (1958): p 269.
Nondestructive Testing (Summer
1951): p 33-39. Errata, Vol. 11, No. 1 Wolf, R.V. and K.P.W. Wolf. “An
(Summer 1952): p 34. Investigation of the Application of
Iridium 192 Gamma Radiation to the
“Radiography with Cobalt-60.” Nucleonics. Radiography of Light Metal Castings.”
Vol. 5, No. 6. New York, NY: Nondestructive Testing. Vol. 12, No. 1.
McGraw-Hill (1949). Columbus, OH: American Society for
Nondestructive Testing
Reed, M.E. Cobalt-60 Radiography in (January-February 1954): p 26-29.
Industry. Boston, MA: Tracerlab
Incorporated (1954). Yeomans, C. and S. Bellanca. “Iridium-192
Proves Useful Inspection Tool in the
Schwinn, W.L. Economics and Practical Aircraft Industry.” Nondestructive
Applications of Cobalt-60 in Testing. Vol. 14, No. 4. Columbus, OH:
Radiographic Inspection of Steel American Society for Nondestructive
Weldments. ASME Special Technical Testing (July-August 1956): p 32, 34.
Publication 112. New York, NY:
American Society of Mechanical Selenium-75
Engineers (1951): p 112.
Grimm, R. and J.J. Munro III. “Gamma
Iridium-192 Radiography Using the Radioisotope
Selenium 75 in the Chemical and
Halmshaw, R. “Use and Scope of Petroleum Industry.” ASNT’s
Iridium-192 for the Radiography of International Chemical and Petroleum
Steel.” British Journal of Applied Physics. Industrial Inspection Technology (ICPIIT)
Vol. 5. London, United Kingdom: IV Topical Conference [Houston, TX].
Institute of Physics (1954): p 238. Columbus, OH: American Society for
Nondestructive Testing (June 1995):
Johns, M.W. and S.V. Nablo. p 51-53.
“Disintegration of Iridium 192 and
Iridium 194.” Physics Review. Vol. 96,
No. 6. Melville, NY: American Physical
Society (1954): p 1599-1607.

Isotope Radiation Sources for Gamma Radiography 87

McCutcheon, D. “Experimental Work Morrison, A. and E.M. Nodwell. “Radium
Employing Radioisotopes Cobalt and Radiography of Thin Steel Section.”
Selenium.” Non-Destructive Testing. ASTM Bulletin. No. 127. West
Vol. 7, No. 3. Columbus, OH: Conshohocken, PA: ASTM
American Society for Nondestructive International (1944): p 29.
Testing (Winter 1948-49): p 7-14.
Radon
Thallium-204
Morrison, A. “Use of Radon for Industrial
Kereiakes, J.G. and G.R. Kraft. Radiography.” Nondestructive Testing.
“Thallium-204 X-Radiography.” Vol. 6, No. 2. Columbus, OH:
Nondestructive Testing. Vol. 16, No. 6. American Society for Nondestructive
Columbus, OH: American Society for Testing (Fall 1947): p 24-26.
Nondestructive Testing (1958): p 490.
Pullin, V.E. “Radon, Its Place in
Thulium-170 Nondestructive Testing.” Welding.
Vol. 18. London, United Kingdom:
Carpenter, A.W. “Complete Portable Field Institute of Welding (1950): p 166.
X-Ray Unit.” Army Medical Research
Laboratories Report No. 168 (1954).

Clarke, E.T. “Gamma Radiography of
Light Metals.” Nondestructive Testing.
Vol. 16. Columbus, OH: American
Society for Nondestructive Testing
(1958): p 265.

Graham, R.L., J.L. Wolfson and R.E. Bell.
“The Disintegration of Thulium-170.”
Canadian Journal of Physics. Vol. 30.
Boucherville, Canada: National
Research Council (1952): p 459.

Halmshaw, R. “Thulium-170 for Industrial
Radiography.” British Journal of Applied
Physics. Vol. 6. London, United
Kingdom: Institute of Physics (1955):
p 8.

West, R. “Low-Energy Gamma Ray
Sources.” Nucleonics. Vol. 11, No. 2.
New York, NY: McGraw-Hill (1953):
p 20.

Radium

“Exposures for Radium Radiography of
Steel.” Metals Progress. Vol. 57.
Materials Park, OH: ASM International
(1950): p 780.

Gezelius, R.A. and C.W. Briggs. Radium for
Industrial Radiography. New York, NY:
Radium Chemical Co., Inc. (1946).

Johns, H.E. and C. Garrett. “Sensitivity
and Exposure Graphs for Radium
Radiography.” Nondestructive Testing.
Vol. 8, No. 3. Columbus, OH:
American Society for Nondestructive
Testing (Winter 1949-50): p 16-25.

Kahn, N.A., E.A. Imbembo and J. Bland. A
Universal Exposure Calculator for Radium
Radiography and Its Application to
Current Radiographic Films and
Techniques. ASME Special Technical
Publication 96. New York, NY:
American Society of Mechanical
Engineers (1950).

Radiological Health Handbook, revised
edition. PB 121784R. Washington, DC:
United States Department of Health,
Education and Welfare (1960).

88 Radiographic Testing

5

CHAPTER

Radiation Measurement1

Frank A. Iddings, San Antonio, Texas
William B. Rivkin, Highland Park, Illinois
Gerald C. Wicks, Durham, North Carolina

PART 1. Principles of Radiation Measurement

Emissions from radioactive nuclei and TABLE 1. Effect of detected and measured ionization.
radiation from that portion of the
electromagnetic spectrum beyond the Effect Type of Instrument Detector
ultraviolet energies can cause the
ionization of atoms and molecules. Electrical ionization chamber gas
Electrical proportional counter gas
Ionizing radiation occurs as three Electrical geiger müller counter gas
forms: (1) charged particles such as alpha Electrical solid state semiconductor
particles, beta particles and protons, Chemical film emulsion photographic
(2) uncharged particles such as neutrons Chemical chemical dosimeter solid or liquid
and (3) electromagnetic radiation in the Light scintillation counter crystal or liquid
form of X-rays and gamma rays. Light cerenkov counter crystal or liquid
Thermoluminescence dosimeter crystal
Radiation Detection Heat calorimeter solid or liquid
Systems

Some forms of radiation, such as light and
heat, can be detected by human sense
organs; ionizing radiation, however, can
be detected only by the aftereffect of its
ionizing properties. If ionizing radiation
does not interact with matter, its
detection and measurement is impossible.
For this reason, the detection process uses
substances that respond to radiation, as
part of a system for measuring the extent
of that response.

The ionization process is used by a
large class of detection systems, including
ion chambers, proportional chambers,
geiger-müller counters and semiconductor
devices (Table 1).

Some systems depend on the excitation
and molecular dissociation that occur
with ionization. These processes are useful
in scintillation counters and chemical
dosimeters. Although other types of
detection systems exist, they are not
generally used in radiation survey
instruments.

Radiation Detection for
Safety

Several widely used technologies for
personnel dosimetry are discussed in the
chapter on radiation safety.

90 Radiographic Testing

PART 2. Ion Chambers and Proportional Counters

Principles of Ionization geometry is cylindrical, a cylindrical
cathode enclosing the gas and an axial,
The mechanism most widely used in insulated rod anode (Fig. 2).
radiation survey applications is the
ionization principle: charged particles Charged particles, photons or both pass
producing ion pairs by direct interaction. through the chamber and ionize the
These charged particles may (1) collide enclosed gas. When an electric field is
with electrons and remove them from applied to the gas, ions drift along the
their atoms or (2) transfer energy to an electrical lines of force to produce an
electron by the interaction of their electric ionization current. Under normal
fields (Fig. 1). If the energy transfer is not conditions, electrons drift at speeds of
sufficient to completely remove an about 104 m·s–1 (22 000 mi·h–1). The drift
electron, the atom is left in a disturbed or velocity of positive ions is many orders of
excited state. magnitude less.

Gamma and X-ray photons interact When the electric field is increased
with matter mainly by photoelectric slightly from zero and a detector is placed
absorption, compton scattering and pair in the constant radiation field the
production, each of which produces collected ions still will be few in number
electrons and ions that may be collected because many recombine. As the voltage
and measured. The average energy is further increased, recombination yields
expended in the creation of an ion pair, to ionization, where all ions are collected
in air and most gases, is about 34 eV. (Fig. 3).

The number of ion pairs produced per FIGURE 2. Basic ionization chamber with high
unit of path length is called specific value resistance R and voltage V.
ionization. Specific ionization is affected
by the energy of the particle or photon by Anode ∆V
its change and by the nature of the
ionized substance.

Ionization Chambers Cathode R
V
In an ionization chamber, an electric field
is applied across a volume of gas, between
two electrodes. Often the chamber’s

FIGURE 1. Ion pair (showing ejected electron
and vacancy in electron orbit of atom).

FIGURE 3. Pulse size as function of voltage in gas ion
chamber.

Legend Pulse size
Recombination
= electron
= vacancy Ion changer
Proportional region

Limited proportionality
Geiger threshold
Geiger-müller

Continuous discharge

Voltage

Radiation Measurement 91

Ion current chambers have a response radiation must penetrate the wall of the
magnitude proportional to the absorbed chamber to ionize the gas volume,
energy and are therefore widely used for chambers are chosen for the specific
making dose measurements. When radiation energy being evaluated. When
(1) recombination is negligible, (2) gas considering a particular instrument the
amplification does not occur and (3) all energy response curve should always be
other charges are efficiently collected, consulted (Fig. 4). Some instruments may
then the steady state current produced is also have an angular dependence (more
an accurate measurement of the rate at sensitivity in some directions), which
which ion pairs are formed within the should also be considered when making
gas. Measurement of this ionization measurements. Radio frequency shielded
current is the principle behind the direct ionization chambers are available for
current ion chamber. measurements made near high level radio
frequency sources.
Ion chambers may be constructed of
several different materials and, because

FIGURE 4. Energy and directional response of typical ion chamber survey meters: (a) example
of response curve; (b) comparison of several response curves.
(a)

1.2

Ratio of indicated to 1.0
actual response
0.8

0.6

0.4 10 100 1000 2000
5

Effective photon energy (keV)

(b) 1.3 40 kV CP 100 kV CP

1.2 7 kV CP 50 kV CP

Ratio of indicated to actual response 1.1 20 kV CP

1.0 150 kV CP

0.9 50 kV CP
200 kV CP

0.8
0.7 250 kV CP Cesium-137

0.6 Cobalt-60

0.5

0.4

0.3

0.2

0.1

0

6 8 10 20 40 60 80 100 200 400 600 1000

6.7 1250

Effective photon energy (keV)

Legend

= Parallel to long axis, cap on
= Parallel to long axis, cap off
= Perpendicular to long axis
CP = constant potential

92 Radiographic Testing

Output Current compared to the time constant of the
Measurements circuit. The induced alternating current
voltage is proportional to the ionization
The ionization current collected in the current (Fig. 6).
ion chamber flows through an external
circuit for measurement. Although in Integrating Instruments
principle an ammeter could be placed in
the external circuit to read the ion The instruments described above (Fig. 7)
current, in practice the ammeter is not are generally rate meters; that is, they
placed there, for the current is very small. indicate the radiation at the time of
A 440 cm3 (27 in.3) ion chamber typically exposure and, depending on its time
produces about 4 × 10–15 A·µSv–1 constant, will return to background levels
(4 × 10–14 A·mR–1) at standard temperature as the source is removed.
and pressure. A high valued load resistor
(on the order of 1010 Ω) is placed in the Some instruments may have an
circuit and the voltage drop across the integration switch that introduces a
resistor is measured with a sensitive capacitor to the circuit to accumulate the
electrometer. A metal oxide silicon field charge. Leaving such an instrument at an
effect transistor (MOSFET) is used in some operator’s location will indicate the total
electrometers. The metal oxide silicon amount of ionizing radiation that area has
field effect transistor produces an input received, from the time the instrument is
impedance on the order of 1015 Ω to engaged.
amplify the collected current (Fig. 5).
Personnel Monitoring
Vibrating Reed Instruments
Electrometers
Pocket Chambers
An alternative approach to ion current
measurement is to convert the signal from Personnel monitoring instruments, some
direct current to alternating current at an the size of a ball point pen, are usually
early stage. This allows a more stable
amplification of the alternating current FIGURE 6. Principle of vibrating reed
signal in subsequent operations. The electrometer; oscillations of capacitance
conversion is accomplished in a dynamic induce alternating current voltage
capacitor or vibrating reed electrometer, proportional to steady state signal current.
by collecting the ion current across a
resistive capacitive circuit. The Signal
capacitance is then changed rapidly, current

FIGURE 5. Operational configuration of current amplifier.

Feedback Capacitor Resistor Alternating
element current electrometer

Ion chamber – Current + 1000 FIGURE 7. Examples of ionization chambers
300 located externally on survey instruments.
– Metal oxide Protective caps are removed, showing thin
+ silicon field 100 windows for low energy X-ray or beta
effect transistor Direct detection.
Collection electrometer current
potential
Calibration 30 gain
control selector

+ 10
Meter
3
–

Radiation Measurement 93

the integrating type and contain an Figure 10 demonstrates the energy
ionization chamber. One version, the response of self-reading pocket
pocket chamber, requires the application dosimeters. Table 2 lists performance
of an initial charge of 150 to 200 V by an specifications of dosimeters in general.2
external instrument. Zero dose is then
indicated on a scale contained in the Proportional Counters
charging unit. Exposure of the chamber to
ionization decreases the initial charge. If the electric field in an ion chamber is
When the chamber is reconnected to the raised above the ionization potential but
charging unit the reduced charge is below saturation potential, enough energy
translated to the level of exposure (Fig. 8). is imparted to the ions for production of
secondary electrons by collision and gas
Direct Reading Dosimeters amplification.

The direct reading dosimeter operates on FIGURE 9. Cross section of pocket (direct
the principle of the gold leaf electroscope reading) ionization chamber.
(Fig. 9). A quartz fiber is displaced
electrostatically by charging it to a Eyepiece Connection
potential of about 200 V. An image of the between
fiber is focused on a scale and viewed Scale charger and
through a lens at one end of the Charger fiber
instrument. Radiation exposure of the Mercury drop
dosimeter discharges the fiber, allowing it
to return to its original position. Objective
Personnel dosimeters may have a full scale
reading of 1 to 50 mSv (100 mR to 5 R)
and may have other scales according to
applicable regulations.

Chambers are available with thin walls
for sensitivity to beta radiation over
1 MeV and may be coated on the inside
with boron for neutron sensitivity.

