ASNT NDT Handbook Volume 10 OVERVIEW

LIQUID PENETRANTTESTING I 83

coverage is easily obtained by dipping parts. However, pud- list are considered equivalent in meeting generic specifica-
dling of the developer in recesses of complex parts must be tion requirements. Consequently, any manufacturer's prod-
avoided so that all water evaporates during drying and the uct listed on a qualified/approved list may be substituted for
developer coating has a uniform thickness. any other product listed under the same classification. In its
listings of materials, a qualified/approved list may formalize
Water suspendible developers are similar to the water the classification of types of penetrants, methods of
soluble type except that the powder, which is insoluble in removal, classes of solvents and forms of developers with
water, remains in suspension until it precipitates onto the number or letter designations for easy reference.
part as it is dried. Aqueous suspendible developers are used
in the same manner as the aqueous soluble developers. A typical example of a qualified/approved list is
However, suspendible developers must be constantly agi- QPL-25135,' a qualified products list generated by the
tated to keep the developer suspended. Removal of dried United States Air Force for the Department of Defense.
suspendible developers may be more difficult because they This QPL lists materials that meet or exceed the require-
are not soluble in water. All aqueous developers necessarily ments of military specification MIL-I-25135 for penetrant
contain biocides, corrosion inhibitors and wetting agents. testingmaterials. A QPL can be based on any specification.
For example, SAE International also has an Aerospace
Nonaqueous (solvent suspendible) developers are sup- Materials Specification (AMS) for penetrant testing materi-
plied in the ready-to-use condition, frequently in aerosol als. The current tendency is to have consensus industry
cans. This type of developer is the most sensitive form specifications instead of military specifications, so the
because the solvent action contributes to the absorption and Department of Defense (the customer) may choose the
adsorption mechanisms of the developer by entering the AMS for its penetrant product requirements.
discontinuity and dissolving into the penetrant. This action
increases the volume and reduces the viscosity of the pene- Listing of penetrant materials on a QPL does not guar-
trant and leaches it to the surface. Nonaqueous developers antee that subsequent products of the same formulation will
are suitable for inspection of installed parts or in be acceptable. Listing on a QPL merely indicates that the
non-assembly line situations involving few parts or small original raw materials, formulation and compounding prac-
areas. With the discontinued use of chlorinated solvents, tice can result in an acceptable product. There are many
flammable solvents may be the only choice. factors and conditions involved in compounding and manu-
facturing penetrants that can affect their performance.
Many propellants in aerosol cans of penetrant materials Specifications for penetrant testing materials may include
are also flammable, as are propellants used in billions of an option for a procuring activity to contractually require a
cans of consumer products sold in the United States each manufacturer to provide quality conformance test results
year. Concerns about aerosol flammability have been voiced for the lot or batch to be supplied and/or a sample of the
over many years, and there have been extensive investiga- material for independent testing.
tions by the Consumer Products Safety Commission and
other agencies into accidents involving aerosol cans. These Sensitivity
investigations have invariably shown that the aerosol cans
are safe to use and that accidents are rare except in cases of When applied to penetrant testing processes, the term
extreme negligence, such as discharging an aerosol can near sensitivity refers to the ability to detect discontinuities. In
an open flame or throwing a can into an open fire. this context, sensitivity is a relative factor. Higher sensitivity
indicates that inspection processes can detect smaller dis-
Qualified/Approved Penetrant continuities than less sensitive processes. Several factors
Materials influence the sensitivity of a given penetrant testing process.
The cleaning methods, the selected penetrant materials and
Qualifying/approving Jlgencies were alluded to above. the procedure by which the materials are processed affect
Although such an agency may be an independent organiza- the sensitivity of the penetrant testing process.
tion designated and approved by a governing body or end.
user, a qualifying/approving agency is ultimately a customer The sensitivity of a penetrant testing process is not to be
that requires penetrant materials pass designated tests in confused with sensitivity level of a penetrant system, which
order to be used for inspecting products it buys. A customer is a term used to classify relative sensitivity of penetrant sys-
may designate specific penetrant materials to be used to tems that are processed according to a standardized proce-
inspect its products or may allow organizations performing dure. A penetrant system is normally designated by type of
inspections to make appropriate selections from a list of dye and method of removal. A postemulsifiable penetrant
qualified/approved penetrant materials to which the cus- system includes the emulsifier. The sensitivity level facili-
tomer subscribes. All materials in a given category on such a tates the selection of a system to obtain the desired inspec-
tion process sensitivity.

84 I NONDESTRUCTIVE TESTING OVERVIEW

PART 3

PENETRANT TESTING PROCESSES

Flow charts for the four basic penetrant processes are Users are urged to consider the following important fac-
illustrated in Figs. 6 to 9. Because the processes for fluores- tors and to discuss them with suppliers of penetrant test
cent penetrant and visible dye penetrant are similar, the equipment and penetrant: materials before deciding on the
process flow charts are applicable to both. Processing vari- type of penetrant testing system to install and/or use:
ables, including penetrant dwell time, emulsification time,
rinse water temperature and pressure, drying temperature 1. composition of parts to be inspected;
and time, and ultraviolet/visible light intensity, are estab- 2. number of parts or test areas to be inspected per unit
lished by customer designated processing specifications. It
is very important to adhere to the established parameters in of time;
order to control the quality of the inspection. 3. size and weight of parts to be handled;
4. location at which inspection is required - that is,
Selection of a Penetrant
Material/Process receiving department, in-process along a production
line, during assembly, as final inspection, in the field
The methods capable of the greatest sensitivity are often during erection, at maintenance facilities or in service;
the most costly. Many inspections require the ultimate sen- 5. types of discontinuities to be expected, for example,
sitivity. However, many times high sensitivity is not required porosity, cracks, seams or laps;
and may even produce misleading results.
FIGURE 7. Postemufsifiable (lipophilicJ
FIGURE 6. Water washable penetrant process
penetrant process
CLEAN
CLEAN
APPLY PENETRANT
APPLY PENETRANT
DWELL
DWELL
WATER WASH
LIPOPHILIC EMULSIFIER

DWELL

WATER WASH

DRY DRY DRY AQUEOUS DRY
DRY DEVELOPER DRY DEVELOPER DEVELOPER
NONAOUEOUS NONAOUEOUS
DWELL DEVELOPER DWELL DEVELOPER
iNSPECT INSPECT
POSTCLEAN

LIQUID PENETRANT TESTING I 85

6. size of discontinuities to be detected - for example, Advantages and Limitations of
small, deep, large, shallow, clustered or scattered; Penetrant Materials and
Techniques
7. surface condition of parts to be inspected (such as as-
cast, as-forged, machined, ground, lapped, polished, Fluorescent penetrants are generally more sensitive than
plated, painted, corroded, oily, covered with combus- visible penetrants. However, reliable detection of fluores-
tion, scratched or scaled parts); and cent indications requires a darkened inspection area and a
sufficiently intense (long wavelength) ultraviolet light. Use
8. condition to which parts will be subjected after of the latter requires precautionary measures to minimize
inspection; for example, medical, nuclear or oxygen personnel hazards.
systems; welding, plating or finishing operations; high
temperature, aerospace, industrial or transportation PostemulsifiableMethod
uses; inaccessible locations; or consumer products.
This method is the most reliable penetrant testing
Control of a Penetrant Process method. It will locate wide, shallowdiscontinuities as well as
tight cracks. However, application of the emulsifier requires
Part of the initial selection of a penetrant testing process additional operations (i.e., prerinse and emulsification),
involves demonstrated ability to detect rejectable disconti- which increase cost. The hydrophilic emulsifier/remover
nuities. However, just as with the penetrant materials, the process is more forgiving of variations in processing than
inspection process must continually demonstrate its capabil- both the lipophilic and water washable processes.
ity to detect known discontinuities. Process control parts
with documented flaws should be periodically processed to Solvent Removable Method
monitor quality of penetrant materials and control of pro-
cess variables. In a similar manner an inspector's perfor- This method is useful when water cannot be conve-
mance should be periodically monitored by the inspector's niently used or when the penetrant materials must be taken
processing test parts containing a statistically valid number
of discontinuities, known to the examiner only.

FIGURE 8. Solvent removable penetrant FIGURE 9. Postemulsifiable (hydrophilic)
process penetrant process

CLEAN CLEAN
APPLY PENETRANT APPLY PENETRANT

DWELL DWELL
PRE RINSE
INITIAL DRY WIPE HYDROPHILIC REMOVER
SOLVENT WIPE DWELL
Fl~DRYWIPE WATER WASH
NONAOUEOUS DEVELOPER
AQUEOUS DRY
DWELL DEVELOPER
INSPECT NONAOUEOUS
POSTCLEAN DEVELOPER

DWELL
INSPECT
POSTCLEAN

86 I NONDESTRUCTIVETESTING OVERVIEW

to the part. If caution is used when removing excess· pene- depends upon the ability of penetrant to enter and exit the
trant, this method is more sensitive than the water washable discontinuity, and the resulting indication must be readily
method. The method is very labor intensive, so its use is distinguishable from the background. Surface conditions,
usually limited to small areas or parts that have to be such as coatings or soil contamination, can reduce the effec-
inspected in place. tiveness of the inspection by interfering with the entry and
exit process or by producing a high residual fluorescent
Water Washable Method background. Penetrant testing is reliable only when the
parts to be inspected are free of contaminants. Foreign
Properly controlled water washable penetrant processes material, either on the surface or within the discontinuity,
will challenge the reliability of postemulsifiable processes. can produce erroneous results. Any interfering conditions
The process often will not detect open, shallow discontinu- must be removed by proper cleaning or surface treatment
ities because overwashing caused by excessive rinse time or before penetrant application. In addition, the cleaning solu-
water pressure will remove penetrant from flaws. This tions must be removed before penetrant testing so that they
method requires less time to complete than other methods do not interfere with the penetrant process.
and does not require a prerinse or an emulsifier/remover.
Cleaning is a broad term covering methods and materials
Pretesting, Cleaning and used to remove contaminants or soils from a surface. Clean-
Postcleaning ing is routinely used for corrosion control and to prepare
surfaces for other treatments. There are no special methods
The condition and treatment of parts before inspection or materials specifically dedicated to penetrant testing. Dif-
have a significant influence on the inspection results. Non- ferent materials . and parts require separate or individual
destructive inspection personnel must be aware of the cleaning processes. No one cleaning method is equally
preinspection condition of parts, surface treatment pro- effective on all contaminants. The selection.of a suitable
cesses and the effects that both have on the penetrant test- cleaning process is complex and depends on a number of
ing process. factors, examples of which follow.