FIGURE 8. Cross section of quartz fiber pocket Quartz fiber
dosimeter. Support

Ionization
chamber

1

3 2 Commutator Hermetic
56 associated joints
with hood

4

78

10 11 FIGURE 10. Energy dependence of response of different
commercial self-reading dosimeters.
9 Dosimeter reading (percent of true dose)
12 300

Legend 200

1. Low atomic number wall 100 ± 30
2. Graphite coated paper shell
3. Aluminum terminal head 30 40 50 70 100 150 200 300
4. Aluminum terminal sleeve
5. Polystyrene support bushing
6. Central electrode, graphite coated
7. Polyethylene insulating washer
8. Polystyrene fixed bushing
9. Electrode contact
10. Retaining ring
11. Aluminum base cap
12. Polyethylene friction bushing

Quantum energy (keV)

94 Radiographic Testing

TABLE 2. General performance specifications for dosimeters.2

Characteristic Performance Specification

Accuracy ±12 percent at 95 percent confidence

Energy dependence ±10 percent over given range

Sensitivity adjustment sealed

Exterior surface smooth

Ruggedness withstands drop of 1.2 m (4 ft)

Temperature +50 to –10 °C (+122 to +14 °F)

Humidity 0 to 90 percent

Discharge no more than 2 percent of full scale in 24 h

Angular dependence more than 70 percent at angles greater than 50 degrees from direction of
maximum response

Operation at this electric potential
overcomes the difficulty of the small
currents in the ionization region yet takes
advantage of pulse size dependence for
separating various ionizing energies.
When an ionization chamber is operated
in this region it is called a proportional
counter.

The size of the output pulse is
determined by, and proportional to, the
number of electrons collected at the
anode and the voltage applied at the
detector. By careful selection of gases and
voltages, a properly designed proportional
counter can detect alphas in the presence
of betas, or higher energy beta and
gamma radiation in the presence of lower
energies. Proportional counters are often
used in X-ray diffraction applications.

Radiation Measurement 95

PART 3. Geiger-Müller Counters

Operating Voltage Level is about 100 ms, which must be corrected

Increasing voltage beyond the at high level readings.
proportional region (Fig. 3) will eventually Resolving time τ of a counter may be
cause the gas avalanche to extend along
the entire length of the anode wire. When determined by counting two sources
this happens, the end of the proportional
region is reached and the geiger-müller independently (R1 and R2), then together
region begins. (R1, R2). The background count is Rb.

An instrument operating in this (1) τ= R1 + R2 − R1,2 − Rb
voltage range, using a sealed gas filled R12,2 − R12 − R22
detector, is referred to as a geiger-müller
counter, a GM counter or simply a geiger Correct counting rate R can be calculated
tube. This instrument was introduced in from observed counting rate Ro and
1928 and its simplicity and low cost have resolving time τ in the following equation
made it the most popular radiation for nonparalyzable systems:
detector since then. Geiger-müller
counters complement the ion chamber (2) R = Ro
and proportional counter and comprise
the third category of gas filled detectors 1 − Roτ
based on ionization.
Dead Time
Properties
The relationship of resolving time to dead
Extension of the gas avalanche increases time and recovery is illustrated in Fig. 11.
the gas amplification factors so that 109 to Resolving time may be a function of the
1010 ion pairs are formed in the discharge. detector alone or of the detector and its
This results in an output pulse large signal processing electronics. Its effect on
enough (0.25 to 10 V) to require no the real counting rate depends on
sophisticated electronic amplification whether the system design is paralyzable
circuitry for readout. At this voltage, the or nonparalyzable.
size of all pulses, regardless of the nature
of the ionization, is the same. Nonparalyzable Systems

When operated in the geiger-müller In Fig. 12, a time scale is shown
region, a counter cannot distinguish indicating six randomly spaced events in
among the several types of radiation and the detector.3
therefore is not useful for spectroscopy or
for the detection of one energy event in At the bottom of the illustration is the
the presence of another. An external corresponding dead time behavior of a
shield is often used to filter out alpha and
beta particles in the presence of gamma FIGURE 11. Resolving time, dead time and recovery time for
energies. geiger-müller system.

Resolving Time Ionizing
events
As an ionizing event occurs in the
counter, the avalanche of ions paralyzes Trigger level
the counter. The counter is then incapable Potential difference
of responding to another event until the Pulse amplitude
discharge dissipates and proper potential
is established. The time it takes to Dead time
reestablish the electric field intensity is Resolving time
referred to as the resolving time. Average Recovery time
resolving time for a geiger-müller counter

96 Radiographic Testing

detector assumed to be nonparalyzable. Electronic Quenching
A fixed time τ follows each event that
occurs during the live period of the Electronic quenching is accomplished by
detector. Events occurring during the dead introducing a high value of resistance into
time have no effect on the detector, the voltage circuit. This will drop the
which would record four counts from the anode potential until all the positive ions
six interactions. have been collected.

Paralyzable Systems Self-Quenching Gas

The top line of Fig. 12 illustrates a A self-quenching gas is one that can
paralyzable system. Resolving time τ absorb ultraviolet (UV) photons without
follows each interaction, whether it is becoming ionized. One way to use this
recorded or not. Events that occur during characteristic is to introduce a small
resolving time τ are not recorded and amount of organic vapor, such as alcohol
further extend the dead time by another or ether, into the tube. The energy from
period τ. The chart indicates only three the ultraviolet photons is then dissipated
recorded events from the six interactions. by dissociating the gas molecule. Such a
In this case, τ increases with increased tube is useful only as long as it has a
number of interactions.3 sufficient number of organic molecules to
dissociate, generally about 108 counts.
It can be demonstrated that with a
paralyzable system (at increasingly higher To avoid the problem of limited
interaction rates), the observed counting lifetime, some tubes use halogens
rates can actually decrease with an (chlorine or bromine) as the quench gas.
increased number of events. When using The halogen molecules also dissociate in
a counting system that may be the quenching process but they are
paralyzable, extreme caution must be replenished by spontaneous
taken to ensure that low observed recombination at a later time. Halogen
counting rates correspond to low quench tubes have an infinite lifetime
interaction rates, rather than very high and are preferred for extended
interaction rates with accompanying, long applications.
dead time. It is possible for a paralyzable
system to record the first interaction and Reaction products of the discharge
then be paralyzed, recording zero counts often produce contamination of the gas or
in high radiation fields. deposition on the anode surface and
generally limit the lifetimes of
geiger-müller tubes.

Quenching Design Variations

As positive ions are collected after a pulse, Geiger-müller counters (Fig. 13) are
they give up their kinetic energy by available in various shapes and sizes. The
striking the wall of the tube; ultraviolet most common form is that of a cylinder
photons and/or electrons are liberated, with a central anode wire. If low energy
producing spurious counts. Prevention of beta or alpha particles are to be counted, a
such counts is called quenching.
FIGURE 13. Assortment of geiger-müller
Quenching may be accomplished counters demonstrating availability of sizes
electronically (by lowering the anode and shapes. Smallest counter shown is
voltage after a pulse) or chemically (by about 30 mm (1 in.) long.
using a self-quenching gas).

FIGURE 12. Processing of detector interactions in paralyzable
and nonparalyzable systems.3

τ

Dead Paralyzable
live

Events in detector Time
Nonparalyzable
τ

Dead
live

Legend
τ = resolving time

Radiation Measurement 97

unit with a thin entrance window (1 to contain a small tube to minimize
4 mg·cm–2) should be selected. resolving time of the system; large volume
detectors may require significant
For surveying large surfaces, pancake or correction.
large window counters are available. High
count rate instruments, greater than A geiger-müller counter response to
0.14 mSv·s–1 (50 mR·h–1), generally gamma rays occurs by way of gamma ray
interaction with the solid wall of the tube.
FIGURE 14. Dose rate ratio versus effective energy for The incident gamma ray interacts with
personnel radiation monitor. the wall and produces a secondary
electron that subsequently reaches the
1.2 gas. The probability of gamma ray
interaction generally increases with higher
Ratio of indicated to density wall material.
true dose rate
1.0 Alarming Rate Meters
(Personnel Monitors)
40 60 80 100 200 300 400 500 1000
Small geiger-müller tubes are used in
Cesium-137 Cobalt-60 pocket-sized units for personnel
monitoring. They generally emit a high
Effective energy (keV) frequency chirp at a rate proportional to
the subjected dose rate. United States

FIGURE 15. Typical energy response curves for geiger-müller counters (a) shielded versus unshielded;
(b) radiation incident on side versus front; (c) exposure ratio close to ideal with radiation incident normal to
long axis of probe; (d) radiation incident normal to long axis of probe.

(a) Bare tube (c) Cesium-137 Cobalt-60
Probe shield open
10 Ratio of indicated to actual Cesium-137 Cobalt-60 Ratio of indicated to actual
8 exposure rate 2exposure rate
6
4

2

1

0.8

0.6 Tube in 1

0.4 probe, beta 0
shield open 10

Beta shield closed 6 Probe shield closed
0.2 5
4
Probe axis perpendicular to incident radiation 3
2
0.1 1 50 100 500 1000

10 50 100 500 1000 10

Effective energy (keV)

(b) Effective energy (keV) (d)
Cesium-137 Cobalt-60
2.2Ratio of indicated to actual Ratio of indicated to actual Cesium-137 Cobalt-60
2.0 exposure rate Side exposure rate Probe shield open
1.8 Front
1.6
1.4 Probe shield closed
1.2
1.0
0.8
0.6
0.4
0.2

50 100 500 1000

20 50 100 500 1000

Effective energy (keV)

Effective energy (keV)

98 Radiographic Testing

regulations specify an alarm threshold of
500 mSv·h–1 (500 mR·h–1) for field gamma
radiography. The energy dependence
curve for one such instrument is shown in
Fig. 14.

Applications

Geiger-müller counters are the most
widely used, general purpose radiation
survey instruments. It must be
remembered that geiger-müller counters,
unlike current ionization chambers, read
pulses (regardless of their energy or
ionizing potential) and register in counts
per minute. Some instruments have a
scale calibrated in milliroentgens per hour
(mR·h–1); however, this is an arbitrary
scale calibrated on the radiation from
radium-226, cesium-137 or some other
energy (Fig. 15). Another scale is
microsieverts per second (µSv·s–1). A
sensitivity versus energy table should
always be consulted before making
measurements with a geiger-müller
instrument.

Radiation Measurement 99

PART 4. Scintillation Detectors

Soon after the discovery of X-rays and and solids, as well as inorganic gases and
radioactivity, it was observed that certain solids, are common scintillators. Organic,
materials emit visible light photons after solid scintillators are available as crystals,
interacting with ionizing radiation. These plastics and gels. Inorganic solid
light photons appear to flash or sparkle scintillators are usually alkali halide
and the materials are said to scintillate. crystals. The scintillation process in
Scintillators commonly used with inorganic materials requires the presence
radiation survey instruments are solid of small amounts of an impurity, or
materials. Being denser than gases, these activator. Inorganic solid scintillators are
scintillators have greater detection commonly used with radiation survey
efficiencies and are useful for low level instruments and are listed in Table 3.
measurements. For gamma photons,
scintillators have detection efficiencies 106 Desirable Scintillator
times greater than typical gas ionization Characteristics
chambers. Detection of alpha and beta
particles, neutrons and gamma photons is A useful and practical scintillator needs to
possible with various scintillator systems have most of the characteristics listed
(Table 3). below. Not all of these characteristics are
ideally satisfied by each scintillator and
Scintillation Process often a compromise is acceptable.

Radiation interactions with matter 1. The scintillator should be of high
produce excitation as well as irrigation. density and large enough to ensure
Ionization refers to the removal of an adequate interaction with the ionizing
electron from an atom and excitation radiation.
refers to the elevation of an electron’s
energy state. The return of excited 2. Efficient conversion of the electron’s
electrons to their normal, lower energy kinetic energy into visible light is
state is called deexcitation. Scintillators required and the light yield should be
excited by ionizing radiation return to linearly related to the deposited
lower energy states quickly and emit electron kinetic energy.
visible light during the deexcitation
process. Radiation detection is possible by 3. The scintillator should be of good
measuring the scintillator’s light output optical quality, transparent to its
(Fig. 16). emitted light and free of hydroscopic
effects, and should have an index of
Materials and refraction close to that of glass.
Characteristics
4. The wavelength of the emitted light
Scintillation materials come in gaseous, should be appropriate for matching to
liquid and solid forms. Organic liquids a photomultiplier tube.

TABLE 3. Common scintillators. Photomultiplier Tubes

Before the advent of photomultiplier
tubes (PMTs), scintillation light photons
had to be visually counted. This limited
the use and development of scintillators.
In the 1940s, the photomultiplier tube

Scintillator a Chemical Radiation FIGURE 16. Energy diagram of scintillation process.
Symbol b Type Detected

Sodium iodide NaI(Tl) gamma Band gap Conduction band Activator excited states
Lithium iodide LiI(Eu) gamma, neutrons Scintillation photon Activator ground state
Zinc sulfide ZnS(Ag) alpha
Bismuth germanate Bi4Ge3O12 gamma Valence band

a. Many other scintillators are available but are not commonly used with
radiation survey instruments.

b. Parentheses indicate impurity used as activator.