Pretesting 1. Type of soilts) or contaminant(s) - any matter on a
part or component that may affect the penetrant test-
Some nonmetallic parts, such as plastic, rubber or plexi- ing process. Contaminants, such as paint or corrosion
glass, may react with chemicals contained in liquid pene- prevention compounds, may be intentionally applied;
trant testing materials. These chemicals can cause swelling, may result from prior processes, such as machining,
softening, distortion, crazing or other surface effects result- heat treating or cleaning; or may be the consequence
ing in damage to the part. The purpose of pretesting is to of service, such as corrosion, carbon deposits, lubri-
ensure that parts to be inspected will not be damaged by cating fluids or dirt particles. The effects of contami-
penetrant materials. All nonmetallic parts that have not nants on the penetrant testing process depend on the
been previously inspected and which do not have approved type of soil and whether it is on the part surface or
penetrant testing procedures, should be pretested. Process entrapped in a discontinuity.
an extra part or a small area if no extras are available. Some
materials may not show effects until they are subjected to 2. Composition of a part - chemical effects on materi-
service conditions (aging, cold, heat, moisture). Consult als differ. Some nonferrous metals, such as aluminum
with materials engineering personnel if any questions exist. and magnesium, present problems because strong
Water based penetrants may be useful for testing these alkaline or acid cleaners attack the metals. Steels,
materials. especially in the heat treated condition, are likely to
become embrittled by acid cleaners. Other metals,
Cleaning such as titanium and high nickel alloys, can be
attacked by cleaning agents containing halogen and
Proper chemical cleaning operations require special skill sulfur compounds if residual amounts of such com-
and knowledge. Improper methods, materials or procedures pounds are present when these metals are exposed to
can result in severe damage to part surfaces. Nondestructive high temperatures.
inspection personnel may not be trained or experienced in
performing certain cleaning operations. Surface cleaning 3. Test surface accessibility and geometry - complex
processes should be done only by qualified personnel. shapes make it difficult to clean all of the surfaces,
and soils lodged in restricted areas may escape the
Proper preparation of parts before inspection is critical. effects of cleaning.
Successful detection of discontinuities by penetrant testing
4. Required degree of cleanliness - determined by the
surface treatment that will follow or what service con-
ditions will be encountered by the cleaned part.

LIQUID PENETRANT TESTING I 87

5. Test s~rface condition - rough surfaces tend to hold Developers are the last material applied in the penetrant
soil, making it harder to remove. process and may take one of several forms. The form of
developer applied (dry powder, nonaqueous, water sus-
6. Availability and adequacy of cleaning facilities - for pendible or water soluble) greatly influences method and
example, a large part cannot be placed in a small alka- difficulties of removal. One point common to most develop-
line or ultrasonic cleaning tank. ers is the increase in adherence with time on the part. The
longer a developer remains on a part, the more difficult it is
Contaminants remaining from previous machining pro- to remove; Removal of developer coating should be done as
cesses are mentioned above. Some cleaning processes also soon as possible after completing the penetrant testing. The
involve intense mechanical working processes (sand blast, method of'removal depends upon the form of developer.
giit blast, wire brushing) for removing heavy or tenacious
contaminants, such as scale or rust. The less severe mechan- Dry powder developer adheres to all areas where
ical working processes (tumbling, liquid honing, vapor blast- applied, Some dry powder may lodge in recessed areas, fay-
ing) are frequently used to remove light oxides and residual ing surface joints, or crevices. Dry powder particles can be
combustion products. Mechanical working removes soils and removed with a water soluble detergent wash followed by a
contaminants by physical action. Physical action may also water rinse. Dry developer particles adhering to penetrant
remove or deform the part surface. Deformation is in the bleedout will be removed during the residual penetrant
form of metal flow or displacement on the part surface. The removal process described below.
amount of deformation depends on the type and severity of
the working plus the ductility of the part. Even a small Nonaqueous developer is usually applied by spraying
amount of deformation, such as that caused by fine san.ding from an aerosol can. The majority of applications involve a
or vapor blasting, will reduce the surface opening of s~all relatively small area. This makes it advantageous to remove
discontinuities. This deformation can reduce the effective- by initially hand wiping the surface with a dry cloth or paper
ness of the penetrant testing process. towel to remove most of the developer. The remaining
traces of developer can then be removed with a water or
Chemical etching should be done when penetrant test- alcohol moistened rag or paper towel. The inspected area
ing follows less severe mechanical working processes such may contain threads, crevices, and surface recesses where
as those mentioned above. Severe mechanical working pro- wiping will not remove all of the developer particles. These
cesses,· such as metal removal, shot peening or grit blasting, areas should first be wiped to remove as much developer as
can seal or close the surface openings of large discontinu- possible, and then pressure sprayed with a water and deter-
ities, which prevents the formation of penetrant indications. gent solution. Solvent spraying is not particularly effective
Penetrant testing should be accomplished before mechani- as the developer is usually insoluble. A vapor degreaser
cal work processes, such as machining, shot peening, grit should not be used because the elevated temperature bakes
blasting, plastic media bead blasting or coarse sanding, that or hardens the developer coating.
severely displace surface metal. If it is not feasible to per-
form penetrant testing before these processes, then another Water soluble developer is the easiest to remove. The
inspection method should be considered. An exception to developer coating readily redissolves in water. Water soluble
this requirement is when penetrant testing is performed to developer should be removed by immersion or spraying
detect discontinuities formed by mechanical working, such with water. Water suspended developer is very similar to
as machining tears or grinding cracks. nonaqueous developer in removal characteristics. The best
method of removal is immersion and pressure spraying with
Postcleaning after Penetrant Inspection a hot detergent solution. It can also be removed with a plain
water spray and hand scrubbing with a fiber bristle brush.
Penetrant testing residues can have several adverse
effects on subsequent processing and service. Removal of The amount of residual penetrant is small, consisting of
residual penetrant and developer following the inspection is penetrant retained in discontinuities, crevices and part sur-
always required whensthe materials could detrimentally face irregularities. Some of the developer removal pro-
affect either subsequent processing operations on a part or cesses, such as detergent or alkaline cleaning, only remove
the intended function of the part. Developer and penetrant some of this entrapped penetrant. Penetrant residues gen-
residues, if not removed, have detrimental effects on the erally can be removed by liquid solvents and more thorough
application of surface finishes such a painting, plating and detergent or alkaline cleaning.
anodizing. Penetrant residues in discontinuities can seri-
ously affect weld quality if not removed before r~pair.we.ld- Protection of Parts after Penetrant Inspection
ing. Developer residues can interfere with the functionm_g
of the part if they involve a moving or wear surface. Inaddi- The penetrant testing process and subsequent removal of
tion, developer materials can absorb and retain moisture, inspection residues leave parts with a chemically clean sur-
resulting in corrosion of the part. face. These surfaces, especially ferrous materials, are highly
reactive and may corrode from the moisture in air. Ferrous

88 I NONDESTRUCTIVETESTING OVERVIEW

parts should receive a corrosion protection treatment as single most widely used nondestructive testing method. It is
soon as practical, during or after the penetrant process. popular because it has a wide range of applications, it is
comparatively easy and inexpensive to use and it requires a
Summary minimum amount of training for its routine use. Its funda-
mental limitation is, of course, that it is useless for detecting
Liquid penetrant testing has become popular through- discontinuities present within a part but not open to its sur-
out industry, and next to visual inspection it is probably the face. Within its normal application, penetrant testing has
proved to be both sensitive and reliable in the hands of
properly trained and experienced technicians.