100 Radiographic Testing

was developed and dramatically increased gain up to 1010 per emitted
the use of scintillators, to the point where photoelectron. Figure 17 illustrates the
scintillators are preferred over other structure of a photomultiplier tube.4
radiation detectors for many survey
applications. System Electronics

The photomultiplier tube’s function is Once the output pulse from a
to convert the scintillator’s light output photomultiplier tube is generated, it is
into a electrical pulse. The amplified and analyzed. The pulse height,
photomultiplier tube is composed of a or amplitude, is proportional to the
photosensitive layer, called the amount of energy deposited within the
photocathode, and a number of electron scintillator and can be correlated to a
multiplication structures called dynodes. count rate or scale of microsievert per
Conversion of the scintillation light into second (µSv·s–1) or milliroentgen per hour
photoelectrons is accomplished by the (mR·h–1) when calibrated against a known
photocathodes through the photoelectric energy source. (See Fig. 18.)
effect. To maximize the information
contained in the scintillation light, the FIGURE 18. Comparison of sodium iodide (thallium activated)
photomultiplier tube photocathode and germanium detectors for gamma spectroscopy.
should be matched to the scintillator; the
scintillator and photomultiplier tube Sodium iodide
should be optically coupled to minimize (thallium activated)
light losses. detector

Electron multiplication, or gain, is Activity (log scale) 0.662 1.17
accomplished by positively charging the
dynodes in successive stages, so that the 1.09 1.33
total voltage applied to the
photomultiplier tube is around 1000 V. 1.29
Electrons emitted by the photocathode
are focused toward the first dynode; more
electrons are emitted than were initially
incident on the dynode. This is repeated
at each dynode stage. The photocathode
and dynodes are positioned in a glass
enclosed vacuum so that air molecules
will not interfere with the collection of
electrons. The net result of the
photomultiplier tube may be an electron

FIGURE 17. Cutaway drawing of photomultiplier tube, Germanium detector
showing crystal, photocathode, collecting dynodes and
voltage divider network.4

Output pulse

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Energy (MeV)

Dynodes High
voltage

Photoelectron Focusing electrode
Photocathode
Path of primary ionizing particle
Reflector
Crystal
Light photon
Ionizing event

Incident photon

Radiation Measurement 101

PART 5. Luminescent Dosimetry

Thermoluminescent readout time and (7) near tissue
Dosimetry equivalence. Disadvantages include the
loss of information after readout and lack
Thermoluminescence is the emission of of information about the incident
light from previously irradiated materials radiation energy.
after gentle heating. The radiation effect
in thermoluminescent (TL) materials is Both gamma photons and neutrons
similar to that observed in scintillators, produce ionization indirectly. Gamma
except that light photon emission does photons interact with matter, releasing
not occur in thermoluminescent materials electrons that in turn cause ionization.
until some heat energy is supplied Lithium fluoride undergoes interactions
(Fig. 19).5 Measurement of the light with gamma photons and is therefore
photons emitted after heating permits used in gamma dosimetry. Slow neutrons
correlation to the amount of ionizing require the presence of the lithium
radiation energy that was absorbed in the fluoride enriched with lithium-6 for
thermoluminescent material. detection of the (n, α) nuclear reaction.
Thermoluminescent dosimetry (TLD) is Fast neutron detection with lithium
possible for beta, gamma and neutron fluoride would only be possible if the fast
radiations, if the appropriate neutrons were slowed down to thermal
thermoluminescent material is used. energies before reaching the lithium
fluoride thermoluminescent dosimeter.
Lithium Fluoride Properties Nearly complete elimination of neutron
response in lithium fluoride is possible
The most common thermoluminescent with lithium fluoride enriched with
phosphor used in gamma and neutron lithium-7. In a mixed gamma and slow
personnel dosimetry is lithium fluoride. neutron field, distinction of gamma and
Other thermoluminescent phosphors are neutron doses is possible by comparing
available for personnel dosimetry but, for the readings of two lithium fluoride
various reasons, are not as well suited as thermoluminescent dosimeters with
lithium fluoride. The advantages of different lithium-6 contents.
lithium fluoride include its (1) usefulness
over a wide dose range, (2) linear dose FIGURE 20. Typical glow curve. Integrated
response, (3) near dose rate independence, area under curve is measure of radiation
(4) reusability, (5) stability, (6) short exposure.5

FIGURE 19. Thermoluminescence process.5 Photon Light emitted (relative unit)

Unstable state
Metastable state

X-rays

Ground state Irradiated Heated
Unirradiated Time
0 8 16 24
Legend Time (s)
x = electron
o = electron vacancy

102 Radiographic Testing

Thermoluminescent Dosimetric
Readout Systems

Thermoluminescent dosimetric readout
systems are commonly made up of a
sample holder, heating system,
photomultiplier tube (light detector), high
voltage supply, signal amplifier and a
recording instrument. The
thermoluminescent dosimetric sample is
heated indirectly, using electrical
resistance heat applied to a pan or
planchette. The photomultiplier tube
converts the light output into an
electronic pulse that is then amplified
before recording. The recording
instrument may be a plotter or any other
instrument that can measure the
amplified photomultiplier tube output
signal. A plot of the output signal versus
time is equivalent to emitted light
intensity versus heat and results in a glow
curve. The area under the glow curve is
proportional to the absorbed dose
(Fig. 20).5

Uses of thermoluminescent
measurement of radiation include
personnel dosimetry, medical dosimetry,
environmental monitoring and
archeological and geological dating.

Optically Stimulated
Luminescence Dosimetry

Optically stimulated luminescence
dosimeters typically have aluminum oxide
detectors and are available in plastic
holders, or body badges, that are worn at
collar level to measure full body dose.
They can measure gamma ray and X-ray
doses from 10µSv to 10Sv (1 mrem to
1 krem).

Radiation Measurement 103

PART 6. Neutron Detection

Characteristics usually uses radionuclides that emit alpha
or gamma photons and produce neutrons
The neutron is a part of the nucleus, has by (α, n) and (γ, n) reactions with various
no charge and is somewhat larger in mass target materials.
than the proton. It is similar to the
photon in that it has no charge and Neutron Detectors
produces ionization indirectly; it is
different from the photon because it is a There are several mechanisms and devices
nuclear particle and not a unit of used to detect neutrons of various
electromagnetic energy. Because the energies. Ionization chambers,
neutron is an uncharged particle, its proportional counters, scintillators,
interactions with matter are different from activation foils, track etch detectors, film
those of charged particles or photons. emulsions, nuclear emulsions and
thermoluminescent phosphors are some
Ionization by neutrons is indirect: as a of the many devices used to detect
result of neutron interactions with matter, neutrons. The main mechanisms used to
recoil nuclei, photons or charged particles detect neutrons in these devices are the
are produced and then interact with (n, α), (n, p), (n, d), (n, f ) and (n, γ)
matter by various mechanisms that cause nuclear reactions.
ionization.
Proportional Neutron Detectors
Neutron Sources
Many fast and slow neutron counters use
Neutrons are classified according to their proportional counting chambers filled
energies as shown in Table 4. with boron trifluoride (BF3) gas, often
enriched in boron-10. The interaction of
Some radionuclides (such as thermal (slow) neutrons with boron gas
californium-252) may decay by releases an alpha particle of several
spontaneous fission and emit neutrons megaelectronvolts that is easily detected
with fission fragments, photons and in the proportional mode. Fast neutrons
electrons. Induced fission reactions, such are detected by a similar counter, in
as those occurring in a nuclear reactor which thermal neutrons are absorbed in
with uranium, emit about 2.5 neutrons an external cadmium shield; the fast
per fission. Fission neutrons range in neutrons that pass through the shield are
energy from 0.025 eV to about 16 MeV. thermalized in hydrogen rich material
Other neutron sources are the result of and counted in the proportional
various nuclear reactions and produce chambers.
either a spectrum of neutron energies or
monoenergetic neutrons. Common Scintillation
neutron producing nuclear reactions are
the (γ, n), (α, n), (p, n), (d, n) and (α, 2n) Scintillators containing lithium-6,
reactions and may use radionuclide boron-10 and hydrogenous plastics have
emissions or accelerated particles to been used as neutron detectors. Lithium-6
initiate the reaction. Neutron radiography is used as lithium iodide (europium
activated) and in lithium glasses to detect
TABLE 4. Neutron classification. slow and fast neutrons. Scintillators
loaded with boron-10 are used for slow
Class Energy neutron detection. Plastic scintillators
with high hydrogen content are used in
Thermal < 0.3 meV fast neutron detection and spectroscopy
Epithermal >1 eV by measuring the energy deposited by
Slow 30 meV to 100 eV recoil protons.
Intermediate 100 eV to 10 keV
Fast 10 keV to 10 MeV Activation Foils
Relativistic greater than 10 MeV
Introducing certain materials to an
incident neutron flux will result in these
materials becoming radioactive. The

104 Radiographic Testing

process is called activation and gaining
information about the incident neutron
flux and energy is possible by analyzing
the radiations emitted from the activated
foil. Activation foils rely on (n, γ), (n, p),
(n, α), (n, f ) and other nuclear reactions
to cause the activation. Selection of the
proper activation foil can give a rough
estimate of the neutron energy spectrum.
In high neutron flux fields, where
instruments would fail, activation foils are
used as integrating detectors.

Miscellaneous Neutron Detectors

Track etch detectors, nuclear emulsions
and film have all been used to detect
neutrons. Various neutron interactions
with the detector material or foils in
intimate contact with the detectors allow
these systems to operate as integrating
dosimeters.

Radiation Measurement 105

PART 7. Semiconductors

Certain semiconductor crystals, when natural movement of electrons and holes
exposed to ionizing radiation, become (reverse bias), the potential barrier across
conductors and may be used as radiation the junction is increased and a depletion
detectors. Semiconductors are most often region is produced.
used for low level spectroscopic
measurements of alpha particles, beta This depletion region is the sensitive
particles and gamma rays in laboratory part of the detector and is analogous to
settings and in X-ray diffraction the gas volume in a gas ionization
equipment (Table 5). detector. Charged particles, on entering
the depletion region, produce electron
The most widely used semiconductor hole pairs analogous to the ion pairs
devices are diffused p-n junction, surface produced in gas ionization chambers.
barrier and lithium drifted detectors. Because an electric field exists in this
Semiconductor detectors have found their region, the charge produced by the
broadest application in the field of ionizing particle is collected, producing a
spectroscopy, although lithium drifted pulse of current. The size of the pulse is
detectors are also being used for gamma proportional to the energy expended by
ray detection. the particle.

Diffused p-n Junction Surface Barrier Detectors
Detector
The operation of surface barrier and
The diffused p-n junction detector lithium drifted detectors is the same as for
(Fig. 21a) gets its name from its the p-n junction: a depletion region is
manufacturing process. A slice of p type produced, in which there exists an electric
(electron depleted) silicon or germanium field. The means of producing the
crystal, with a layer of n type (electron depletion region (as well as its dimension
rich) impurity (usually phosphorus) and location within the crystal) vary from
deposited on the surface, is heated to one type of detector to another.
form a p-n junction just below the surface.
The phosphorus may also be painted onto The operation of a surface barrier
the silicon and made to diffuse into it by detector (Fig. 21b) depends on the surface
applying heat. Because the n type material conditions of the silicon or germanium.
has an excess of electrons and the p type At the surface of a piece of pure crystal,
has an excess of holes (holes may be an electric field exists such that both
thought of as unit positive charges), the holes and electrons are excluded from a
natural action of the combined materials thin region near the surface. For n type
tends to align the electrons on one side of crystals, the field repels free electrons. If a
the junction and the holes on the other. metal is joined to the crystal, the free
Thus a difference of potential is built up electrons are still repelled but a
across the junction. concentration of holes is produced
directly under the surface. If a reverse bias
By applying an external voltage to the is then applied, a depletion region is
crystal of such polarity as to oppose the produced.

TABLE 5. Radiation detector types. Detector Type

____________R_a__d_ia_t_i_o_n__T_y_p_e______________ Silicon surface barrier detectors
Charged Particle Gamma Ray X-Ray Silicon (lithium activated) detector systems for X-ray detection exclusively below 30 keV
Coaxial germanium (lithium activated) detectors
x —— —— Coaxial pure germanium detectors
—— —— x Planar, pure germanium detectors: low energy photon spectrometer for energy range
—— of 2 to 200 keV
—— x ——
—— x x
x x

106 Radiographic Testing

Surface barrier detectors give better but must be operated at liquid nitrogen
resolution for particle spectroscopy than temperatures. For these reasons, coupled
p-n junctions but wider depletion regions with the small sensitive volumes
are possible with the latter. (The wider the obtainable to date, semiconductor
depletion region, the higher the energy of detectors have not received widespread
particles can be analyzed because a application in radiation survey
particle must expend all its energy in a instruments.
depletion region.)

Lithium Drifted Detectors FIGURE 22. Cross section of lithium drifted detector.

The lithium drifted detector (Fig. 22) is Uncompensated p type
produced by diffusing lithium into low
resistivity p-type silicon or germanium. Lithium
When heated under reverse bias, the diffused layer
lithium ions serve as an n type donor.
These ions drift into the silicon or Incident particle ±±±±
germanium in such a way that a wide
layer of the p type material is Gold contact Lithium
compensated by the lithium, yielding an surface barrier compensated layer
effective resistivity comparable to that of
the intrinsic material. Wider depletion Guard ring
regions can be obtained with the lithium
drift process than by any other means.
Consequently, lithium drifted detectors
are most useful in gamma spectroscopy
work.

Silicon detectors can be operated at
room temperatures but exhibit low
efficiency for gamma rays. Germanium
detectors have higher gamma efficiencies

FIGURE 21. Cross sections: (a) diffused p-n junction detector;
(b) surface barrier detector.

(a) Charged particles n type region produced by
enter from this side diffussion in phosphorus
Contact for
electrical leadoff

Depletion
region

p type silicon

Electrical lead Metal electrode

(b) Charged particles
enter from this side
Contact for
electrical leadoff

Thin gold electrode

Depletion
region

n type silicon

Electrical lead Metal electrode

Radiation Measurement 107

PART 8. Film Badges1,5-7

One of the most important uses of through a film emulsion and can render
radiographic film as a means of measuring many grains developable. The number of
radiation is in film badges. Individuals grains exposed per photon interaction
who work with isotope radiation sources varies from one (for X-radiation of about
and X-ray machines are required by codes 10 keV) to 50 or more (for a 1 MeV
to wear badges indicating cumulative photon).
exposure to ionizing radiation. Film
badges are discussed in this volume’s Because a grain is completely exposed
chapter on radiation safety and by the passage of an energetic electron, all
elsewhere.2,5,6 X-ray exposures are, as far as the
individual grain is concerned, extremely
Latent Image Formation short. The actual time that an electron is
within a grain depends on the electron
Latent image formation is a very subtle velocity, the grain dimensions and the
change in the silver halide grain of film. squareness of the hit. A time on the order
The process may involve the absorption of of 10–13 s is representative. (In the case of
only one or, at most, a few photons of light, the exposure time for a single grain
radiation and this may affect only a few is the interval between the arrival of the
atoms out of some 109 or 1010 atoms in a first photon and the arrival of the last
typical photographic grain. Formation of photon required to produce a stable latent
the latent image, therefore, cannot be image.)
detected by direct physical or analytical
chemical means. The process that made Development
an exposed photographic grain capable of
transformation into metallic silver (by the Many materials discolor with exposure to
mild reducing action of a developer) light (some kinds of wood and human
involved a concentration of silver atoms skin are examples) and could be used to
at one or more discrete sites on the record images. Most of these materials
photographic grain. react to light exposure on a 1:1 basis —
one photon of light alters one molecule or
In industrial radiography, the image atom. In the silver halide system of
forming effects of X-rays and gamma rays, radiography, however, a few atoms of
rather than those of light, are of primary photolytically deposited silver can, by
interest. The agent that actually exposes a development, be made to trigger the
film grain (a silver bromide crystal in the subsequent chemical deposition of some
emulsion) is not the X-ray photon itself 109 or 1010 additional silver atoms,
but rather the electrons (photoelectric and resulting in an amplification factor on the
compton) resulting from an absorption order of 109 or greater. This amplification
event. process can be uniform and reproducible
enough for quantitative radiation
The most striking difference between measurements.
X-ray and visible light exposures arises
from the difference in the amounts of Development is essentially a chemical
energy involved. The absorption of a reduction in which silver halide is
single photon of light transfers a very converted to metallic silver. To retain the
small amount of energy to the crystal — photographic image, however, the
only enough energy to free a single reaction must be limited largely to those
electron from a bromide (Br–) ion. Several grains that contain a latent image; that is,
successive light photons are required to to those grains that have received more
make a single grain developable (to than a prescribed minimum radiation
produce within it, or on it, a stable latent exposure.
image). The passage of an electron
through a grain can transmit hundreds of Compounds that can be used as
times more energy than the absorption of photographic developing agents are those
a light photon. Even though this energy is in which the reduction of silver halide to
used inefficiently the amount is enough metallic silver is catalyzed (speeded up) by
to make the grain developable. the presence of metallic silver in the
latent image. Those compounds that
In fact, a photoelectron or compton reduce silver halide, in the absence of a
electron can have a fairly long path catalytic effect by the latent image, are
not suitable developing agents because

108 Radiographic Testing

they produce a uniform overall density on
the processed film.

Closing

More information on the radiographic
latent image, its formation and processing
are available elsewhere.1,7 The correct use
of film badges is especially important for
safety in the conduct of radiographic
testing programs and is discussed in this
book’s chapter on radiation safety and
elsewhere.2,5,6

Radiation Measurement 109

References

1. Rivkin W.B. and G. Wicks. Ch. 4, Radiation Safety
“Radiation Detection and Recording.”
Nondestructive Testing Handbook, Aerna, V. Ionizing Radiation and Life. Saint
second edition: Vol. 3, Radiography and Louis, MO: C.V. Mosby Company
Radiation Testing. Columbus, OH: (1971).
American Society for Nondestructive
Testing (1985): p 152-185. Alpen, E.L. Radiation Biophysics. Upper
Saddle River, NJ: Prentice Hall (1990).
2. ANSI N13.5-1972 (R1989), Direct
Reading and Indirect Reading Pocket Basic Radiation Protection Criteria. NCRP
Dosimeters for X- and Gamma-Radiation, Report 39. Washington, DC: National
Performance, Specifications for. New Council on Radiation Protection and
York, NY: American National Measurements (1971).
Standards Institute (1989).
Hine, G. Instrumentation in Nuclear
3. Knoll, G.F. Radiation Detection and Medicine. New York, NY: Academic
Measurement, second edition. New Press. Vol. 1 (1967).
York, NY: John Wiley and Sons (1989).
Instrumentation and Monitoring Methods for
4. Cember, H. Introduction to Health Radiation Protection. NCRP Report 57.
Physics, second edition. New York, NY: Washington, DC: National Council on
Pergamon Press (1983). Radiation Protection and
Measurements (1978).
5. Cameron, J.R., N. Suntharalingam and
G.N. Denney. Thermoluminescent International Commission on Radiological
Dosimetry. Madison, WI: University of Protection. Ann. ICRP 21 (1-3), 1990
Wisconsin Press (1968). Recommendations of the International
Commission on Radiological Protection.
6. Bush, J. Gamma Radiation Safety Study ICRP Publication 60. Oxford, United
Guide, second edition. Columbus, OH: Kingdom: Pergamon Press (1991).
American Society for Nondestructive
Testing (2001). Ionizing Radiation: Sources and Biological
Effects. New York, NY: United Nations
7. Quinn, R.A. and C.C. Sigl, eds. Scientific Committee on the Effects of
Radiography in Modern Industry, Atomic Radiation (1982).
fourth edition. Rochester, NY: Eastman
Kodak Company (1980). Martin, A. and S.A. Harbison. An
Introduction to Radiation Protection,
Bibliography third edition. London, United
Kingdom: Chapman and Hall (1986).
Radiation Measurement
Moe, H.J. Radiation Safety Technician
Attix, F.H. and W.C. Roesch. Radiation Training Course. Prepared for the
Dosimetry. Vol. 2. New York, NY: United States Atomic Energy
Academic Press (1966). Commission under contract
W-31-109-Eng-38. Argonne, IL:
Attix, F.H., ed. Luminescence Dosimetry. Argonne National Laboratory (May
Symposium Series No. 8. Washington 1972).
DC: Atomic Energy Commission
(1967). Morgan, K.Z. and J.E. Turner. Principles of
Radiation Protection. New York, NY:
A Handbook of Radioactivity Measurements John Wiley and Sons (1973).
Procedures. NCRP Report 58.
Washington, DC: National Council on National Research Council, Committee on
Radiation Protection and the Biological Effects of Ionizing
Measurements (1978). Radiations. Health Effects of Exposure to
Low Levels of Ionizing Radiations.
Lapp, R.E. and H.L. Andrews. Nuclear Washington, DC: National Academy
Radiation Physics. Upper Saddle River, Press (1990).
NJ: Prentice-Hall (1972).
Personnel Dosimetry Systems for External
Price, W.J. Nuclear Radiation Detection, Radiation Exposures. Technical Report
second edition. New York, NY: Series No. 109. Vienna, Austria:
McGraw-Hill (1964). International Atomic Energy Agency
(1970).

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Rivkin, W.B. Personnel Monitoring Radiation ASTM E 666-97, Standard Practice for
Safety and Protection in Industrial Calculating Absorbed Dose from Gamma
Applications: Proceedings of a or X Radiation. West Conshohocken,
Symposium. DHEW Publication PA: ASTM International (1997).
No. (FDA) 73-8012. Washington, DC:
Department of Health, Education and IEEE/ANSI N323-1978, American National
Welfare (1973). Standard Radiation Protection
Instrumentation Test and Calibration.
Rollo, F.D., ed. Nuclear Medicine Physics, New York, NY: Institute of Electrical
Instrumentation and Agents. Saint Louis, and Electronics Engineers (1978).
MO: C.V. Mosby and Company (1977).
IEEE 309-1999 / ANSI N42.3-1999, IEEE
Shapiro, J. Radiation Protection: A Guide for Standard Test Procedures and Standard
Scientists and Physicians. Cambridge, Bases for Geiger-Mueller Counters. New
MA: Harvard University Press (1990). York, NY: Institute of Electrical and
Electronics Engineers (1999).
Shleien, B. and M. Terpilak. The Health
Physics Handbook. Olney, MD: Nuclear IEEE N42.20-1995, ANSI Performance
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Monitors. New York, NY: Institute of
Simmons, G.H. A Training Manual for Electrical and Electronics Engineers
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DMRG 70-3. Washington, DC: Bureau
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Turner, J. Atoms, Radiation, and Radiation Occupational Safety and Health
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United States Government Printing
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ANSI N13.15-1985, Dosimetry Systems,
Performance of Personnel
Thermoluminescence. New York, NY:
American National Standards Institute
(1985).

ANSI N13.2-1969 (R1982), Administrative
Practices in Radiation Monitoring (A
Guide for Management). New York, NY:
American National Standards Institute
(1982).

ANSI N13.27-1981 (R1992), Dosimeters and
Alarm Ratemeters, Performance
Requirements for Pocket-Sized Alarm.
New York, NY: American National
Standards Institute (1992).

ANSI N13.7-1983 (R1989), Photographic
Film Dosimeter Performance, Criteria for.
New York, NY: American National
Standards Institute (1989).

ANSI N42.5-1965, American National
Standard for Bases for GM Counter
Tubes. New York, NY: Institute of
Electrical and Electronics Engineers
(1965).

ANSI N43.3-1993, General Radiation Safety
— Installations Using Non-Medical
X-Ray and Sealed Gamma-Ray Sources,
Energies up to 10 MeV. New York, NY:
American National Standards Institute
(1993).

ASTM E 1894-97, Standard Guide for
Selecting Dosimetry Systems for
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West Conshohocken, PA: ASTM
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Radiation Measurement 111

6

CHAPTER

Radiation Safety1

William D. Burnett, Albuquerque, New Mexico
Garry L. Balestracci, Balestracci Unlimited,
Charlestown, Rhode Island
Frank A. Iddings, San Antonio, Texas

PART 1. Management of Radiation Safety

MOVIE. Introduction Radiation Safety
Radiation Inspections and Audits
injury. There are many considerations involved
in setting up and outfitting a safe Government Licensing6
radiographic facility. Commercial
consulting firms specializing in personnel Most manufacturers specify that radiation
dosimetry and radiation protection may producing devices should be operated
help with this goal. Regardless of who only by qualified personnel. Most states
establishes or monitors the program, it is require the registration of radiation
vitally important that radiation exposures machines and provide survey services
to personnel be reduced to as low a level during compliance audits. Licenses to
as is practical. To this end, each possess byproduct materials (radioisotopes
radiographic facility should appoint a other than radium) are issued by the
radiation safety officer, who is responsible Nuclear Regulatory Commission (NRC) or
for systematically assuring management states operating under its rules (agreement
that a safe operation exists. The functions states).
of the radiation safety officer are discussed
later in this part. Radiation Safety Officer

In the twenty-first century, some Personnel responsible for work with
publications of the 1970s2-5 are still useful radiation are also responsible for radiation
to document information in later safety. A radiation safety officer (RSO) needs
publications. However, all guidelines, to be appointed if fields may be
standards, regulations and handbooks experienced in excess of 1 mSv
have a shelf life beyond which some of (100 mrem) per work week in accessible
their information is obsolete. It is the regions inside or outside externally
duty of inspectors and safety personnel to applied shielding. The radiation safety
become familiar with the literature and officer is responsible for: (1) technical
refer to up-to-date documents for critical assistance in planning and execution of
decisions. work insofar as radiation safety is
concerned, (2) appraisal of safe operation
Because of potential changes in safety of the radiation source through surveys
requirements, radiation safety officers and and personnel monitoring,
all personnel active in the field of (3) notification of personnel working
radiography should consult the most around the source of any special hazards,
up-to-date publications and regulations (4) reporting of radiation hazards or
before making a determination on the unsafe practices to the proper authorities,
safety of a radiographic facility. Many (5) seeking advice from qualified experts
publications are written specifically to when necessary, (6) keeping records of
describe in detail the requirements and personnel exposures and area dose levels,
techniques involved. The following (7) keeping informed of any changes in
discussion is an overview of radiation the mode of operation of the source and
safety and personnel protection and does (8) periodically providing radiation safety
not attempt to duplicate the information training.
available elsewhere — for example, in the
works cited in the references and A good radiation safety officer has the
bibliography at the end of this chapter. confidence and support of company
Unsealed radioactive sources and the management and the radiography
associated health protection requirements, personnel. Fair and honest treatment,
internal dosimetry, instrumentation and knowledge of the regulations and open
related subjects are not covered in this mindedness to ideas and needs of those
chapter. Note also that safety regulations involved builds a good working
may vary with locality. relationship. This relationship helps to
ensure that corrective actions are taken,
however unsavory. The radiation safety
officer must have access to any level of
management necessary to ensure the
compliance with regulations and

114 Radiographic Testing

procedures to provide for a safe work radiation protection program, an annual
environment. review of the quality assurance program
and a continuous review of the company
Written Procedures program to keep personnel exposures as
low as reasonably achievable (ALARA).
All radiographic work must be covered by Audit procedures for gamma radiography
written procedures that are reviewed and or X-radiography are basically the same,
updated annually. The radiation safety just as observations of temporary field
officer needs to work with operating sites are conducted in a manner similar to
personnel and management in preparing cell or permanent facility audits. These
these procedures so that adequate safety components make up the internal
procedures are integrated with the needs inspection system.
and goals of the workplace environment.
The radiation safety officer can recommend The single most important part of the
approval of a written procedure but only internal inspection system is the radiation
management can approve the procedure safety officer. The radiation safety officer
with a signature. should have sufficient experience and
expertise to observe radiography
The level of management required for operations and immediately recognize
approval depends on the level of risk for infractions or violations as well as good
operation. Where first level management practices. The radiation safety officer
is delegated to approve some procedures, should be able to make a valid assessment
a written delegation of authority from top of the conditions observed and provide
management should be on file in the corrective actions or recommendations to
radiation safety officer’s records. those involved. Any and all discrepancies
should immediately be pointed out to the
Emergencies responsible individuals with a followup
notification to the appropriate
Written procedures should exist for supervision.
actions to be taken in case of an
emergency. While the radiation safety The radiation safety officer should
officer may have considerable authority in conduct audits in person and take
a radiation emergency situation, the appropriate actions to stop violations or
written procedures should make it clear unsafe practices. Unfortunately some
that management is responsible for regulations are instituted as a result of the
assuming the level of risk for any action actions of a few individuals. The integrity
taken in case of an emergency. of the radiation safety officer and the
radiographers are important to a good
The case of a radiographic source that radiation safety program. A good
because of mechanical problems cannot relationship between regulators and
be returned to its storage container licensees is also important to a quality
provides an example. In this situation, all program. Regulators should not be feared
personnel should know from existing or shunned: avoidance gives the
general procedures to evacuate to a safe impression that people have something to
distance or location where a specific, hide.
written procedure, even handwritten, can
be prepared and approved for restoring A number of factors can affect how an
the source. individual reacts to situations. Very few
people start out with the intention to
In a case where an injured or break the rules. But good intentions, lack
unconscious person is exposed to a of training, lack of proper equipment or
hazardous radiation dose rate, time is very misunderstanding of the requirements
important. Written procedures prepared in can result in problems. Many factors can
advance with assignments of roles and contribute to the situation, such as tight
responsibilities, combined with periodic schedules, cost implications and the
training and practice scenarios, can mental health or morale of the persons
facilitate the rapid recovery of an involved. Maybe there is a bonus offered
immobile person without unacceptable to finish the job early. Radiographers that
radiation exposures to recovery personnel. circumvent the regulations or take short
cuts around procedural requirements run
Internal Inspections7 greater risk of accidents or overexposures
than those that continuously operate by
An internal inspection system is essential the book. Audits are necessary to detect
to maintaining a quality industrial and correct breaches of safety procedure.
radiography program. Internal inspection
programs are mandated by regulations What makes up an audit or observation
and are vital to ensure safe operations and varies. Simply questioning a radiography
the welfare of radiography workers as well crew can often provide a false idea of how
as of the general public. the crew normally operates. An
experienced auditor can usually perceive
Required internal inspections consist of more while approaching a radiography
semiannual radiographer audits, an job site and observing the normal work
annual overview audit of the entire