LIQUID PENETRANT TESTING I 89

BIBLIOGRAPHY

1. Boisvert, B. Principles and Applications of Liquid Pene­ · 11. Mlot-Fijalkowski., A. "Proper Pre-Cleaning Tech-
trant Testing:A ClassroomTrainingText. Columbus, OH: niques Improve the Reliability of Penetrant Inspec-
American Societyfor Nondestructive Testing (1993). tion," 11th World Conference on Nondestructive
Testing [Las Vegas, NV, November 1985]. Vol. 1.
2. Boisvert, B.W., G. Hardy, J.F. Dorgan, R.H.Selner. Columbus, OH: American Society for Nondestructive
"The Fluorescent Penetrant Hydrophilic Remover Pro- Testing (1985): p 337-339.
cess." Materials Evaluation. Vol. 41, Nos. 1 and 2 (Jan-
uary and February 1983): p 34, 36, 38, 40-42 and 12. Moon, J.F. "Factors Influencing the Choice and Per-
134-137. . formance of Penetrant Systems." Proceedings of the
12th World Conference on Non­Destructive Testing
3. Dubose, P. "Degreasing before Penetrant Inspection: A [Amsterdam, Netherlands, April 1989]. J. Boogaard
Key Factor in Quality." Materials Evaluation. Vol. 51, and G.M. Van Dijk, eds. Vol. 1. Amsterdam, Nether-
No. 8. Columbus, OH: American Society for Nonde- lands: Elsevier Science Publishers (1989): p 403-405.
structive Testing (AugusU993): p 892-895.
13. Prokhorenko, P., N. Migun and N. Dezhkunov.
4. Flaherty, J.J. "History of Penetrants." Materials Evalua­ "Development of Penetrant Test Theory Based on
tion. Vol. 44, No. 12. Columbus, OH: American Society New Physical Effects." Non­Destructive Testing 92:
for Nondestructive Testing (November 1986): Proceedings of the 13th World Conference on Non­
p 1,371-1,374, 1,376, 1,378, 1,380, 1,382. Destructive Testing [Sao Paulo, Brazil, October 1992].
Vol. 1. C. Hallai and P. Kulcsar, eds. Amsterdam,
5. Hardy, G.L. and W.E. Mooz. "The Impact of Environ- Netherlands: Elsevier (1992): p 538-542.
mental Concerns on Penetrant Inspection Materials."
Materials Evaluation. Vol. 50, No. 11. Columbus, OH: 14. Sherwin, A. "Penetrant System Performance Check."
American Society for Nondestructive Testing (Novem- Materials Evaluation. Vol. 44, No. 12. Columbus,
ber 1992): p 1,283-1,285. OH: American Society for Nondestructive Testing
(November 1986): p 1,443-1,445.
6. Holden, W.O., J.L. Rhoads-Roberts and C.E. Moss.
"Radiation Safety and Visual Enhancement in the Fluo- 15. Sherwin, A. "Still a Good Rule: Visible Penetrant
rescent Dye Penetrant Processes." Materials Evaluation. Inspection Not to Precede Fluorescent." Materials
Vol. 44, No. 12. Columbus, OH: American Society for Evaluation. Vol. 48, No. 12. Columbus, OH: Ameri-
Nondestructive Testing (November 1986): p 1,435-1,442. can Society for Nondestructive Testing (December
1990): p 1,456-1,457.
7. Iddings, F.A. "The Basics of Liquid Penetrant Testing."
Materials Evaluation. Vol. 44, No. 12. Columbus, OH: 16. Wein, J.A. and T.C. Kessler. "Development of Process
American Society for Nondestructive Testing (Novem- Control Procedure for Ultrahigh-Sensitivity Fluores-
ber 1986): p 1,364. cent Penetrant Inspection Systems." Materials Evalu­
ation. Vol. 48, No. 8. Columbus, OH: American
8. Lord, R.J. "Assessment of Penetrant and Eddy Current Society for Nondestructive Testing (August 1990):
Methods for the Detection of Small Cracks." Materials p 991-994.
Evaluation. Vol. 51, No. 10. Columbus, OH: American
Society for Nondestructive Testing (October 1993): 17. Wheeler, D.G. "Penetrant Systems, Minimisation
p 1,090-1,094. of Adverse Environmental Impact." 6th European
Conference on Non Destructive Testing [Nice,
9. Lovejoy, D. Penetrant Testing:A Practical Guide. Lon- France, 24-28 October 1994]. Vol. 1. Paris, France:
don, United Kingdom: Chapman and Hall (1991). French Confederation for Nondestructive Testing
(COFREND) for the European Council for Nonde-
10. McMaster, R.C., eq1 Nondestructive Testing Handbook, structive Testing (1994): p 699-702.
second edition: Volume 2, Liquid Penetrant Tests.
Columbus, OH: American Society for Nondestructive
Testing (1982).



4SECTION

RADIATION PRINCIPLES AND
SOURCES

Frank A. Iddings, San Antonio, Texas

92 I NONDESTRUCTIVETESTING OVERVIEW

PART 1

ELECTROMAGNETIC RADIATION

The Photon TABLE 1 . Photoncharacteristics

The German physicist Wilhelm Conrad Rontgen discov- Velocity c
ered X-rays in 1895 while working with a high voltage Frequency
gaseous discharge tube. Electrons were emitted from a Wavelength v = c/):
cathode and accelerated toward a target, which they struck Mass ')., = c/v
with a high velocity. He found that a very penetrating radia- Momentum
tion was emitted from this bombarded target. Almost imme- Energy hv/c2
diately these X-rays were used to penetrate the human body
and solid materials, and radiology and radiography were hv/c
well on their way to becoming important parts of their hv
respective fields: medicine and nondestructive testing.
spectrum is shown in Fig, 1. Only a very small region of the
Further research over the many years since Roentgen's chart is occupied by,the visible spectrum.
discovery indicated that the radiation photon has a dual
character, acting sometimes like a particle and at other X-Rays and Gamma Rays
times like a wave. This duality was hinted at by the quantum
theory as put forth by Planck at the turn of the century. X-rays are a form of electromagnetic radiation having
Planck postulated that the photon energy was contained in wavelengths in the region of 0.001 to 1.000 nm (4 x 10-14 to
an electromagnetic packet of radiation, or quantum, and 4 x 10-11 in.). They are usually produced by allowing a
stream of high energy electrons to impinge on a metallic tar-
was proportional to its frequency. His equation E = hv get, producing photons by deceleration of the electrons.
(where E is the energy associated with the quantum, v is its They may also be produced by acceleration of electrons,
frequency and h is Planck's constant) has been used success- usually tangential acceleration of high energy electrons by a
very strong magnetic field. Gamma rays are electromagnetic
fully to explain many physical phenomena. radiation originating from the nuclei of atoms and have very
All radiant energy has similar characteristics and varies short wavelengths. X-rays originate in the extranuclear
structure of the atom; gamma rays are emitted by atomic
only in frequency. Photons all have equal velocity, have no
electric charge and have no magnetic moment. Photon
characteristics are listed in Table 1. A chart of the radiation

FIGURE 1. The electromagnetic spectrum

RADIATION WAVELENGTH
nanometers

l 06 l 05 104 l 03 102 10 JO-' I 0-2 10-3 J0--4 10-5 10-0

-- RADIO INFRARED :::J): X-RAYS. COSMIC RAYS
i 10 102 103
~>\::9:; ULTRAVIOLET GAMMA RAYS
'I --- 10-1

10-9 10...i 10-7 10-6 10-s 10--4 10-3 10-2

PHOTON ENERGY
megaelectronvolts

RADIATION PRINCIPLES AND SOURCES I 93

nuclei in the state of excitation. The emission of gamma rays from an atom. When an impinging electron has lost some of
usually occurs in close association with the emission of alpha its energy in this way and then is suddenly stopped by an
and beta particles. atomic nucleus, the energy transformed into an X-ray pho-
ton is less than the original kinetic energy eV of the electron.
Generation of X-Rays The quantum of radiation produced in this manner has a
wavelength greater than Amin- In general, X-rays of many
X-rays are emitted whenever electrons are rapidly accel- wavelengths are emitted. The X-ray spectrum is continuous.
erated or decelerated. Usually, deceleration is used, as when
matter is bombarded bv a stream of electrons. If it is When an.electron is removed from an atom of the target,
assumed that an electro~ starts with zero velocity at the the atom is l~ft in an unstable state with greater than normal
cathode surface, itskinetic energy upon arrival at the target energy. If an electron replaces the ejected electron, the
of an electrostatic X-ray tube is: atom returns to its normal state. It emits one or more pho-
tons with an energy he/A corresponding to a wavelength A
w ­21 me v2 eV (Eq. 1) characteristic of the element. These narrow bands of wave-
lengths, called characteristic spectral lines, are of higher
intensities than the continuous spectrum. Both phenomena
occur at the same time in the operation of an X-ray tube. A
typical X-ray spectrum is shown in Fig. 2.

Where:

W electron energy, in joules (J); Production of Monochromatic X Radiation
me mass of electron (9.1 x 10-28 g);
Homogeneous, monochromatic X-rays may be obtained
v electron velocity (in m-s", small compared with
the velocity of light); from such spectra by using either a double spectrometer or
filters.
e charge of electron (1.6 x 10-19 C); and At a given angular setting e of a crystal grating with con-
V applied potential difference, in volts, between stant spacing d, only rays with a certain wavelength A can be

cathode and target. reflected at the definite angle 20 from the primary undevi-
ated beam, according to Bragg's law:
Note that l eV = 1.6 x 10-19 J.

Transformation of Electron Energy into X-Rays n'A. 2d sine (Eq. 3)

When an electron with kinetic energy eV strikes the tar- FIGURE 2. Typical X-ray spectrum
get of an X-ray tube, the energy may be transformed in sev-
eral ways. The simplest transformation occurs when the ~ CHARACTERISTIC
electron interacts directly with the nucleus of a target atom. zV)
The electron is stopped by the nucleus, which, because of its ~LI-!J
heavy mass, is not appreciably disturbed and so gains no
energy. Hence, all the kinetic energy of the electron is trans- 5>L!J
formed into a quantum of radiation whose minimum wave-
length is: LU
Cl::'.
he 12,395 (Eq. 2)
O. l 0.2 0.3
v
WAVELENGTH
Where [nanometers)

h = Planck's constant (6.6 x 10-33 J-s); and
c = the velocity oflight (3 x 108 m-s ").

Production of Continuous X-Radiation 0.4

Most of the impinging electrons interact with electrons
associated with the target atoms. Only a part of the energy
of a high spe~d electron is required to remove an electron

94 I NONDESTRUCTIVETESTING OVERVIEW

TABLE2. Bragg angles for various crystalsand Ka radiations

RadiationKa
(degrees)

Crystals Reflections Molybdenum Copper Nickel Cobalt Iron Manganese Chromium

a-alumina 002 1.8 3.9 4.2 4.6 4.9 5.4 5.8
Gypsum 020 2.7 5.8 6.3 6.8 7.3 8.0 8.7
~-alumina 004 3.6 7.9 8.5 9. l 9.9 10.8 l l.8
Pentaerythritol 002 4.7 10.l 10.9 l l.8 12.8 13.9 15.2
Quartz !00 6. l 13.3 14.4 15.6 16.9 l 8.4 20.1
Fluorite 11 l 6.5 14. l 15.2 16.5 17.9 19.5 2 l.3
Urea nitrate 002 6.5 14.2 l 5.3 16.6 18.0 19.6 21.4
Calcite 200 6.7 14.7 15.8 17.1 18.6 20.3 22.2
Rock salt 200 7.2 15.9 l 7. l 18.5 20. l 21.9 24.0
Diamond l 11 99 22.0 23.8 25.8 28.1 30.8 39.9