Radiation Safety 115

practices than can be obtained by overall conditions at the work site affect
spending eight hours sitting on a job and the operations. Distances to radiation area
interviewing radiographers. During that boundaries need to be calculated and
amount of time when the auditor’s posted as required to prevent
presence is not known, work ethics are unauthorized entry into the radiography
demonstrated and the real story is told. area. Conditions may require that
Followup interviews should be conducted nonradiography personnel must work in
to verify the details that must be noted: close proximity to the radiography
serial numbers, calibration dates and boundaries. Surveillance is required to
items that need to be checked and maintain control of the established area.
validated. Specific transportation requirements and
regulations mandate how the radiographic
This is not to suggest in any way that exposure device and equipment are
observations should be conducted, as transported to the work location.
some audits are conducted, from a long Radiation surveys must be performed to
distance by hidden auditors with ensure compliance with established
binoculars. Audits should be open procedural requirements. Peak readings
exchanges of information. All parties need to be documented. Emergency
involved should be treated with the procedures and points of contact should
dignity and respect expected in any be reviewed to afford timely response in
business encounter. All involved should the event of an accident or emergency. By
participate in a professional manner. the nature of the operation, an
overexposure or other accident is more
The radiographers should be aware that likely during a temporary field operation.
the sole purpose of the radiation safety
officer, observer or auditor at the job site Permanent facilities are constructed
is to validate that the radiography team is and evaluated to determine restrictions
operating to the established procedures for use. These restrictions allow some
and within the restraints of governing relaxation of the requirements associated
regulations, not to try to catch the with temporary field site operations. If
participants committing infractions. permanent cells are used within the
Systematic or generic deficiencies should parameters established, radiation levels
be addressed to appropriate management outside the facility will always be at
for long term corrective actions. The audit acceptable limits. The safety inspector
process should be a positive experience must confirm that activities are within the
rather than a traumatic one. A more established parameters. Exposure cells
casual, relaxed, audit allows an must be outfitted with alarms and
opportunity to experience the way things warning devices and these devices now
are done. require a daily operability check. Accesses
to the facility must be locked or guarded
Careful observation of details, such as while exposures are being completed.
radiation levels at the posted boundaries,
can be conspicuously determined while In industrial radiography operations,
approaching the job site. Proper high radiation exists in permanent
surveillance techniques, area control exposure cells — for example, facilities
procedures and adherence to proper equipped with cobalt-60 exposure devices
operating procedures should become of 14 TBq (385 Ci). Some permanent
obvious as the auditor approach the facilities also serve as long term storage
radiography operation. areas for radiography exposure devices.
When established as a storage area,
The better the auditor understands additional radiation surveys and postings
operations, the better the ability to are required and should be checked.
identify existing or potential problems. When it is necessary to operate an
Experience provides a higher potential to exposure cell outside of the established
ensure the safety of personnel involved as parameters for use, the cell can be
well as the general public. Large scale established as a temporary field site.
operations with many radiographers or Additional considerations needed for a
multiple locations may require assistant temporary site will then apply. If an alarm
radiation safety officers or radiation safety or warning device malfunctions, a
officer delegates to be assigned to provide permanent facility may be used as a
the support and coverage needed to temporary field site but current
ensure compliance. regulations must be checked to find out
how long.
Temporary Field Sites versus
Permanent Facilities for Isotopic Semiannual Isotopic Source
Sources7 Audits7

At temporary field sites specific restraints Field audits of radiography are required to
apply. Generally each field site operation be conducted semiannually, quarterly in
offers a new challenge. The site should be some locations. Every person,
examined and assessed to determine
problems that might arise. Location and

116 Radiographic Testing

MOVIE. radiographer, radiographer’s assistant or Personnel Certification for
Survey meters. radiation safety officer that operates Radiation Safety
radiography equipment or participates
directly in a radiography operation must The United States Nuclear Regulatory
be observed. A checklist should be used to Commission (NRC) has published rules
ensure that each specific point is properly that govern the use of nuclear, or gamma,
addressed. A regular semiannual radiation in those states that choose to
inspection should cover the following. follow federal regulations, the NRC states.
In contrast, states that wish to use their
1. Determine the source and exposure own regulations, which must meet or
device being used. Verify the serial exceed Nuclear Regulatory Commission
number of the source and the requirements, are known as agreement
exposure device. states and their regulations are in force for
nuclear radiation in those states. Because
2. Check that the source is safe from X-rays (unlike gamma rays) are not
unauthorized removal or tampering. generated by nuclear materials, the
Nuclear Regulatory Commission does not
3. Check the condition of the equipment have jurisdiction over X-ray radiography
in use. Are a sufficient number and each state is responsible for regulating
properly functioning, calibrated survey X-radiography. Radiographers working in
meters available on the job site? Are any state must be aware of who has
the exposure device, control assembly jurisdiction over radiation safety and must
and source guide tubes in good adhere to the requirements that govern in
working condition? Does the that state. In some instances, large
equipment appear to have received metropolitan areas also have requirements
adequate inspection and maintenance and these must also be met when working
for the conditions of use? in those areas.

4. Check to ensure that the equipment is Safety Personnel Certification
being operated properly and in
accordance with established In May 1997, the Nuclear Regulatory
procedures. Are good collimators and Commission published a rule requiring
shielding being used? Are practices that all industrial radiographers using
being followed to keep exposures as radioactive materials be certified through
low as reasonably achievable? Are either an approved independent certifying
trainees and assistants being properly organization (ICO) or an agreement state
supervised? program that complied with the criteria in
10CFR [Code of Federal Regulations:
5. Do all persons involved with the Title 10], Part 34, Appendix A.8 The final
operation have required personnel deadline for compliance was set as
monitoring devices? Is each dosimeter July 1999 for Nuclear Regulatory
within calibration, not discharged Commission states and as July 2000 for
beyond its range? Is a agreement states.
thermoluminescent dosimeter badge
or film badge available and being The American Society for
used? Is an alarming rate meter Nondestructive Testing (ASNT), in an
available and within calibration? effort to provide a service to industry,
developed the American Society for
6. Ensure that the area is adequately Nondestructive Testing’s Industrial
posted in accordance with applicable Radiography Radiation Safety Personnel
procedures. Signs must be posted for (IRRSP) program,9,10 which was sent to the
restricted and high radiation area Nuclear Regulatory Commission for
boundaries. review in late 1997. In May 1998, The
Nuclear Regulatory Commission formally
7. Check to ensure that the high approved the American Society for
radiation is under constant direct Nondestructive Testing as an independent
surveillance at all times while the certifying organization and accepted the
source is exposed. Are adequate radioactive materials (RAM) portion of the
controls established to keep Industrial Radiography Radiation Safety
unauthorized personnel out of the Personnel examinations.
radiography area?
The Nuclear Regulatory Commission
8. Are procedures being properly does not take responsibility for radiation
followed? Are surveys being taken as producing machines, such as X-ray
required? Do the people involved machines used in radiographic testing.
display adequate competence for the Each individual state was responsible for
tasks involved? determining their own certification
requirements for radiographers using
9. Check the records to ensure that the X-radiation. The agreement states, to
source use log agrees with the source
and equipment in use. Is all required
information properly documented?
Are the transportation records in
order?

Radiation Safety 117

minimize duplication and establish their knowledge of safety regulations by
uniformity between the States’ successfully completing a safety
certification requirements, formed the examination on the type of radiation to
Conference for Radiation Control be used in the course of their work. To be
Program Directors (CRCPD). In early eligible to sit for these safety
1998, the American Society for examinations, radiographers must be able
Nondestructive Testing asked the to show that they have had adequate
Conference for Radiation Control training and experience in performing
Program Directors to review the Industrial radiography.
Radiography Radiation Safety Personnel
program to determine if it would meet the Transportation of
requirements of the agreement states. Radioactive Materials

In September 2001, after detailed Radioactive material is considered
review and some revision of the program, hazardous material. As a result its
the Conference for Radiation Control shipment within the United States is
Program Directors formally approved the controlled by the Department of
American Society for Nondestructive Transportation under the Code of Federal
Testing as an independent certifying Regulations, Title 49, Subtitle B,
organization and recommended Parts 171-177.11 These regulations
acceptance of the radioactive materials prescribe the rules and procedures for
examinations and X-ray examinations for packaging, marking, labeling, placarding
use by agreement states. This decision was and shipping.
sent to all agreement states, because each
state makes its own decision whether or Additional requirements for the
not to accept recommendations of the international shipment of such materials
Conference for Radiation Control by air are set forth by the International
Program Directors. Air Transport Association (IATA).

Radiographer Certification Except for very minor quantities, use of
the Postal Service for transport of
Radiographers are generally required to radioactive materials is prohibited.
carry two types of certification, one based
on technical competence and the other Finally the Inter-Governmental
based on the knowledge of safety Maritime Consultative Organization
regulations. The requirements listed in (IMCO) and the International Atomic
commercial codes, standards and Energy Agency (IAEA, an office of the
specifications are predominantly technical United Nations) represent the collection
and rely on the contractor (the of nations around the world that regulate
radiographer’s employer) to ensure that all the international transport of dangerous
applicable safety requirements are met. goods by sea.
The safety requirements are detailed by
the local, state or federal government Disposal
regulatory agencies that have jurisdiction
over radiography in the locale where the The disposal of leaking sources,
work is to be performed. contaminated equipment or sources
decayed below useful levels must be
Technical certification is required by according to the Code of Federal
the code or standard governing a specific Regulations, Title 10.12 Generally, a
project. The purpose of this certification is commercial radioactive waste disposal
to ensure that the radiographer can make service licensed by the Nuclear Regulatory
proper exposures and accurately interpret Commission is used for this purpose,
radiographs in accordance with the either directly by the owner of the source
requirements of the governing code or or indirectly by returning the source to
specification. Each code or specification the manufacturer.
has varying technical requirements and
each will specify that a radiographer be
certified somehow before working on
projects governed by those documents. A
certified radiographer will be able to
produce acceptable radiographs that
accurately show that the quality of
workmanship required by the designer
has been achieved.

Safety certification is required by local,
state and federal regulatory agencies.
Because of the dangers of penetrating
radiation, these agencies want to ensure
the safety of the general public and
require that all radiographers demonstrate

118 Radiographic Testing

PART 2. Dose Definitions and Exposure Levels

Radiation Quantities and from the old centimeter-gram-second
Units (CGS) system. In the SI system, the unit
for radiation dose is the gray (Gy). The
Radiation is measured by the gray is useful because it applies to doses
International System of Units (SI), absorbed by matter at a particular
described elsewhere in this volume. SI location. It is expressed in energy units
units include the becquerel, coulomb, per mass of matter or in joules per
sievert and gray. The literature for kilogram (J·kg–1). The mass is that of the
radiation safety also uses older units, such absorbing body. One gray equals 100 rad
as roentgen, curie, rad and rem. Because equals 10 000 ergs per gram
of the widespread use of the older units in (1 Gy = 100 rad = 10 000 erg·g–1).
the United States, especially in regulatory
documents dealing with health and safety, Dose Equivalent. Dose equivalent H is a
the United States Department of quantity used for radiation protection
Commerce in 1998 accepted these older that expresses on a common scale for all
units with SI.13 All these units are irradiation incurred by exposed persons.
discussed briefly below.1,14 The SI unit of dose equivalent is the
sievert, equal to 100 rem (1 Sv = 100 rem).
Disintegration Rate. Disintegration rate is The SI system’s unit for the dose absorbed
the rate at which a radionuclide decays. by the human body (formerly rem for
In SI, the unit for radioactivity is the roentgen equivalent man; also known as
becquerel (Bq), one disintegration per ambient dose equivalent, directional dose
second. Because billions of disintegrations equivalent, dose equivalent, equivalent dose
are required in a useful source, the and personal dose equivalent) is similar to
multiplier prefix giga- (109) is used and the gray but includes quality factors
the unit is normally seen as gigabecquerel dependent on the type of radiation. This
(GBq). An older unit is the curie (Ci), absorbed dose has been given the name
simply the radiation of 1 g of radium. A sievert (Sv) but its dimensions are the
curie is equivalent to 37 GBq, that is, to same as the gray (J·kg–1), that is,
3.7 × 1010 disintegrations per second. 1 Sv = 1 J·kg–1.

Exposure. Exposure is a measure of Quality Factor. Quality factor15-18 is a
X-radiation or gamma radiation based on modifying factor used in determining the
the ionization produced in air by X-rays dose equivalent. The quality factor
or gamma rays. The unit for quantity of corrects for the dependence of biological
electric charge is the coulomb (C), where factors on the energy and type of the
1 C = 1 A × 1 s. The original roentgen (R) radiation. A formerly commonly used
was the quantity of radiation that would term, relative biological effect, is restricted
ionize 1 cm3 of air to 1 electrostatic unit in use to radiobiology. For practical
(ESU) of charge (where 1 ESU =
3.3356 × 10–10 C) of either sign. TABLE 1. Radiation weighting factors.31
A roentgen is equivalent to
258 microcoulombs per kilogram of air Radiation Type Quality Factor a
(1 R = 258 µC·kg–1 of air). This
corresponds to 1.61 × 1015 ion pairs per X-rays 1
1 kg of air, which has then absorbed
8.8 mJ (0.88 rad, where rad is the obsolete Gamma rays 1
unit for radiation absorbed dose, not the
SI symbol for radian). Beta rays 1

Absorbed Dose. Absorbed dose is the Neutrons 2 to 11b
mean energy imparted to matter by
ionizing radiation per unit mass of Neutrons of unknown energy 10
irradiated materials at the place of
interest. The roentgen (R) was an intensity High energy protons 10
unit but was not representative of the
dose absorbed by material in the radiation Alpha particles 20
field. The radiation absorbed dose (rad)
was first created to measure this value and Multiple charged particles 20
was based on the erg, the energy unit
Fission fragments 20

Heavy particles of unknown charge 20

a. Value of quality factor at point where dose
equivalent is maximum in 300 mm (12 in.) diameter
cylinder tissue equivalent phantom.

b. Quality factor depends on energy of neutron.

Radiation Safety 119

purposes the quality factors in Table 1 are Permissible Doses
conservative. For example, consider an
absorbed dose in the lens of the eye of Concept of ALARA (As Low As
1 mGy (0.1 rad) from 2 MeV neutrons. Reasonably Achievable)19
The dose equivalent is:
All persons should make every reasonable
(1) H =  Dose in  × Quality effort to maintain radiation exposures as
milligray   low as is reasonably achievable, taking
 factor  into account the state of technology and
the economics of improvements in
= 1 mGy × 10 relation to benefits to the public health
and safety. In this sense, the term
= 10 mSv permissible dose is an administrative term
mainly for planning purposes.
Compound Units
Prospective Annual Limit for
Roentgens could be measured with an Occupationally Exposed Personnel
ionization chamber that, when placed
1.0 m (39 in.) from the radiation source, The maximum permissible prospective
provided necessary information — one dose equivalent for whole body irradiation
roentgen per hour at one meter (1 R·h–1 at is 50 mSv (5 rem) in any one year.15 The
1 m), for example. The roentgen per hour Nuclear Regulatory Commission19 has
(R·h–1) was used to designate the exposure further restricted for its licensees the rate
to an ionizing radiation of the stated at which this planned annual dose may
value. The SI unit used for this exposure be received by averaging over calendar
rate is the sievert (Sv), 100 times as large quarters rather than calendar years. This
as the compound unit it replaces: maximum dose and limits for other parts
1 Sv·h–1 = 100 R·h–1. The radiation of the body are summarized in Table 2.
received from 1 R·h–1 appeared equal to
about 1 rem, so the relationship is Permissible Levels of Radiation in
approximated as Unrestricted Areas19
1 R·h–1 = 0.01 Gy·h–1 = 10 mGy·h–1.
Nonoccupationally exposed personnel or
A previously popular unit, roentgen per all personnel in unrestricted areas (see
curie per hour at one meter (R·Ci–1·h–1 at below) shall not receive more than
1 m), is expressed in SI units as 1.0 mSv (0.1 rem) to the whole body in
millisievert per gigabecquerel per hour at any period of one calendar year.
one meter (mSv·GBq–1·h–1 at 1 m), such
that 1 mSv·GBq–1·h–1 at 1 m = Restricted Areas
3.7 R·Ci–1·h–1 at 1 m. In this relationship,
roentgen converts to millisievert on a A restricted area needs to be established
one-to-ten basis. where either (1) a dose in excess of 20 µSv
(2 mrem) can be received in any 1 h or
Exposure charts were often made by (2) a dose in excess of 1.00 mSv
using curie minutes at a squared distance (100 mrem) can be received in a calendar
from source to sensor in inches. This was year.
written Ci·min·in.–2. Exposure charts
made in SI use gigabecquerel minutes Exposure of Minors19
for a squared distance from source
to sensor in centimeters, where An individual under 18 years of age must
1 Ci·min·in.–2 = 50 GBq·min·cm–2. not be exposed to greater than 10 percent
of the limits for occupationally exposed
TABLE 2. Maximum permissable dose per quarter of workers, that is, 10 percent of 12 mSv
calendar year (3 mo) for whole body irradiation.19 (1.25 rem) per quarter to the whole body
and similarly for the hands, forearms,
___D_o__s_e_p_e__r_Q__u_a_r_te__ra___ feet, ankles and skin of the whole body.

Radiation Workers mSv (rem) Exposure of Females

Whole body; head and 12 (1.25) During the entire nine months of
Active blood forming organs 12 (1.25) gestation the maximum permissible dose
Lens of eyes 12 (1.25) equivalent to the fetus from occupational
Gonads 12 (1.25) exposure of the declared pregnant woman
Hands and forearms b 188 (18.75) should not exceed 5 mSv (0.5 rem) evenly
Feet and ankles 188 (18.75) distributed over the entire pregnancy.15-21
Skin of whole body 75 (7.5)

a. These numbers are obtained by dividing annual doses of 5, 75 and 30,
respectively, by 4.

b. All reasonable efforts should be made to keep exposure of hands and
forearms within the general limit for skin.2,15-18

120 Radiographic Testing

PART 3. Radiation Protection Measurements

MOVIE. Personnel Dosimetry21,22 gamma radiation and about 0.8 µC·kg–1 (a
Check few mR) of 100 keV X-rays. A useful range
equipment. Requirements is from about 0.8 µC·kg–1 (a few mR) to
500 mC·kg–1 (2 kR) can be covered by two
Personnel monitoring must be performed commonly available films or two
on all occupationally exposed persons emulsions of different sensitivity on one
who may receive in a calendar quarter film base. For energies below 200 keV,
more than one fourth of the applicable film overresponds where, for example, the
doses in Table 2. Occasional visitors to photographic density per roentgen at
restricted areas, including messengers, 40 keV is about 20 times higher than for
servicemen and deliverymen, can be 1 MeV photons. Metallic filters covering
regarded as nonoccupationally exposed portions of the film provide additional
persons who do not need to be provided readings that help determine the incident
personnel monitors when it is improbable radiation energy and afford a means of
that they would receive in one year a dose computing a dose from appropriate
equivalent exceeding the nonoccupational calibration curves. Film has several
limit of 5 mSv (0.5 rem). Long term undesirable characteristics. Fogging may
visitors in an installation should be result from mechanical pressure, elevated
regarded as occupationally exposed if they temperatures or exposure to light. Fading
are likely to receive a dose equivalent of the latent image may result in
greater than 5 mSv (0.5 rem) per year.
FIGURE 1. Radiation survey meter incorporates air filled
X-Rays, Gamma Rays and ionization chamber vented to atmosphere, with five
Electrons selectable linear ranges: 0 to 50 µSv·h–1 (0 to 5 mR·h–1),
0 to 500 µSv·h–1 (0 to 50 mR·h–1), 0 to 5 mSv·h–1
For radiation protection measurement, the (0 to 500 mR·h–1), 0 to 50 mSv·h–1 (0 to 5 R·h–1),
choice lies among ionization chambers, 0 to 500 mSv·h–1 (0 to 50 R·h–1).
film badges, photoluminescent glasses
and thermoluminescent dosimeters.
(These and other dosimetric technologies
are discussed in the chapter on radiation
measurement.)

Ionization Chambers. The principal
advantages of ionization chambers
(Fig. 1), particularly those of the
self-reading type, are the simplicity and
speed with which readings are made. They
are useful, therefore, particularly for
monitoring exposures during nonroutine
operations or during transient conditions
or for monitoring short term visitors to an
installation. Chambers should be tested
for leakage periodically and those that
leak more than a few percent of full scale
over the period of use should be removed
from service. Most of these ionization
chambers are small, about the size of a
pencil, and are charged on a separate
device. They read from a few hundredths
to a few sievert (a few tens to a few
hundred milliroentgen) of exposure.

Film Badges. Small badges containing
special X-ray films are popular personnel
dosimeters (Fig. 2a). The sensitivity of
available emulsions is sufficient to detect
about 2.6 µC·kg–1 (10 mR) of cobalt-60

Radiation Safety 121

decreased sensitivity but may be installations. Very small
minimized by special packaging to thermoluminescent dosimeters can be
exclude moisture and by storage in a used to measure exposure to specific parts
refrigerator or freezer before distribution. of the body. They probably represent the
Film dosimeters also exhibit directional technique of choice for measurement of
dependence, particularly for the densities finger, hand or eye dose. They have a
recorded behind metal filters. useful range down to 1 µC·kg–1 (several
Photoluminescent Glasses. Silver activated mR) for lithium fluoride and even lower
metaphosphate glasses, when exposed to for more exotic thermoluminescent
ionizing radiation, accumulate fluorescent dosimetric materials.
centers that emit visible light when the
glass is irradiated with ultraviolet light. Others. Electronic dosimeters and hybrid
The intensity of the light is proportional technologies are also available.
to radiation exposure up to 250 mC·kg–1
(1000 R) or more. Glass dosimeters exhibit Neutrons
energy dependence below 200 keV and
are also subject to fading. They are useful For neutron fields the practical devices are
down to only 250 µC·kg–1 (1 R). nuclear track film, thermoluminescent
Thermoluminescence. A common dosimeters containing lithium-6 fluoride
technique of personal radiation exposure and fission track counting systems. The
measurement is thermoluminescent nuclear track films do not respond to
dosimetry (Fig. 2b). The desirable neutrons below 0.5 MeV in energy; in
characteristics of thermoluminescent practice, a substantial fraction of the
dosimeters (TLDs) include their wide neutrons may be below this energy. Track
linear range; short readout time; relative counting is a relatively insensitive
insensitivity to field conditions of heat, technique of neutron dosimetry. For low
light and humidity; reusability; and for doses, counting of a statistically
some phosphors, energy independence. significant number of tracks is too time
Response is rate independent up to consuming to be warranted. On the other
1 GSv·s–1 (100 GR·s–1), which can be hand, at high doses it is difficult to
useful in flash X-ray radiographic distinguish tracks from one another so
that they can be counted. Fading occurs
FIGURE 2. Clip-on personal radiation and, as a result, short tracks may
dosimeters: (a) film badges; disappear. For these reasons, nuclear track
(b) thermoluminescent dosimeters (TLDs).21 film is more useful in demonstrating that
large neutron doses have not been
(a) received than in measuring actual low
doses.
(b)
The lithium-6 fluoride and fission track
counting systems do not suffer from these
disadvantages and will provide
measurements at permissible dose levels.
These techniques are sensitive down to
doses of about 30 or 40 µGy (3 or 4 mrad)
and down to thermal neutron energies.

Boron trifluoride neutron radiation
detector tubes provide high gamma
rejection up to about 5 Sv·h–1 (500 R·h–1)
and detect neutrons with energies from
thermal to about 10 MeV (Fig. 3).23 Other
means of neutron dosimetry, including
ion chambers, have been investigated or
developed.23-25

Radiation Detection and
Measurement22

In an area survey, measurements are made
of radiation fields to provide a basis for
estimating the dose equivalents that
persons may receive. Changes in
operating conditions (such as beam
orientations and source outputs) can
cause changes both in field intensity and
pattern. The number of measurements
depends on how much the radiation field
varies in space and time and on how

122 Radiographic Testing

much people move about in the field. Measurements close to radiation
Measurements made at points of likely sources of small dimensions or of
personnel occupancy under the different radiation transmitted through holes or
operating conditions are usually sufficient cracks in shielding require special
to estimate dose equivalent adequately for attention. The general location of
protection purposes. discontinuities in shielding should be
determined by scanning with sensitive
Detection instruments are used in detection instruments. More precise
radiation surveys and area monitoring to delineation of the size and configuration
warn of the existence of radiation or of the discontinuities can be obtained by
radiation hazard and, as distinct from using photographic film or fluorescent
measuring instruments, usually indicate screens for X-ray, gamma ray or electron
count rate rather than dose rate or leakage. Measurements may then be made
exposure rate. They should be used only in any of three ways:
to indicate the existence of radiation.
1. Film may be used at the point of
Measurement interest, provided it has been properly
calibrated for the types and energies of
At points of particular interest, individual the radiations present.
determinations of dose or exposure rate
should be made with calibrated measuring 2. An instrument may be used that has a
instruments. Dose integrating devices detector volume small enough to
(dosimeters) may be mounted at points of ensure that the radiation field
interest and left for an extended period of throughout the sensitive volume is
time to improve the accuracy of the substantially uniform.
measurement.
3. An instrument with a large sensitive
Information concerning the volume may be used, if an appropriate
dimensions, dose rate and location of correction factor is applied. Only
primary beams of radiation in relation to when Achamber is larger than Abeam,
the source is important in determining multiply the reading by the ratio of
direct external exposure from the beam the instrument chamber cross section
and the adequacy of protective measures. area to the beam cross section area:
The dose or exposure rates within the
beam at specific distance from the source (2) Reading × Achamber = Corrected
should be measured and compared with Abeam reading
expected values.
Choice of Instruments22
FIGURE 3. Boron trifluoride neutron radiation
detector tube provides high gamma The following general properties should
rejection up to about 5 Sv ·h–1 (500 R·h–1) be considered.
and detects neutrons with energies from
thermal to about 10 MeV. Energy Response. If the energy spectrum
of the radiation field differs significantly
from that of the calibration field, a
correction may be necessary.

Directional Response. If the directions
from which the radiations arrive at the
instrument differ significantly from those
in the calibration field, correction may be
necessary. If the dose equivalents being
determined are small in comparison to
permissible doses, large errors are
acceptable and correction may not be
necessary.

Rate Response. Instruments that measure
dose or exposure are called integrating
instruments; those that measure dose rate
or exposure rate are called rate instruments
or rate meters. If the dose rate or exposure
rate differs significantly from that in the
calibration field, correction may be
necessary. Ordinarily, an integrating
instrument should be used only within
the rate ranges for which the reading is
independent of the rate. Rate instruments,
similarly, should be used only within the
rate ranges in which the reading is
proportional to the rate. A few
instruments will become saturated at very

Radiation Safety 123

high rates; that is, they will cease to need only to indicate the average rate for
function and the reading will drop to zero radiation protection purposes.
or close to zero. It is particularly necessary
to know the rate response of instruments Mixed Field Response. Because some
to be used near machines that produce radiations (such as neutrons) have higher
radiation in short pulses. Rate instruments quality factors than others, mixed field
used near repetitively pulsed machines monitoring is necessary. This can be done
either by using two instruments that are

FIGURE 4. Gamma and X-radiation sensing devices incorporating geiger-müller tubes:
(a) survey meter for range selectable from 0 to 20 mSv·h–1 (0 to 2 R·h–1) and automatic aural
alarm over 2.5 mSv·h–1 (250 mR·h–1); (b) survey meter with on/off switch for aural
monitoring; (c) for high noise areas, personal rate alarm with flashing light and optional
earplug for aural alarm; (d) area monitor with standard 20 µSv·h–1 (2 mR·h–1) trip point,
audio piezo alert and large red strobe warning light; (e) visual alarm for gamma and X-rays
from 80 keV to 1.5 MeV and adjustable alarm threshold.

(a) (c)

MOVIE.
Personnel
monitoring
devices.

(d)
(b)

(e)

124 Radiographic Testing

each sensitive to only one radiation or by Ionization Chambers. Many gamma ray
using two instruments that are sensitive and X-ray exposure rate measurements are
to both but to a different extent. made with portable ionization chambers
(Fig. 1). Ionization chambers with separate
Unwanted Response. Interference by readers are useful for measuring either
energy forms that an instrument is not very high or very low exposure rates. Ion
supposed to measure can be a problem. chambers made of plastic or other low
Response to heat, light, radiofrequency atomic number materials usually give
radiations and mechanical shock are exposure readings independent of photon
examples. energy down to 50 keV. Ionization
chambers are available for exposure rates
Fail Safe Provision. To avoid unknowingly to over 20 Sv·h–1 (3 or 4 kR·h–1).
exposing personnel to radiation,
malfunctions of an instrument should be Geiger-Müller Counters. The dead time in
readily recognizable or should always geiger-müller counters (Fig. 4) sets a limit
result in readings that are too high. to their count rate that, in turn, limits
their use to exposure rates up to about
Precision and Accuracy. Typically, precision 0.03 nSv (a few µR·h–1). The counters
of a few percent should be obtained on respond to the number of ionizing events
successive readings with the same survey within them independent of energy and
instrument. At the level of a maximum thus do not yield equal count rates for
permissible dose a measurement accuracy equal exposure rates of different energies.
specified by regulations should be Geiger-müller counters are better suited
achieved. At levels less than 0.25 the for radiation detection than for
maximum permissible dose a lower level measurement.
of accuracy (say, a factor of 2) is
acceptable. Scintillation Instruments. Scintillation
devices (Fig. 5) also have count rate
Calibration. Instruments used for limitations because of the duration of the
radiation protection are not absolute light flashes but can count much faster
instruments; that is, they require than geiger-müller counters. In the same
calibration in a known radiation field or exposure field, scintillation count rates are
comparison with instruments whose higher than geiger-müller count rates, so
response is known. Many users of scintillation counters are useful for
radiation protection instruments must locating weak X-ray and gamma ray fields.
rely on the manufacturer to calibrate their
instruments properly. Users should FIGURE 5. Radiation detector with scintillation
arrange a reproducible field in which the counter measurement of low energy gamma
instruments are placed and read radiation.
frequently  at least semianually. The
possibility of reading error due to
imprecision is minimized by computing
the mean of several readings. If changes
in the mean reading are detected, the
instruments should be recalibrated
promptly.