For a simple cubic lattice the diffracted intensity is approxi- molybdenum target t~be is excited at 30 kV, the spectrum
mately: shows the K series lines superimposed on the continuous
spectrum. If the first part of this band and-the K~ lines
In (h,k,l) ::::: 1 (Eq. 4) could be suppressed, the Ka line would be left in essentially
n(h2+k2+Z2) undiminished intensity. Zirconium has a K critical ?bs9rp-
tion wavelength of 68.88 picometers (pm) (0.6888 A), lying
where n is the order of diffraction and h, k, l are the Miller between K~ and Ka wavelengths of molybdenum, i.e.,
indices. Consequently, a second spectrometer or other between 63.1 and 70.8 pm (0.631 and 0.708 A). A thin zirco-
apparatus can be adjusted to receive the purely monochro- nium screen will absorb practically all radi~tion with wave-
matic beam. Table 2 shows the Bragg angles for various lengths shorter than 68.88 pm (0.6888 A), but will be
crystals and Ka radiations. transparent to the most intense Ka line. Inpractice, a zirco-
nium filter is used with a molybdenum target, nickel with a
Some metals absorb more of certain wavelengths than copper target, and manganese with an iron target.
others do and show characteristic absorption edges. If a

RADIATION PRINCIPLES AND SOURCES I 95

PART 2

RADIATION ABSORPTION

Categories of Absorpiton (Eq. 6)

Radiation absorption can be separated into three major where 10 is the intensity of the incident beam. The number
categories: the absorption of photons, the absorption of
charged particles and the absorption of neutrons. Np/A"of atoms per cubic centimeter n can also be expressed as
where N is Avogadro's number, A is the atomic weight
The absorption of photons has played the most impor- of the absorption material and p is the density of the mate-
tant role in the field of nondestructive testing. Almost from rial. The exponential factor in Eq. 6 can be expressed as:
the very day that Roentgen announced his experimental
results, applications have come forth in the fields of radiol- ­ncrx = -(:cr)px = -(N;p)x (Eq. 7)
ogy, radiography, medicine, experimental physics and thick-
ness gages, to name a few. Therefore, the major emphasis Where:
here is on the physical characteristics of the absorption of
electromagnetic radiation, or photons. NNcrcpr//AA(5 atomic attenuation coefficient;
µ/p = mass attenuation coefficient; and
Absorptionof Photons µ = linear attenuation coefficient.

A beam of X-rays or gamma rays exhibits a characteristic The total attenuation coefficient consists of the sum of
exponential absorption in its passage through matter. This is the attenuation coefficients for each of the various processes
a consequence of the fact that usually a photon is removed mentioned above. That is, the total atomic attenuation coef-
from a beam by a single event. Such an event is a result of
the photon's interaction with a nucleus or an orbital electron ficient is given by:
of the absorbing element and can be classified as one of
three predominant types: the photoelectric effect, scatter- (Eq. 8)
ing and pair production. A fourth but minor contributor to
absorption is photodisintegration. Where:

The following analysis of photon absorption assumes a attenuation coefficient due to photoelectric
narrow beam geometry; that is, any photon that is deflected, effect;
however small the angle, is considered completely attenuation coefficient due to scattering;
absorbed. Because the number of photons removed from a attenuation coefficient due to pair production;
monoenergetic beam at a thickness x of an absorber is pro- and
portional to the intensity at that thickness I(x), the number attenuation coefficient due to
of atoms per cubic centimeter n and the incremental thick- photodisintegration.
ness of material traversed dx, the change in the beam inten-
sity in dx may be expressed as: Photoelectric Effect

dl (x) = I(x) no dx (Eq. 5) The photoelectric effect is defined as that process in
which a photon of energy E0 transfers its total energy to an
Here, o is a proportionality constant and is interpreted electron in some shell of an atom (Fig. 3). This energy may
as the total probability (more often referred to as the cross be only sufficient to move the electron from one shell to
section because it has the dimensions of an area) per atom another, or it may be sufficient to remove the electron com-
for scattering or absorption of a photon of the original pletely from (i.e., to ionize) the atom. Inthe latter case, the
energy. Integration of Eq. 5 gives: kinetic energy of the ejected electron is just the difference
between the photon's energy and the binding energy of that
particular electron in the atom.

96 I NONDESTRUCTIVETESTING OVERVIEW

FIGURE3. Photoelectricinteractionof incident FIGURE4. Absorptioncurvesfor uranium,
photon with orbital electron showingcomponentsof total attenuation
coefficientas functionof energy (to convert
massattenuationcoefficientto squaremeters
per kilogram,multiplyby 1 OJ

JOO

- L1• L2, AND L3 ABSORPTION EDGES

,.._ TOTAL
10

As the photon energy is increased from zero, the photons -+-- K ABSORPTION EDGE
are absorbed by electrons in deeper lying shells of the atom.
When the photon energy reaches the binding energy of a -- \
particular shell of electrons, there is an abrupt increase in
the absorption. The energy at which this sharp change 0. I \
occurs for K electrons, called the K absorption edge, repre- COMPTON /' • -: ~' \
sents a situation where the kinetic energy of the ejected K
electron is zero. Further increase of the photon energy SCATTERING ".,i,\',,
causes the absorption to decrease approximately inversely
with the cube of the energy. The photoelectric attenuation PHOTOELECTRIC -, '\ ', ', , , ,' ,'~, PAIR
coefficient for an atom may be expressed as the sum of the \ :<_ PRODUCTION
coefficients representing the contributions of the K, L, etc. ,' ',
electron shells. The energy dependence for the photoelec- 0.01 ~---~---~~------~ \ ' 'I \ 100
tric absorption in uranium is shown in Fig. 4. The solid
curve represents the total attenuation coefficient and the 'I
dotted curves show the components. The total scattering
curve includes the Compton effect, coherent scattering, and 0.01 0. I JO
a correction for the average binding of the electrons in their
shell structure. The absorption increases so rapidly with the ENERGY
atomic number Z (between 24 and 25) that for heavy ele- megaelectronvolts
ments it remains appreciable to energies of the order of a
few million electronvolts.

Scattering of Photons Compton Scattering

Upon increasing the photon energy past the K edge, the Analysis of the Compton process shows that the energy
main process contributing to absorption changes from the of a scattered photon is always less than that of the primary
photoelectric effect to the Compton effect. This is really not photon; the effect is, therefore, one of incoherent scatter-
true absorption, since part of the photon's energy is not ing. The famous analysis made by Compton was based on
absorbed but merely redirected (Fig. 5). In the Compton the particle properties of the photon and the conservation of
effect a photon collides with an electron. Instead of giving energy and momentum during the collision. The energy
up all its energy to the electron as in the photoelectric pro- shift predicted depends only on the angle of scattering and
cess, however, the photon only shares its energy with the not on the nature of the scattering medium, the larger
struck electron.1 The binding energy of the electron is usu- energy shifts being due to the larger angles through which
ally considered negligible compared with the photon's the incident photon is scattered. A complete analysis of the
energy. Compton energy angle relationship and the Klien-Nishina
formula for calculating the associated scattering cross sec-
tions may be found in the literature.

RADIATION PRINCIPLES AND SOURCES I 97

FIGURE 5. Compton scattering in which FIGURE6. Coherentscatteringof photon
incident photon ejects electron and lower without lossof energy
energy scattered photon

Figure 4 illustrates the decrease in the attenuation coef- FIGURE7. Pair productionof electronand
ficient, attributable to the Compton effect, as the photon positronfrom incidentphoton
energy increases. This decreasing of the number of scat-
tered photons is equivalent, of course, to an increase in the E0 = E+ + E_ + 2 mc2
penetrating power of the photons with increasing energy.
E
Coherent Scattering
increases approximately logarithmically with energy above
If a photon does not experience an energy shift upon the threshold value and then levels off for extremely high
being scattered by an atom, the process is spoken of as being energy photons. This process may also occur in the field of
coherent. This phenomenon is often referred to as the an orbital electron and in this case is sometimes referred to
Rayleigh process. It can only occur for soft radiation for as triplet production. However, the third electron, which is
which the binding energy of the electrons in their atomic often seen because of its large recoiled energy, is not cre-
shells is important. Classically speaking, the atomic elec- ated in the process but is the orbital electron. The combined
trons are set into oscillation by the absorption of the inci- effect of these two pair production processes is plotted in
dent photon. Then, acting as a common source, they emit a Fig. 4.
photon of the same frequency as the incident photon
(Fig. 6). The contribution of this type of coherent scattering Photod isi ntegration
to the total attenuation coefficient is never greater than In addition to the three dominant photon absorption
about 20 percent.
processes already discussed, the effects of photodisintegra-
Coherent scattering can also occur from the atoms in tion of nuclei are of interest. Here, a photon is captured by a
crystalline structure. However, because of the many ways an nucleus, which then loses one or more of its constituent par-
atomic system may be ordered in nature and because this ticles (Fig. 8). This may be thought of as the nuclear analog
type of scattering is primarily contained in very small angles of the photoelectric effect. This effect, however, is very
with respect to the incident photon, the effect has little small and its first maximum, usually between 10 and
importance in increasing the attenuation coefficient, and 20 MeV,is only a few percent of the attenuation coefficient.
therefore it will not be considered further here.
Z Dependence and Energy Dependence
Pair Production
It is apparent that the attenuation coefficient depends
Very high energy photons are absorbed in matter by a very strongly upon the atomic number Z, varying approxi-
process in which a photon is converted in the electrical field mately as Z(Z+l) for pair production, directly with Z for the
of a nucleus into an electron and a positron (Fig. 7). This is
pair production. Because both members of the pair have a
nonzero mass, there is a minimum energy corresponding to
creation of the rest masses of the two particles, below which
pair production cannot occur. The value of this threshold is
1.02 MeV Any energy in excess of that needed for creating
an electron-positron pair appears as kinetic energy of the
pair particles. The probability of occurrence of the process

98 I NONDESTRUCTIVETESTING OVERVIEW

FIGURE8. Photodisintegrationin which Artificially produced radioactive neutron sources are of
incidentphoton ejects electronfrom nucleus two general types: those formed by the interaction of alpha
(a) particles with the light elements, e.g., beryllium, boron,
Compton effect, and between Z4 and Z5 for the photoelec- or lithium; and those formed by interaction of gamma (y)
tric effect. The attenuation coefficient decreases with photons with neutron producing targets. Neutrons from
increasing energy for the photoelectric effect as £-3·5, these radioactive sources usually have energies in the. 1 to
decreases for the Compton effect in the region of interest 10 MeV energy range.
roughly as E-1 and increases approximately logarithmically
with energy for pair production. Spontaneous fission is a popular source of neutrons.
These sources produce a fission spectrum, including unfor-
AttenuationCoefficients of the tunately many alpha particles and gamma radiation for each
Elements neutron produced. An example is californium-252. The use
of these neutron sources requires great care and shielding
Attenuation coefficient tables give the radiation attenua- and is discussed in more detail elsewhere.1
tion coefficients of most of the elements of importance in
nondestructive testing.1 For elements not listed, the attenu- Particle accelerators capable of producing beams of pro-
ation coefficients can be calculated by direct interpolation, tons, deuterons and alpha particles can produce neutrons by
using the proper Z dependence and energy dependence for target interactions. An accelerator capable of imparting
the various components of the total attenuation coefficients. energies up to 10 MeV can produce monoenergetic neu-
Some care should be exercised in using these tables because trons at energies up to 27 MeV. Very high energy accelera-
they are based on narrow beam absorption. tors (>100 MeV) can .produce large numbers of neutrons
from heavy materials because of multiple nuclear interac-
Neutron Irradiation tions within the target material, notably photo-neutron and
photo-fission caused by secondary reactions from the pri-
Production of Neutrons mary particle beam. When large particles or elements are
The most frequently used source of neutrons is the formed, the process is usually referred to as spallation.
Fusion reaction neutron sources use energetic deuterium
nuclear reactor. Here the neutrons are formed by fission, ion beams impinging on tritiated targets to produce 14 MeV
where an unstable heavy element isotope (uranium or plu- neutrons. Special, small vacuum-sealed machines are built
tonium) formed by neutron capture disintegrates into sev- specifically for this purpose.
eral elements with the release of several neutrons. This
chain reaction reactor produces a continuous Maxwellian Neutron Absorption
energy spectrum with a long energy tail up to about 10 MeV,
but peaking in the thermal energy range or somewhat The absorption of neutrons relies on completely differ-
higher depending on the type of reactor. Typical intensities ent mechanisms than that of charged particles. The neu-
available range from 1014 to 1018 neutrons per square cen- tron, carrying no charge, does not have any Coulomb
timeter per second (neutrons cm-2-s-1 ). interactions, and is free to travel through material until it
has a direct collision with a target nucleus or an orbital elec-
tron. In the latter case, the electron is so small as to con-
tribute only negligibly to the total absorption. In general,
neutrons interact with matter in two ways, the neutron is
either scattered by the nucleus or is absorbed into the
nucleus.

Because of many reactions possible for absorbing neu-
trons and their complicated energy and mass dependencies,
there is no simple way to present the total absorption effect.
However, the probability of any interaction between neu-
trons and matter can be made qualitative by means of the
concept of cross sections. The cross section o is the effective
target area of the nucleus as seen by the impinging neutron
of a given energy. The number of interactions per unit time
will be nvNcr, where n is the number of neutrons per unit
volume moving with velocity v towards the target of N
nuclei. The quantity nv is the neutron flux density. The cross
section cr is usually expressed in barns (l0-22 mm2).

RADIATION PRINCIPLESAND SOURCES I 99

PART 3

BASIC GENERATOR CONSTRUCTION

A conventional X-ray generator consists of three main although still in common use, are far more susceptible to
components: (1) X-ray tube; (2) high voltage source and thermal and mechanical shock than the metal-ceramic
(3) control. While examining each of the major components envelope and are currently being replaced in many indus-
in some detail, it should be remembered that all conven- trial applications with more durable metal-ceramic tubes.
tional units will have similar construction.
The vacuum envelope of the metal-ceramic tubes con-
X-Ray Tubes sists of a metal cylinder capped on both ends with ceramic
disks, usually composed of aluminum oxide. These ceramic
Early X-ray tubes made use of gas filled tubes and a cold insulators are designed to allow for more effective use of the
cathode from which electrons were freed by positive ion insulation characteristics of both the ceramic and the high
bombardment. Modem tubes used in radiography are of the tension grease used in sealing connections between the high
high vacuum variety, allowing for reduction in size, voltage source and the tube. This design allows for reduc-
extended tube life and more stable operation. tion in the size of the tube housing, especially important for
higher energy units.
Electrons are supplied by thermionic emission from the
filament. The accelerating potential (in volts) and the tube Cathode
current (in amps) can then be independently varied, with
the exception that, at low accelerating voltages, tube amper- The cathode includes the tungsten filament which pro-
age is affected by the space charge that accumulates around vides thermal electrons for acceleration. The filament is usu-
the cathode. ally powered by alternating current (50 to 60 Hz) from a
separately controlled transformer, although in some units fil-
Envelope ament current is fixed or automatically controlled to main-
tain a constant tube current. Normally, filament currents
Envelopes for X-ray tubes are usually of the glass or range from 1 to 10 A. Tube current, passing between cathode
metal-ceramic type (Figs. 9 and 10). Glass envelope tubes, and anode by means of high speed electrons, ranges from
several hundred microamperes (µA) for microfocus units up
to 20+ mA for conventional industrial radiographic units.

FIGURE9. GlassX-ray tube FIGURE 1 0. Metal-ceramicX-ray tube

LEGEND LEGEND
1 . GLASS ENVELOPE 1 . METAL ENVELOPE
2. FOCUSING CUP 2. CERAMIC DISK INSULATOR
3. FILAMENT 3. FILAMENT MOUNTING STRUCTURE
4. TARGET MOUNTING STRUCTURE
5. TARGET 4. FILAMENT
5. TARGET MOUNTING STRUCTURE
6. TARGET

100 I NONDESTRUCTIVETESTING OVERVIEW

Beam Focusing be a solution to geometrical unsharpness, but this approach
is limited by the durability of the filament.
At times the filament is located in a recess in the cathode
called afocusing cup. This surrounds the emerging beam of One alternative, called line focusing, is to project the
electrons with an electric field that repels the beam away approximately rectangular beam produced by the filament
from the cup wall and into a more localized form. onto a target angled with respect to the beam (approxi-
mately 21 degrees). This projects an X-ray beam that
The importance of a well defined beam of electrons appears to issue from a focal spot with approximately equal
arises from the fact that the sharpness or unsharpness of an lateral dimensions (Fig. 12). In practice, this method allows
image depends on the focal spot size (Fig. 11). The relation- production of units with focal sizes in the range of 1.0 to
ship for geometric unsharpness Ug is: 3.0 mm (0.04 to 0.12 in.).

ug = By use of a deep focusing cup, advantage can also be
taken of the screen effect (Fig. 13). This refers to the

(Eq. 9)

Where: FIGURE 1 2. Diagram of line focusingsetup

Ug geometric unsharpness (the measure of focal spot's ELECTRON BEAM SIZE
penumbra effect);
(
F focal spot size;
D0 distance from target (focal spot to object); and

d distance from front of object to recording medium.

Note that d = thickness of object when the recording medium

(film) is flush against the back of the radiographed object.
From the equation, we see that U increases directly as

the focal spot size increases. Because tlie beam originates at
the filament, reduction of the filament size might seem to

FIGURE 11 . Illustrationof geometric ACTUAL
unsharpness U9 FOCAL SIZE

EFFECTIVE FOCAL SIZE

D FIGURE 1 3. Graph of screen effect

KILOVOLT PEAK

d

---J u, f- 0 0.5 1.0
lMAGE TIME (HALF CYCLES)

RADIATION PRINCIPLES AND SOURCES I 1 01

removal of the lower energy electrons that are produced For higher energy units in continuous use, it is usually
necessary to cool the anode by injecting coolant directly into
during that portion of the AC cycle where the potential dif- it. This is accomplished by hollow construction of the anode
conductor.
ference between cathode and anode is significantly less than
Another way of alleviating the problem oflocalized heat-
maximum. In practice this improvement is not without cost ing of the target is by use of a rotating anode in which the
target, a tungsten disk, is driven as shown in Fig. 14. This
to the output of the unit. A loss of approximately 25 percent allows the tube current to be increased by as much as ten
times the value for a stationary target. The focal spot on
is experienced in units with high screen effect. This can be such units can be reduced to less than 1 mm (0.04 in.) for
short exposure times, which is of value in medical as well as
compensated for, in part, with higher filament current some specialized industrial applications, including flash
radiography.
though this adversely affects the lifetime of the filament. An
Target
alternate method of removing low energy components of
In radiographic applications, the target is usually tung-
the electron beam is found in the discussion on constant sten and is bonded to the copper anode; however, analytical
units make use of several other target materials to take
potential (CP) units. advantage of the characteristic X-rays produced. Some of
If still further focus of the beam is desired, as in microfo- these materials include copper, iron and cobalt.

cus radiography and some analytical applications, additional The orientation of the target with respect to the electron
methods may be used: conversion of the conventional diode beam strongly influences the size and shape of the focal
arrangement of cathode and anode into a triode arrange- spot. Orientations from O degrees to 30 degrees are used for
ment including a focusing electrode or grid; and electro- various applications. For example, zero is the angle used for
static or magnetic deflection systems. panoramic units. An angle of 20 degrees is commonly
selected for directional units because, in this case, the distri-
For the triode arrangement, which is used widely in the bution of X-rays is predominantly perpendicular to the tube
microfocus industry, a negative bias of up to -150 V is axis. This is shown graphically in Fig. 15. The actual maxi-
applied to the third element of the tube to further focus the mum of intensity occurs at + 12 degrees. For radiography of
beam and remove lower energy components. This configu- objects whose lateral dimensions are less than one half the
ration allows a reduction of beam size, producing focal spots
smaller than 50 µm (0.002 in.), and a subsequent drop in
tube current.

In the case of electrostatic deflection, even more ele-
ments are included within the envelope, while a magnetic
deflection system is external to the tube. These types of
deflection systems have an additional advantage in the fact
that the beam may also be deflected to various areas of the
target for added service life. Units that incorporate their
own vacuum systems usually allow for replacement of both
filament and target components. These types are available
as standard microfocus and analytical units.