Time Constant. An important
characteristic of a rate instrument is the
time constant, an indication of the time
necessary for the instrument to attain a
constant reading when suddenly placed in
a constant radiation field. Time constants
are generally given as the time required to
arrive at 1 – e–1 (that is, 0.63) of the final
reading. Typical time constants of good
rate meters are 1 s or less. The response
time of a rate instrument is defined as the
time necessary for it to reach 90 percent
of full response. It is equal to 2.3 time
constants.

Radiation Surveying and Area
Monitoring

Various technologies for radiation
surveying and area monitoring are
available. The following can be used for
sealed gamma ray sources and for sources
of X-rays. (More information on these
technologies can be found in the chapter
on radiation measurement.)

Radiation Safety 125

Instrument Calibration contact with the source will also reveal
leakage if it is contaminated.
The National Institute of Standards and
Technology (NIST) is the point of record Leak tests of devices from which the
for reference standards. Laboratories encapsulated source cannot be removed or
calibrate according to the National is too large to handle should be made by
Institute of Standards and Technology. wiping the accessible surface or aperture
Laboratory standard instruments for of the device nearest to the storage
measuring exposure from photons of position of the source.
higher energies from 1 to 1000 mSv
(0.1 to 100 R) or exposure rate from 0.1 to Detection of contaminants on the
150 mSv·min–1 (0.01 to 15 R·min–1) can housing or surface of a neutron source
be calibrated by the National Institute of may not indicate source leakage but may
Standards and Technology by comparison be due to induced activity. Confirmation
with either cesium-137 or cobalt-60 of leakage may require identification of
calibrated sources. These laboratory the contaminant.
standard instruments or secondary
standards may then be used to calibrate In leak testing of radioactive sources,
radiation protection survey instruments special equipment may be necessary for
by comparison in radiation fields of radiation exposure control. Depending on
similar quality. Consideration must be the activity of the source, shielding may
given to beam width, uniformity of be required to keep the leak tester’s
radiation over the calibration area and exposure as low as possible. The actual
changes in radiation quality caused by leak test wipe should be done by using
scattered radiation. tongs or forceps and not the fingers.
Rubber gloves should be used to minimize
Neutron instrument calibration can be hand contamination. The wipes should be
afforded by exposure to fields from taken quickly and the source returned to
National Institute of Standards and its designated container.
Technology calibrated neutron sources.
One type of such a source is made by
mixing a radionuclide such as plutonium,
polonium or radium with a material such
as beryllium or boron. The neutrons are
produced in (α, n) reactions in the latter
materials. Radium sources are difficult to
use because they also emit intense gamma
radiation.

Leak Testing of Isotope Sealed
Sources26

All sealed sources must be tested for
leakage of radioactive material before
initial use, at intervals not exceeding six
months, whenever damage or
deterioration of the capsule or seal is
suspected or when contamination of
handling or storage equipment is
detected.

The leak test should be capable of
detecting the presence of 185 Bq (5 nCi)
of removable activity from the source.
Sources that are in the United States and
that are leaking greater than 185 Bq
(5 nCi) of removable activity, based on
the tests described below, should be
reported to the Nuclear Regulatory
Commission within five days. Records of
leak test results should be specific in units
of disintegrations per minute or
microcuries. Leak test records should be
kept until final disposition of the source is
accomplished.

A small sealed capsule may be tested by
washing for a few minutes in a detergent
solution. An aliquot of this solution
should then be counted. An absorbent
liner in the storage container normally in

126 Radiographic Testing

PART 4. Basic Exposure Control

Physical Safeguards and Classes of Installations for
Procedural Controls22 X-Rays and Gamma Rays

As long as the radiation source remains There are four types of nonmedical X-ray
external, exposure of the individual may and gamma ray installations: protective,
be terminated by removing the individual enclosed, unattended and open.3,26
from the radiation field, by removing the
source or by switching off a radiation Protective Installation
producing machine. If the external
radiation field is localized, exposure to This class provides the highest degree of
individuals may be limited readily by inherent safety because the protection
shielding or by denying access to the field does not depend on compliance with any
of radiation. operating limitations. The requirements
include the following.
Physical Safeguards
1. Source and exposed objects are in a
Physical safeguards include all physical permanent enclosure within which no
equipment used to restrict access of person is permitted during irradiation.
persons to radiation sources or to reduce
the level of exposure in occupied areas. 2. Safety interlocks are provided to
These include shields, barriers, locks, alarm prevent access to the enclosure during
signals and source shutdown mechanisms. irradiation.

Planning and evaluation of physical 3. If the enclosure is of such a size or is
safeguards should begin in the early so arranged that occupancy cannot be
phases of design and construction of an readily determined by the operator,
installation. Detailed inspection and the following requirements should
evaluation of the radiation safety of also be provided: (a) fail safe audible
equipment are mandatory at the time of or visible warning signals to indicate
the installation’s initial use. Additional the source is about to be used;
investigations are necessary periodically to (b) emergency exits; (c) effective
ensure that the effectiveness of the means within the enclosure of
safeguards has not decreased with time or terminating the exposure (sometimes
as a result of equipment changes. called scramming).

Procedural Controls 4. The radiation exposure 50 mm
(2.0 in.) outside the surface of the
Procedural controls include all enclosure cannot exceed 5 µSv
instructions to personnel regarding the (0.5 mR) in any 1 h.
performance of their work in a specific
manner for the purpose of limiting 5. Warning signs of prescribed wording
radiation exposure. Training programs for at prescribed locations.
personnel often are necessary to promote
observance of such instructions. Typical 6. No person may be exposed to more
instructions concern mode of use of than the permissible doses. The low
radiation sources, limitations on proximity allowable exposure level necessitates
to sources, exposure time and occupancy of greater inherent shielding. At high
designated areas and the sequence or energies in the megavolt region with
kinds of actions permitted during work high workloads, the required
with radiation sources. additional shielding may be extremely
expensive. For example, in the case of
Periodic area surveys and personnel cobalt-60, the required concrete
monitoring are necessary to ensure the thickness will have to be about 0.3 m
adequacy of and compliance with (1 ft) greater than for the enclosed
established procedural controls. type.

Enclosed Installation

This class usually offers the greatest
advantages for fixed installations with low
use and occupancy. With proper supervision
this class offers a degree of protection

Radiation Safety 127

similar to the protective installation. The 6. Service doors to areas where exposure MOVIE.
requirements for an enclosed installation can exceed the measurements in items Warning tape
include items 1, 2, 3, 5 and 6, above, plus 3 and 4 above must be locked or and sign.
a different item 4. secured with fasteners requiring
special tools available only to qualified
4. The exposure at any accessible and service personnel.
occupied area 0.3 m (1 ft) from the
outside surface of the enclosure does Open Installation
not exceed 100 µSv (10 mR) in any
1 h. The exposure at any accessible This class can only be used when
and normally unoccupied area 0.3 m operational requirements prevent other
(1 ft) from the outside surface of the classes, such as in mobile and portable
enclosure does not exceed 1 mSv equipment where fixed shielding cannot
(100 mR) in any 1 h. This class of be used. Mobile or portable equipment
installation requires administrative used routinely in one location should be
procedures to avoid exceeding the made to meet the requirements of one of
permissible doses. The tradeoff the fixed installation classes. Adherence to
between (1) the intrinsic but initially safe operating procedures is the main
expensive safety of a protective safeguard to overexposure. The
installation and (2) the required requirements include the following.
continuing supervision and
consequences of an overexposure in 1. The perimeter of any area in which
an enclosed installation should be the exposure can exceed 1 mSv
carefully considered in the planning (100 mR) in any 1 h must be posted as
stages of a new facility.22 a very high radiation area.

Unattended Installation 2. No unauthorized or unmonitored
person may be permitted in the high
This class consists of automatic radiation area during irradiation. In
equipment designed and manufactured by cases of unattended operation, positive
a supplier for a specific purpose that does means, such as a locked enclosure,
not require personnel in attendance for shall be used to prevent access.
operation. The requirements for this class
include the following. 3. The perimeter of any area in which
the radiation level exceeds 50 µSv
1. The source is installed in a single (5 mR) in any 1 h must be posted as a
purpose device. radiation area.

2. The source is enclosed in a shield, 4. The equipment essential to the use of
where the closed and open positions the source must be inaccessible to
are identified and a visual warning unauthorized use, tampering or
signal indicates when the source is on. removal. This shall be accomplished
by the attendance of a knowledgeable
3. The exposure at any accessible person or other means such as a
location 0.3 m (1 ft) from the outside locked enclosure.
surface of the device cannot exceed
20 µSv (2 mR) in any 1 h. 5. No person can be exposed to more
than the permissible doses.
4. The occupancy in the vicinity of the
device is limited so that the exposure 6. For reasons of safety and security,
to any individual cannot exceed restricted areas must be clearly defined
5 mSv (0.5 R) in a year. and marked. Means of surveillance to
enforce the restrictions are needed.
5. Warning signs are used.

TABLE 3. Gamma ray sources.3,15,27

Radionuclide Atomic Half Energy __________G__a_m__m__a_R_a__y_C__o_n_s_t_a_n_t__________
Number Life (MeV) mSv·GBq–1·h–1 at 1 m (R·Ci–1·h–1 at 1 m)

(Z)

Cesium-137 55 30 yr 0.662 0.086 (0.320)
Chromium-51 24 28 d 0.323 0.005 (0.018)
Cobalt-60 27 1.17, 1.33 0.351 (1.300)
Gold-198 79 5.3 yr 0.412 0.062 (0.230)
Iridium-192 77 2.7 d 0.136, 1.065 0.135 (0.500)
Radium-226 88 74 d 0.047 to 2.4 0.223 (0.825)
Tantalum-182 73 1622 yr 0.066 to 1.2 0.162 (0.600)
155 d

128 Radiographic Testing

Output of Radiation insignificant scattering or absorption, the
Sources primary beam is the total radiation field.

Table 3 lists some data on gamma ray For example, consider a 3.7 GBq
sources of interest for industrial purposes. (100 mCi) iridium-192 source in air in the
shape of a pencil, 6.3 mm (0.25 in.)
Table 4 lists some typical radiation diameter and 0.13 m (5.0 in.) long. What
machine outputs for varying voltages. would the working time be at 3.0 m? First,
solve for 1 m. From Table 3, the gamma
Working Time ray constant for iridium-192 is
135 µSv·GBq–1·h–1 at 1 m (0.5·Ci–1·R·h–1 at
This is the allowable working time in 1 m). Therefore:
hours per week for a given exposure rate.
For example, for an exposure rate of Exposure 0.135 mSv⋅GBq–1⋅h –1 
100 µSv·h–1 (10 mR·h–1) to the whole (4) rate 
body: =  at 1 m 

× 3.7 GBq

= 0.5 mSv⋅h–1

Permissable occupational 

(3) Working = dose per week (5)  Exposure = 0.5 Rat⋅C1im–1⋅h–1
time Exposure dose rate  rate 

1000 µSv ⋅ wk–1  
= 100 µSv ⋅ h–1 × 0.1 Ci

 = 0.05 R⋅h–1 

 100 mR ⋅ wk–1 
= 
 10 mR ⋅ h–1  Because 3.0 m is obviously more than
10 times 0.13 m (5.0 in.), the inverse
= 10 h ⋅ wk–1 square law applies. Also, scattering is not
a problem. Using the inverse square law
gives the exposure rate at 3 m:

Working Distance (6) Exposure = 0.5 mSv⋅h–1
rate  at 1 m 
The inverse square law applied to
radiation states that the dose rate from a ×  1 m2
point source is inversely proportional to  3 m 
the square of the distance from the origin
of the radiation source provided that = 55 µSv⋅h–1
(1) the dimensions of the radiation source
are small compared with the distance and 
(2) no appreciable scattering or absorption 0.05 aRt⋅1Cmi–1⋅h–1
of the radiation occurs in the media (7)  Exposure = 
through which the radiation travels. In 
practice, the first requirement is satisfied  rate
whenever the distance involved is at least
ten times greater than the largest source  ×  1 m2 
dimension. In situations where there is   3 m  
 
 
 
 = 5.5 mR⋅h–1 

TABLE 4. Forward X-ray intensity from optimum
target.3,27,32

Peak Voltage ______I_n_t_e_n_s_i_ty__a_t__1_m___(_4_0__in_._)______ Equations 8 and 9 finally give the
(MV) working time at 3 m:
kSv·min–1·mA–1 (R·min–1·mA–1)

0.050 0.005 (0.05) (8) Working = 1 mSv⋅wk–1
0.070 0.01 (0.1) time 55 µSv⋅h−1
0.100 0.04 (0.4)
0.250 0.2 (2.0) = 18 h⋅wk–1
1.000 2.0 (20)
2.000 28 (280) (9)  Working = 100 mR⋅wk–1 
5.000 500  
10.000 3000 (5000)  time 5.5 mR⋅h−1 
15.000 (30 000)
20.000 10 000 (100 000)  = 18 h⋅wk–1 
20 000 (200 000)

Radiation Safety 129

PART 5. Shielding

Protective Enclosures designed to shield against the primary
radiation beam; secondary shields are
Because of scattered radiation, protection only thick enough to protect against tube
for the operator and other personnel housing leakage and scattered radiation.
working in the neighborhood often Therefore, the X-ray tube or source should
requires shielding of the part being not be pointed toward secondary shields.
radiographed and any other material For this reason, mechanical stops should
exposed to the direct beam, in addition to be used to restrict tube housing
the shield for the source itself. Preferably orientations toward primary barriers.
the source and materials being examined Operating restrictions, such as not
should be enclosed in a room or hood pointing the beam at certain walls or the
with the necessary protection ceiling, should be spelled out in the
incorporated into the walls (Fig. 6). operating procedures.

Shields can be classified as either Protective materials are available in
primary or secondary. Primary shields are panels so that radiation barriers may be
customized for work areas of various sizes.
FIGURE 6. Rooms offering radiation shielding: Mobile work rooms with modular designs
(a) concrete shooting booth; (b) modular are also available, offering the same
radiation enclosure. flexibility in size and location (Fig. 6b).

(a) When changes in operating conditions
are contemplated, the radiation safety
Concrete Secured officer (RSO) should be contacted for
entrance consultation and for surveys as needed to
(b) determine additional shielding
requirements.