Anode FIGURE14. Rotatinganode

As mentioned previously, heat is the major form of X~YB~---
energy produced as the electrons strike the target. Uncon-
trolled, this heat would quickly cause the surface of the tar-
get to erode, which in tum would reduce the definition of
the focal spot. In addition, the vaporized target material
would reduce the high vacuum of the tube and lead to pre-
mature failure due to conduction within the tube. To avoid
overheating of the target, the anode to which it is attached is
composed of a material with high thermal conductivity, such
as copper. If the cooling demands are relatively low, as for a
low energy unit or intermittent use, cooling is often accom-
plished by means of a conductor that passes through the
tube end for connection to the high voltage source; this
allows for radiation of heat into an oil or gas reservoir sur-
rounding the tube. Although not the most efficient, the
weight of such a tube is minimal because of the absence of
pumps or heat exchangers.

1 02 I NONDESTRUCTIVETESTINGOVERVIEW

focus-to-film distance (objects which subtend an angle of charge buildup because of electrons scattered from the
less than 30 degrees) the variation is seldom consequential. tungsten target or released by the photoelectric effect
(Fig. 17).
Another cause for intensity variation is the electron
beam itself. A cross section of the electron beam from the Removing the unused radiation directly at the anode
filament would resemble Fig. 16a, with relative beam inten- reduces the amount of radiation shielding that must be pro-
sity also shown. Figure 16b shows a similar representation vided externally or incorporated into the tube housing. The
for a microfocus beam. The beam distribution in Fig. 16b is hood, normally constructed of copper, may have materials
said to be Gaussian (bell shaped) because of the shape of with high atomic numbers, such as tungsten, incorporated
the intensity curve. Such a beam profile is required when it to increase absorption. The electrical shielding function of
is necessary to define very closely spaced objects, such as the hood may be improved by the addition of a beryllium
microcircuitry components. window over the X-ray port. A window several millimeters
in thickness will stop electrons with negligible effect on the
Hood overall X-ray beam.

Addition of a hood to the anode has two functions: FIGURE 16. Electron beam distributions:
(1) eliminating a portion of the X-ray beam outside the cen- (a) conventionalbeam; (b) microfocusbeam
tral cone of radiation and (2) electrically shielding the insu-
lating portions of the envelope (glass or ceramic) from

FIGURE 15. X-ray distributiongraph fa)

0.5 1.0

DISTANCE ACROSS TARGET

-20 -13-10 0 JO 13 20 (bJ
31 70 80
ANGLE 104 105 95 ~ 0.5 1.0
(degrees) zwvi
~I- DISTANCE ACROSS TARGET
JOO
~s
INTENSITY
(percent) zen
0~
wti

w_J
0

RADIATION PRINCIPLESAND SOURCES I I03

Rod Anode insert, electrical connections, fittings, pumps, thermal and
high voltage overload sensors and radiation shielding con-
The rod anode (sometimes referred to as an oxtail) is tained in the head. For the tank type unit, the tube head
another adaptation of the anode. This type of tube arrange- also houses the high voltage and filament transformers. If
ment requires special circuit considerations which allow the the unit has separate components, the tube head will also
anode to be grounded. This tube, developed for use through provide for connection to the high voltage source.
small openings, has been partially replaced by a metal-
ceramic tube, which can have a diameter of less than 50 mm High Energy Sources
(2 in.) and tube head diameter as small as 76 mm (3 in.).
The target of such an end grounded tube can be cooled by From line voltages in the range of 100 to 250 V, the high
circulating water in direct contact with the anode. Beam tension circuitry supplies potential differences to the tube
focusing is often required for longer tubes. from 5 kV to as much as 420 kV for the larger industrial
radiographic units. Several standard circuit designs are used
Coolant for various applications. The portable tank type units gener-
ally employ one of the designs shown in Fig. 18.
With the exception of the end grounded configurations
and units designed for low energy output (less than 50 keV), FIGURE 18. Standard high voltage circuit
the tube insert is surrounded by an insulating coolant and designs for portable tank type units: (aJ with
encased in a housing called the tube head. cathode ground; (bJ with center ground;
(cJ with anode ground
The coolant may be highly dielectric gas or oil. If oil is
used, simple convection may be sufficient for lower output fa)
units. For larger units, an oil circulating pump combined
with a heat exchanger, either internal or external to the tube
head, may be used.

For units making use of a fixed amount of oil in the tube
and a circulating pump to circulate it within the tube head,
an oil resistant bellows is incorporated to allow for expan-
sion and contraction of the oil. Because of the compressibil-
ity of insulating gases, this is not required for gas filled
heads, but a pressure gauge is normally included to monitor
possible loss of coolant insulation.

Tank Type Head fb)

The housing itself structurally protects the tube, contains
the coolant and forms the structural support for the tube

FIGURE 1 7. Hooded anode tube

fc)

LEGEND
I . ANODE HOOD··
2. CATHODE

3. BERYLLIUMWINDOW

1 04 I NONDESTRUCTIVETESTING OVERVIEW

These circuits are all self-rectified; the X-ray tube itself transformer peak voltage. Capacitors are charged during
limits the flow of electrons to one direction in the circuit. one half of the cycle and discharged when current passes
While the anode is at negative potential with respect to the through the tube, augmenting the voltage produced by the
cathode, no tube current flows. transformer (Fig. 19a).

One drawback of the self-rectified system is the possibil- Graetz Circuit
ity of tube backfire. If the target or anode overheats, reverse
conduction can occur during the negative half cycle. This The full wave or Graetz circuit allows use of both halves
type of unit is normally used for tubes producing X-rays in of the AC cycle with a substantial increase in tube output
the range of 50 to 300 keV peak and tube currents from 2 to per unit time. This system is widely used in medical applica-
8mA. tions but is used less than constant potential (CP) units in
industrial applications (Fig. 19b).
Cathode Grounded Circuit
Greinacker Circuit
The main advantage of the cathode grounded configura-
tion is that it allowsthe filament transformer to be external to As can see from the output wave form, the Greinacker
the tank, because the cathode is at ground potential and does circuit is of the constant potential type (Fig. 19c). Basicallya
not require isolation. The tube head can be reduced in size variation of the Villard circuit, in which capacitors are
and weight and is often gas filled to further decrease weight. charged during both halves of the cycle, the voltage. is not
only doubled but remains near maximum value throughout
Center Grounded Circuit the cycle. This gives enhanced high energy output and elim-
inates the electrical stress placed on the tube and insulation.
Both center grounded and anode grounded units require Enhanced tube life and about 30 percent reduction in expo-
isolated filament transformers, which must be insulated sure times are the results.
adequately. For the center grounded unit, this is justified by
reduction of the high tension (HT) transformer insulation. A common misconception is that constant potential units
The transformer needs to supply only one half of the poten- provide a beam of constant energy X-rays. Although the
tial difference to each electrode, rather than having either electron beam is nearly monoenergetic, the X-rays are pro-
the cathode or anode held at ground potential and supplying duced during random deceleration processes. The absence
the entire accelerating voltage to the other electrode. of low energy electrons reduces the number of low energy
X-rays but does not eliminate them.
In the range of 200 to 300 kV peak with beam currents to
15 mA, center grounded systems can be made smaller than Alternative Circuit Designs
comparable end grounded units.
A method for improving tube output, which can be used
Anode Grounded Circuit in conjunction with any of the above circuits including those
in tank units, is the use of a higher frequency waveform to
For the anode grounded or end grounded system, the power the high tension (HT) transformer. Although this
advantage lies in the specialized use of the rod anode tube requires additional electronic circuitry or a motor generator,
or metal-ceramic tube for access through small openings. As the core of the high tension transformer can be reduced in
mentioned earlier, cooling of the target is also simplified. size because of increased reactance at higher frequencies.
This can be used to advantage in portable or mobile units.
With the addition of capacitors and diode rectifiers, the Also, if filtering is to be done, the variation or ripple of the
transformer is normally placed in a tank separate from the output voltage can be reduced even further.
head. The additional elements allow the current to be recti-
fied by means of valve tubes or solid state diodes and to be · A variation of this technique is the use of three phase
filtered and smoothed to provide a more nearly constant input power with the high tension transformer. Commonly
accelerating voltage. Several popular circuits and their used in medical X-ray generators, this method is now in use
waveforms are shown in Fig. 19. by several industrial manufacturers as well.

Villard Circuit Another approach is to use an output waveform other
than the standard sine wave. Approximate square wave out-
An extension of the half-wave system, the Villard circuit puts, in conjunction with both phase inversion circuitry and
allows production of accelerating potentials of twice the a high frequency transformer, can provide accelerating
potentials with extremely low ripple characteristics. Such
units are currently available for industrial applications.

RADIATION PRINCIPLESAND SOURCES I 105

High Tension Connections The high tension cables themselves are shielded to provide
protection against electrical shock. Cables used at lower
One remaining topic in our discussion of conventional energies are relatively flexible but as the amount of insula-
high voltage sources is connection of the high tension trans- tion is increased, the flexibility decreases and sharp bends
former to the tube. For tank units this is not a major consid- during installation should be avoided.
eration since the transformer can be connected directly to
tube electrodes. However, for units with separate compo- The cables are inserted into terminations usually made
nents, insulation and connection of leads (which may carry of phenolic (thermosetting plastic) or ceramics and are
voltages in excess of 200 kV) it is an important consideration. sealed against air by use of insulating epoxy materials called
potting compound. The phenolic termination used primarily

FIGURE 19. Typicalanode groundedcircuitsand their waveforms:(a) Villard circuit;(b) Graetz circuit;
(c) Greinackercircuit

(a) TO FILAMENT TRANSFORMER

VILLARD CIRCUIT WAVEFORM
X-RAY TUBE

t3oLI.J

0>
TRANSFORMER

TIME

1~ T

~

TO HIGH VOLTAGE CONTROL

(bJ TO FILAMENT
TRANSFORMER
GRAETZ CIRCUIT WAVEFORM

X-RAY TUBE

oLI.J
t3 '0>
TO HIGH \ _.,.,/ ,_\ I
VOLTAGE \ /
CONTROL \

RECTIFIED

ALTERNATIONS

TIME

(cJ TO FILAMENT GREINACKER CIRCUIT WAVEFORM
TRANSFORMER

X-RAY TUBE

LI.J /-,

l'.J .r>.
t3TO HIGH / \
II ' \ \
jvoLTAGE
\ I \ I
II- ~CONTROL \ \ ,~I
/

TRANSFORMER

TIME

1 06 I NONDESTRUCTIVETESTING OVERVIEW

with glass and lower energy metal-ceramic tubes is of the As stated above, a transformer is used to provide the
form shown in Fig. 20a. potential difference for conventional X-ray units. As the
accelerating voltages are increased toward 1 MeV, standard
These connectors have rather large dimensions in com- transformer and insulation technologies become inade-
parison to the newer style ceramic terminations (Fig. 20b) quate. The major types and their operating principles are
used primarily with higher energy metal-ceramic tubes. discussed below.
Both styles use highly dielectric grease to seal out air at join-
ing surfaces. Because of the tendency for the long male- Resonance Transformer
female connections to trap air and substantially reduce the
insulation capabilities of the grease, these joints are nor- Resonance transformers are used in conjunction with
mally rated at about one tenth their theoretical values or multisection tubes to produce X-rays in the range of 1 to
about 10 kV per centimeter. The linear dimensions of such a 2 MeV. By resonating transformer circuitry at a multiple of
termination must be correspondingly increased. At the input frequency, the ferromagnetic core of the transformer
transformer tank, this size increase is not so important, but can be eliminated. Weight and bulk of the transformer are
at the tube head the increase in size and weight can make reduced not only by removal of the core, but also by removal
the unit very cumbersome and bulky. For a 400 kV tube of the insulation necessary to isolate the core from the wind-
head, 200 kV is applied to each electrode. This requires a ings. In the absence of a core, the tube can be placed on the
termination 175 to 200 mm (7 to 8 in.) long at each end of axis of the high tension windings. Proper spacing of active
the tube for proper insulation. tube segments allows the acceleration of electrons to take
place in several intervals instead of the single active region
The ceramic insulator pictured in Fig. 20b makes full use of the conventional tube. Connections to sequential por-
of the dielectric strength of the insulating grease by provid- tions of the tube and windings are facilitated by the concen-
ing rigid, flat mating surfaces which exclude air from the tric arrangement, and the tube is also electrically shielded
joint. This allows for a substantial decrease in the length of because of its central location. Although bulky in compari-
the joint. This design has been incorporated into tubes used son to more modem generators, resonance transformers
by several equipment manufacturers for units up to 420 kV. have proven to be very durable. Many of the original units
are still in use five decades after manufacture.
FIGURE20. High voltage transformer
terminations:(a) phenolicconnection; Van de Graaff Generator
(bJ ceramicconnection
This generator (Fig. 2la) is unique in that the potential
(a) difference is produced by mechanically transporting a
charge to the high voltage terminal via an insulating belt.
(b) The terminal is surrounded by a case held at ground poten-
tial. Electrons are emitted from an electron gun and pass
23 between the high voltage terminal and the outer case. The
potential difference between the terminal and case causes
LEGEND TO HIGH TENSION GENERATOR the electrons to accelerate to very high speeds.
I . CONNECTOR TO TUBE
2. AIR SPACE Upon striking a target, X-rays are produced in large
3. CONNECTOR quantities because of the increased efficiency ( 10 percent
and higher) of all high energy generators. The accelerating
potential is essentially constant and X-ray output of several
grays (several hundred rads) per minute at 1 mis attainable.
Van de Graaff units are commonly used to provide energies
of up to 8 MeV; but because of the size of the larger
machines, radiographic units seldom produce more than
2.5 MeV. The larger units find applications in research and
development and may be used to accelerate particles other
than electrons.

Betatron

The betatron, synchrotron and cyclotron all share a com-
mon heritage in the use of magnetic fields. The oldest mem-
ber of the family, the cyclotron (Fig. 22) makes use of

RADIATION PRINCIPLES AND SOURCES I 1 07

magnetic deflection to bend a stream of heavy, charged maintain a circular path. This allows replacement of the
particles, such as completely ionized deuterium, into a spi- polar type magnet with a toroidal (doughnut shaped) mag-
ral path. During each cycle, the particles are accelerated as net. The resulting synchronized cyclotron or synchrotron
they pass between oppositely charged, hollow electrodes can produce particles having energies of more than 500 GeV
shaped like the letter D. Using this type of arrangement, it Although such units are used mainly for elementary particle
is possible to produce deuterons with energies of more research and military applications, smaller versions used to
than 15 MeV, in dees that are just over 100 mm (4 in.) in accelerate electrons do have radiographic applications.
diameter.
The betatron is such an adaptation (Fig. 23). Here,
The spiral path of the charged particles is produced by a acceleration of the electrons is produced as a direct effect of
fixed magnetic field. By increasing the field in synchroniza- the increasing magnetic field. As the field of the polar mag-
tion with the acceleration of the particles, it is possible to net is increased, an electric field is created by the process of

FIGURE21 . High energygenerators:(a) Van de Graaff generator;(b) linear accelerator
(aJ (bJ

ii 8
5

3

/l\ 12

LEGEND 7. TARGET '7 \
I. BELT B. PULSE TRANSFORMER
2. HIGH VOLTAGE TERMINAL 9. RADIO FREQUENCY SOURCE
3. OUTER CASE I 0. RADIO FREQUENCY SYSTEM
4. INSULATING GAS I I . ACCELERATOR
5. ELECTRON GUN 12. LASER
6. COLLIMATOR

1 08 I NONDESTRUCTIVETESTING OVERVIEW

induction. The electric field acts much the same as a poten- energies from 2 to 15 MeV with outputs as high as
tial difference would in accelerating the electrons to high several grays (several hundred rads) per minute at 1 m
speeds. During each circuit around the betatron, the elec- (40 in.). Such powerful X-ray beams are capable of useful
trons gain several hundred electronvolts of energy. Follow- radiography of steel sections over 500 mm (20 in.) thick.
ing more than one hundred thousand cycles, the beam of These linear accelerators have been used in materials
electrons can have energies of 20 to 50 MeV. The X-rays research and weapons technology as well as in radiography.
produced by this beam can penetrate 305 to 380 mm ( 12 to
15 in.) of steel in relatively short exposure times. Control Units under 500 keV

Linear Accelerator Line voltage is introduced to the unit through the control
Electrons introduced into the cavity of a linear wave- which is often capable, especially for portable units, of
accepting llO, 220 or 440 V line input. Single phase 50 or
guide (carrying a standing or traveling radio frequency [rf] 60 Hz AC is generally used for portable applications, while
wave) will experience an electric field acting axially along fixed units may be single- or three-phase in design. Units may
the guide. In this field, electrons will increase their kinetic also have some adjustment to compensate for line voltage
energy at the rate of approximately 15 MeV per meter drops such as those caused by use of long extension cords.
(40 in.) of guide. This energy increase allows for construc-
tion of very compact units (Fig. 2lb) that can produce X-ray Aside from allowing .the unit to be turned on and off,
controls usually allow for the adjustment and monitoring of
FIGURE22. Cyclotron the three radiographic variables: exposure time, energy and
tube output. The actual controls and monitors may vary in
LEGEND appearance, or may even be absent from a particular unit,
I. BEAM PORT but these functions will be performed either manually or
2. ACTIVE ACCELERATION REGION automatically in all units.

The control will contain the fuses and circuit breakers,
warning and interlock circuitry, various initiation, termina-
tion and security switches and, on some later models, com-
puterized memories with digital controls. The particular
type of control depends on the range of functions required
of the unit, and just as importantly, the skill and training of
the operator. For a production facility with standard tech-
niques established, a programmable control unit might be
desirable; an independent test lab may require a control that
allows easy adjustment and that is rugged and portable
enough for field use. Such requirements need to be consid-
ered before acquiring any unit.

FIGURE23. Betatron Kilovoltage Adjustment

OVACUUM TOP YOKE X-ray energy is controlled by adjustment of the voltage
supplied to the primary of the high tension transformer.
r::9•-TUBE This may be done electronically or manually. For manually
• ROUND s adjusted units, a variac is commonly used. The variac is a
transformer with a toroidal winding that provides an
tPOLEPIECE adjustable output voltage from zero to about 17 percent
above line voltage. This allows for continuous adjustment of
----+- LEG the kV between minimum and maximum values. It is usually
necessary to reduce the kV setting to its minimum value
• RADIALLY iN before an exposure can be initiated. The variac setting is
LAMINATED then increased gradually to the desired value. This avoids
electrical stress that would otherwise be experienced by the
BOTTOM YOKE tube and insulation.

RADIATION PRINCIPLES AND SOURCES I 1 09

For units which are not continuously variable (for units current is done by measuring the current passing between
that are variable in steps) adjustment during an exposure ground and the secondary of the high tension transformer.
should not be attempted. This introduces transients into the On some units, monitoring and adjustment of the current is
high tension circuitry that can exceed design limits of the done automatically within the control.
tube; arcing between the cathode and anode can occur,
leading to tube failure. Such units normally have provisions For units that use center or anode grounded systems, the
for minor adjustment of kilovoltage, as well as an automatic filament transformer must be isolated and the control will
initiation cycle to gradually apply voltage to the tube. Digi- allow adjustment of the input voltage to the primary of the
tally controlled units will also have an initiation cycle as a filament transformer.
part of their circuitry.
A fact to remember is that kilovoltage and milliamperage
Programmable systems range in complexity from card to
keyboard and will become a more requested item on most settings are not completely independent. At high voltages
control units. the electrons are quickly accelerated away from the cathode
but at lower kilovoltage the X-ray tube operates much less
Milliamperage Adjustment efficiently because of space charge buildup. In addition to
space charge, the screen effect also influences tube output.
Tube output is controlled by regulating tube current For units rated to operate at 5 mA, the kilovolt meter is cal-
which is, in tum, strongly influenced by filament current. As ibrated at 5 mA and will not necessarily be accurate at other
previously discussed, cathode grounded units need not have values. Likewise, if the unit is adjusted to put out 5 mA at
the filament transformer isolated. The filament supply is 100 kV, the tube current may be above limits when the kilo-
often contained directly in the control. Monitoring of tube voltage is raised. Operators should also be aware that some
older units do not monitor tube current at all but instead
measure filament current.