For design purposes, the primary beam
should not be pointed at a high personnel
occupancy space and the distance from
the radiation source to any occupied space
should be as great as is practical. Scattered
radiation usually has a lower effective
energy than the primary beam and may,
therefore, be easier to shield.

Skyshine28

In the design of facilities, there is often a
question concerning the magnitude of
shielding required for the roof over the
building. As an ordinary weather roof

FIGURE 7. Shielding above radiation source reduces radiation

reflected from atmosphere. Such radiation is called
skyshine.28

Alternative Solid angle Skyshine
shielding Ω
positions Observation
d1 point

Radiation Controlled area
source ds

130 Radiographic Testing

provides little if any attenuation for as the half value layer (HVL). Similarly, the
radiation directed up, there is a significant thickness that will reduce the radiation to
probability that radiation reflected back one tenth is referred to as the tenth value
from the atmosphere will be unacceptable layer (TVL). (See Tables 5 and 6 and see
in the immediate area of the facility. See Figs. 9 and 10.1,3)
Fig. 7 for X-rays and gamma rays this
radiation (1) increases roughly as Ω1.3, FIGURE 9. Transmission through lead of gamma rays from
where Ω is the solid angle subtended by selected radionuclides.3
the source and shielding walls,
(2) decreases with (ds)2, where ds is the 1
horizontal distance from the source to the
observation point and (3) decreases with 10–1
(di)2, where di is the vertical distance from
the source to about 2 m (6.5 ft) above the Transmission (ratio) 10–2
roof.
10–3
The shield thickness necessary to
reduce the radiation to an acceptable level 10–4
may be calculated according to published
techniques28 and may alternatively be Iridium-192 Cesium-137
designed into the roof structure or
mounted over the source with a lateral Gold-198 Cobalt-60 Radium
area sufficient to cover the solid angle Ω.
Similar statements apply to neutron 10–5 125 250
skyshine, except that the functional 0 (5) (10)
dependences of the radiation at ds are
slightly different for Ω and ds. Thickness of lead, mm (in.)

Materials FIGURE 10. Transmission through concrete (density of
2.35 g·cm–3 [147 lbm·ft –3]) of gamma rays from radium,
Common materials such as concrete and cobalt–60, gold-198 and iridium-192.3
lead can be used as absorbers or shields to
reduce personnel exposures.29 Beta or 1
electron radiation is completely stopped by
the thicknesses of material shown in
Fig. 8.30 The thickness of any material
that will halve the amount of radiation
passing through the material is referred to

FIGURE 8. Maximum range of beta particles as function of
energy in various materials indicated.30

25 000 (103)

Maximum range of beta particles, mm (in.) Air 10–1

2500 (102)

250 (101) Transmission (ratio) 10–2
25 (100)
2.5 (10–1) Aluminum 10–3 Cesium-137
Water Concrete
0.25 (10–2) Acrylic Cobalt-60
0.025 (10–3) Radium
Glass

10–4 Iridium-192
Gold-198

Copper 10–5
Iron Lead 0

1 2 34 0.25 0.50 0.75 1.00 1.25 1.50 1.75
Energy (MeV) (10) (20) (30) (40) (50) (60) (70)

Concrete slab thickness, m (in.)

Radiation Safety 131

These terms imply an exponential attenuation. Lead, however, requires extra
function for transmitted radiation in structural support because it is not
terms of shield thickness. Figures 9 and self-supporting. Concrete is by far the
10, however, show that the transmission most commonly used shielding material
curves are not completely linear on a for economic, structural and local
semilogarithmic plot.1,3 Hence, the listed availability reasons — in addition to
half value layers and tenth value layers in desirable shielding characteristics. Where
Tables 5 and 6 are approximate, obtained space considerations are important
with large attenuation. depleted uranium shields are expensive
but offer excellent solutions to difficult
Table 7 lists densities of commercial problems.
building materials. For X-radiation and
gamma radiation, the absorption process Table 5 lists half value layers and tenth
depends largely on compton absorption value layers for several commonly used
and scattering, which in turn increase gamma ray emitting radionuclides.
with the atomic electron density. As a first Table 6 lists similar information for X-ray
approximation, electron density varies peak voltages. Figures 9 and 10 show
directly with the mass density of a actual transmission through lead and
material. Hence, the denser building concrete for the gamma ray emitting
materials are usually better shielding radionuclides. Figure 11 shows a
materials for a given thickness of material. representative transmission through
On a mass basis, shielding materials are concrete. Similar charts are available for
much the same above about 500 keV. steel, lead and other materials for X-ray
Where space is a problem, lead is often beams of various peak energies.1,28
used to achieve the desired shield

Table 5. Shielding equivalents: approximate tenth (TVL) and half value (HVL) layer
thicknesses in lead and concrete for several gamma ray sources.3,27

_________________L_e_a_d_________________ _______________C_o_n__cr_e_t_e_______________

H_a__lf_V__a_lu_e__L_a_y_e_r_s T_e_n__th__V_a_l_u_e__L_a_y_e_r_s _H_a__lf_V__a_lu_e__L_a_y_e_r_s_ Te_n__th__V_a_l_u_e__L_a_y_e_rs

Source mm (in.) mm (in.) mm (in.) mm (in.)

Radium-226 56 (2.20) 16 (0.65) 234 (9.2) 69 (2.7)
Cobalt-60 41 (1.60) 12 (0.49) 218 (8.6) 66 (2.6)
Cesium-137 21 (0.84) 157 (6.2) 48 (1.9)
Iridium-192 20 (0.79) 6 (0.25) 140 (5.5) 41 (1.6)
Gold-198 11 (0.43) 6 (0.24) 140 (5.5) 41 (1.6)
3 (0.13)

TABLE 6. Shielding equivalents: approximate half value layers (HVL) and tenth value layers (TVL) for lead
and concrete for various X-ray tube potentials.3,27

Peak ____________________L_e_a_d____________________ __________________C_o_n__cr_e_t_e__________________
Voltage
_H_a__lf_V__a_lu_e__L_a_y_e_r_s_ _T_e_n_t_h__V_a_l_u_e_L_a__y_e_rs_ _H_a__lf_V__a_lu_e__L_a_y_e_r_s_ _T_e_n_t_h__V_a_l_u_e_L_a__y_e_rs_
(kV)
mm (in.) mm (in.) mm (in.) mm (in.)

50 0.05 (0.002) 0.16 (0.006) 4.32 (0.170) 15.10 (0.594)
70 0.15 (0.006) 0.50 (0.020) 8.38 (0.330) 27.95 (1.100)
100 0.24 (0.009) 0.80 (0.031) 15.10 (0.594) 50.80 (2.000)
125 0.27 (0.011) 0.90 (0.035) 20.30 (0.799) 66.00 (2.598)
150 0.29 (0.011) 0.95 (0.037) 22.35 (0.880) 73.60 (2.898)
200 0.48 (0.019) 1.60 (0.063) 25.40 (1.000) 83.80 (3.299)
250 0.90 (0.035) 3.00 (0.118) 27.95 (1.100) 94.00 (3.701)
300 1.40 (0.055) 4.60 (0.181) 31.21 (1.229) 104.00 (4.094)
400 2.20 (0.087) 7.30 (0.287) 33.00 (1.299) 109.10 (4.295)
500 3.60 (0.142) 11.90 (0.469) 35.55 (1.400) 116.80 (4.598)
1000 7.90 (0.311) 26.00 (1.024) 44.45 (1.750) 147.10 (5.791)
2000 12.70 (0.500) 42.00 (1.654) 63.50 (2.500) 210.40 (8.283)
3000 14.70 (0.579) 48.50 (1.909) 73.60 (2.898) 241.20 (9.496)
4000 16.50 (0.650) 54.80 (2.157) 91.40 (3.598) 304.48 (11.987)
6000 17.00 (0.669) 56.60 (2.228) 104.00 (4.094) 348.00 (13.701)
1000 16.50 (0.650) 55.00 (2.165) 116.80 (4.598) 388.50 (15.295)

132 Radiographic Testing

These charts present broad beam reference to tables or by calculations. See
shielding information, which includes all the applicable standard.3,28,29
scattered radiation resulting from
deflection of the primary gamma or In many cases an additional tenth
X-rays within the shield as well as value layer can be induced at little extra
absorption of the primary radiation. Most cost and will increase the margin of safety
engineering applications need to consider considerably. A series of measurements of
broad beam geometry. Narrow beam transmitted radiation in occupied areas,
geometry, where only the primary beam called a radiation survey, is necessary to
needs consideration, is seldom document the adequacy of the facility’s
encountered in practice. design. Such a radiation survey can be
derived from a combination of portable
Thickness of Shielding instrument readings and personnel
Walls dosimeters placed at appropriate locations
in the facility (called badge plants).
The shielding in the walls of the
enclosures should be of sufficient FIGURE 11. Transmission through concrete (density of
thickness to reduce the exposure in all 2.35 g·cm–3 [147 lbm·ft–3]) of X-rays produced by 0.1 to
occupied areas to a value as low as 0.4 MeV electrons under broad beam conditions. Four
reasonably achievable (ALARA). In the
design the desired thickness can be curves shown represent transmission in dose equivalent
determined with reasonable accuracy by
index ratio. First three electron energies were accelerated by
TABLE 7. Densities of commercial building materials.3,27
voltages with pulsed wave form. Fourth electron energy
___A__v_e_r_a_g_e__D_e_n_s_i_t_y___
(0.4 MeV) was accelerated by constant potential generator.
Material g·cm–3 (lbm·ft–3)
Top scale indicates required mass thickness, or mass per unit
area, g·cm–2 (lbm·in.–2). Concrete of different density may be
used if required mass thickness is achieved. Where weight is

considered, this scale can be used in selection of optimum
shielding material.28

Aluminum 2.7 (169) Required mass thickness, g·cm–2 (lbm·in.–2)
Bricks: fire clay 2.05 (128)
Bricks: kaolin clay 2.1 (131) 0 25 50 75 100 125 150 175
Bricks: silica 1.78 (111) 1 (51) (102) (154) (205) (256) (307) (358)
Bricks: clay 2.2 (137)
Cement: colemanite borated 1.95 (122 10–1
Cement: portland and sand a 2.07 (129)
Concrete: barite 3.5 (218) Transmission (ratio) 10–2
Concrete: barite with boron frit 3.25 (203)
Concrete: barite with limonite 3.25 (203) 10–3
Concrete: barite with other b 3.1 (194)
Concrete: iron portland 6.0 (375) 10–4
Concrete: portland c 2.2 (137)
Glass: borosilicate 2.23 (139) 10–5 AB CD
Glass: lead (high density) 6.4 (399)
Glass: plate (average) 2.4 (150) 10–6 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Iron 7.86 (491) 0 (4) (8) (12) (16) (20) (24) (28)
Lead 11.34 (708)
Acrylic (polymethyl methacrylate) 1.19 Legend Concrete slab thickness, m (in.)
Rock: granite 2.45 (74) A. 0.10 MeV.
Rock: limestone 2.91 (153) B. 0.15 MeV.
Rock: sandstone 2.40 (182) C. 0.25 MeV.
Sand 2.2 (150) D. 0.40 MeV.
Sand plaster 1.54 (137)
Steel: Type 347 stainless 7.8
Steel: 1 percent carbon 7.83 (96)
Uranium 18.7 (487)
Uranium hydride 11.5 (489)
Water 1.0 (1167)
(718)

(62)

a. One part portland cement and two parts sand.
b. Barite with calcium aluminate and colemanite.
c. One part cement, two parts sand and four parts gravel.

Radiation Safety 133

PART 6. Neutron Radiographic Safety

Introduction neutrons having energies up to near the
maximum energy of the accelerator.
Neutrons are of interest in radiography
because their interaction with matter is Nuclear Reactor Sources
significantly different from X-rays or
gamma rays. Neutrons are absorbed and Neutron production in reactors occurs as a
scattered more in low atomic numbered result of the fission process. In the usual
(low Z) materials than high Z materials. operating mode the number of fissions
Thus, plastics, explosives and some (and neutrons) is essentially constant in
organic materials can be examined for time. The neutron energies range from
discontinuities with little interference thermal to 15 MeV with the number over
from encapsulating metals and electronic 10 MeV being small.
parts and wiring.
Shielding
Neutron Sources24
Fast Neutrons
Radioactive Neutron Sources
Adequate shielding against neutrons will
Radiation measurement techniques often attenuate gamma radiation to
specific to neutron radiation are discussed acceptable levels at both reactors and
elsewhere.23-25 accelerators. Water and other
hydrogenous shields may constitute an
Spontaneous Fission Neutron important exception to this rule. Ordinary
Sources or heavy aggregate concrete or earth are
the recommended materials in most
These sources are attractive because of installations. Any economy achieved by
their fissionlike spectrum, relatively low water filled tanks is likely to be offset by
gamma ray yield and their small mass. maintenance difficulties. Both paraffin
Californium-252 has been used for and oil, although good neutron absorbers,
stationary and mobile systems. are fire hazards and should not be used in
large stationary shields. Techniques of
Accelerator Sources shielding calculations are discussed in
detail elsewhere.24
Constant voltage accelerators such as van
de graaff and cockcroft-walton The importance of concrete as a
accelerators can produce energies up to structural and shielding material merits
about 20 MeV for protons and deuterons special mention. Its use for gamma and
and still higher energies for alpha particles X-ray shielding has been previously
and heavy ions. Small accelerators using discussed. Because of its relatively high
deuterons of 100 to 200 keV energy can hydrogen and oxygen content, it is also a
produce large numbers of 14 MeV good neutron shield. The subject of
neutrons when using a tritiated target. shielding calculations for neutrons is
High frequency positive ion accelerators complex and should be performed by
include the cyclotron, synchrocyclotron, specialists. Benchmarks include
proton synchrotron and heavy ion linear approximate tenth value layers of
accelerator. These are capable of 250 mm (10 in.) of concrete for 14 MeV
producing a wide range of neutron neutrons and 150 mm (6 in.) for 0.7 MeV
energies. Protons above 10 MeV will neutrons.
produce neutrons when striking almost
any material. Thermal Neutrons

High frequency electron accelerators Generally the energies associated with
such as the betatron produce X-rays thermal neutrons are less than 1 eV. For
through the interaction of the accelerated radiation protection the most important
electrons with the target. The X-rays in interaction of thermal neutrons with
turn produce photoneutrons, most with matter is radioactive capture. In this
energies of a few MeV but with some process, the neutron is captured by the
nucleus with the emission of gamma

134 Radiographic Testing

radiation. A shield adequate for fast
neutrons usually will be satisfactory for
thermal neutrons. The low quality factor
(QF = 2) for thermal neutrons (0.025 eV)
makes their biological consequence
considerably less than for fast neutrons.

Radiation Safety 135

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Radiation Safety 137