11 0 I NONDESTRUCTIVETESTING OVERVIEW

PART4

X-RAY OPERATING RECOMMENDATIONS

Baseline Data material thickness and exposure times are plotted. This is
done to convert the exponentially increasing graph into a
Once a unit is acquired, actual operating characteristics straight line representation. Production of such a graph not
should be checked against quoted values and recorded for only allows for accurate determination of exposure tech-
future reference. This baseline information is useful not only niques but also allows the unit performance to be checked
for technique determination, but also for troubleshooting. at any later date by comparison with established chart val-
ues.
Equipment Literature
Another exposure chart commonly used shows kilovolt-
Information available from the manufacturer might age versus thickness, ,exposure being constant. This chart
include weight and size, energy, tube current, inherent fil- can be made at various film densities.
tration, normal gas pressure and pressure rise (for gas filled
heads), coolant requirements, line current, focal spot size, Focal Spot Size
operating temperature, etc. Service manuals also supply
useful information and should be acquired for all units. This As previously discussed, focal spot size and beam inten-
information should not simply be kept on file; it should be sity are important factors affecting radiographic quality. In
used by personnel operating the unit. A separate specifica- addition, many codes require calculation of geometric
tion for the X-ray tube is often available from the manufac- unsharpness for technique approval.
turer· or from the tube supplier.
For conventional X-ray units up to 300 keV, measure-
Exposure Charts ment is done using a standard pinhole aperture. Using a fine
grained film without screens, an exposure is made of the
For any unit that will be used in a variety of applications, aperture. The film is processed and the image is measured,
a set of exposure charts at different output energies is very using a 5x to lOx calibrated reticule. To facilitate measure-
useful and should be produced for each unit. This should be ment of smaller focal spots, the aperture is usually placed so
done under conditions representative of procedures at the that a magnification of 2x is obtained for focal sizes between
facility using the unit. It is necessary to do this for each unit 1.2 and 2.5 mm (0.05 and 0.10 in.) and a magnification of 3x
(or, at least, for each type of unit) because of differences in for focal sizes between 0.3 and 1.2 mm (0.012 and 0.05 in.).
generator characteristics, tube efficiency and inherent filtra- Smaller focal spots require an alternative measurement
tion. technique in which a standard grid pattern is used in place
of the pinhole camera. For X-rays over 300 keV, other tech-
To produce a chart for a particular energy range and niques are required.
material type, several exposures of a step wedge are made.
All variables including keV, mA, focus-to-film distance, film Additional Data
type, screens, chemistry and processing times are kept con-
stant for each chart; only exposure time is varied. At least Other useful information obtainable through measure-
three, preferably five, exposure times are then selected that ment might include gray (or roentgen) output at various keV
adequately span the range of possible exposure times. The settings, filament current versus tube current and line cur-
exposures are made and processed (fresh developer chemi- rent draw at fixed output.
cals are recommended). Density measurements are then
made for each thickness represented on the radiograph. Selecting a Unit
Usually densities of 1.5 to 3.0 are chosen and corresponding
In addition to size limitations and mobility, the process of
matching the unit to the job is in many cases determined by
equipment availability. However, in those instances where a

RADIATION PRINCIPLES AND SOURCES I 111

choice exists, including equipment purchase, there are times and production rates consistent with the duty cycle
some basic considerations to take into account. and quality.

Application Tube Warmup

The application or range of applications (including the To avoid thermal shock, arcing, backfire due to out-
material and thickness, configuration and accessibility, and gassing of the target or other detrimental effects, it is advis-
production rates) forms the first consideration in selection able (and. generally recommended by manufacturers) to
of a unit. This will dictate the techniques to be used and the follow a warmup procedure when placing a unit into service.
rate at which the exposures are made. The longer the period between uses, the longer the recom-
mended warmup. Starting at approximately 50 percent of
Applicable Specifications the maximum and proceeding in 10 percent increments to
maximum output will generally suffice for overnight or
The government or commercial specification, to which weekend periods. The amount of time at each stage should
the radiograph is being produced, may also influence equip- not be much longer than two minutes and should not exceed
ment selection. As an example, some specifications require values consistent with the rated duty cycle. If fluctuations in
various energy ranges for different materials and thicknesses. output are noticed, such as jumps in tube current (mA), the
Quality and sensitivity requirements are also in this category. keV setting should be reduced until a stable value is
obtained. Warmup should be continued from this point with
Energy Range a decrease in the size of the increments used.

Operating a unit continually at maximum output will Maintenance
shorten the between maintenance. A rule followed by many
experienced radiographers is never to exceed 90 percent of The extent of the maintenance performed by the user is,
maximum keV rating of the tube. This provides a measurable of course, determined by the capabilities of the personnel.
increase in tube, transformer, connector and control life. Some items of maintenance that can and should be per-
formed on a routine basis are listed below:
Likewise, operating a tube at much below half of its max-
imum output voltage, although not taxing on the electrical 1. Unit cleanliness. This includes removal of dirt and oil
components of the unit, is not recommended as the stan- from the tube head, connectors and control. This
dard mode. Again, availability may require such usage, but a should not be considered a cosmetic function by any
unit used in this manner is usually operating at a much means. Minutes spent here may avoid hours of down-
reduced efficiency and may not provide optimum results. time for repair. Few controls are totally sealed against
dirt or moisture and oil can cause insulation to break
As an example, consider a 200 keV unit. If operated at down prematurely.
70 kV, many times the inherent filtration of the tube and
tube head is such that a large fraction of the low energy pho- 2. Visual inspection. Wiping down the unit and power
tons are absorbed before leaving the housing. Consequently, cables also affords an opportunity to inspect the com-
tube current and/or the exposure time must be increased to ponents for damage, such as a broken wire or loose
compensate for the loss. connections. Loose connections and partially broken
wires should not be overlooked, because they can
Duty Cycle introduce transients into the high voltage circuitry.
Burnt or loose pins on connectors are equally impor-
In addition to maximum and minimum output energies, tant. Oil seepage or coolant loss are items that can
duty cycle must also be considered in selection of a unit. also be detected at this time.
Although overheating protection is provided by all major
manufacturers, a competent technician will seldom allow 3. Fuse replacement. Use of the correct size is recom-
the unit to reach cutoff temperature before allowing it to mended. If the unit repeatedly blows fuses, repair is
cool. The possibility of localized overheating of target, enve- indicated. Use of fuses larger than specified may
lope and other components is enhanced any time the rated overload other components.
duty cycles are exceeded. In conjunction with production
requirements, the above items form the basic considera- For advanced maintenance and troubleshooting, refer to
tions in selection of a unit. the manufacturer's service instructions.

A unit should generally be used in the range of 50 to
90 percent of its maximum output voltage with exposure

11 2 I NONDESTRUCTIVETESTINGOVERVIEW

Electrical Safety use of equipment should be, and often are required to be,
supplied to all operators. Operators should be aware of the
X-ray equipment manufactured today must conform to physiological hazards of penetrating radiation. In addition,
many national and international design requirements. In careful attention must be given to personnel monitoring,
order for safety features of the equipment to remain opera- facility design and radiation survey techniques.
tive and effective, proper maintenance and operation is
required. All personnel involved with the operation of X-ray Many states have regulations that require the registra-
equipment should be familiar with the manufacturer's oper- tion of X-ray equipment by serial number. The owners of
ating instructions and the specific safety features of the X-ray equipment should always check with the radiation
machine. General electrical safety considerations are listed division of their state health department to determine
below. whether they're required to register their units and, if so,
the course of action to follow.
Power Sources
Personnel Monitoring
Many X-ray units are adaptable to different line voltages.
Use the appropriate power cable and connector. The volt- State regulations usually specify the extent of this moni-
age selection dial on the control panel must be set for the toring. Film badges or thermoluminescent dosimeters
power in use. Required wiring changes should be per- (TLDs) are required for recording accumulated doses of
formed in accordance with the manufacturer's instructions. radiation exposure. Pocket dosimeters or chambers and/or
other radiation detection equipment should be used and are
Grounding often required. These devices allow for the timely detection
of radiation exposure. Dosimeter construction should be
All X-ray machine power cables must be grounded. Do appropriate to the energy level of the X-rays being used.
not use an ungrounded power circuit. Replace connectors Dosimeter housings made of aluminum· or composite mate-
that have defective or missing ground plugs. Failure to do so rials record radiation exposure to low energy X-rays (100 keV
is a shock hazard and may be a fire hazard as well. or less) more reliably than steel cased dosimeters. Record
keeping of the results of personnel monitoring systems must
Fuses also comply with state regulations.

Replacement fuses should be rated at the amperage Facility Design
required by manufacturer's specifications. Damage to com-
ponents or an electrical hazard to the operator may result Facility design comprises two categories, fixed and
from the use of inappropriate fuses. portable. The design of a fixed facility should be reviewed
by a qualified expert .. Consideration must be given to the
Control Circuits occupancy of adjacent areas, the adequacy of shielding,
warning signals and signs, interlocks, tube head position
Do not bypass or override the overload and overheating restrictions and radiation monitoring procedures. Confor-
circuit breakers or the tube head limit switches. Electrical mance to state regulations is essential and subject to inspec-
overload and excessive heat can damage electrical compo- tion or audit by regulatory agencies.
nents. Tube head limit switches are required for safe and
legal operation of the equipment. Fixed shielding is impractical for portable operations.
The protection of personnel and the public depends almost
X-Ray Safety exclusively on strict adherence to approved, safe operating
procedures. Areas with radiation levels at 5 Cy (500 rad) or
The radiation hazards associated with the use of X-ray more in 1 h at 1 m (40 in.) from a source of radiation or any
equipment can be minimized by adherence to state regula- surface through which radiation penetrates at this level must
tions and the manufacturer's operating and maintenance be controlled and posted with signs bearing the radiation
instructions. Written instructions for the safe and reliable symbol and the words grave danger: very high radiation
area. Areas of radiation in excess of 1 mSv-h-1 (100 mli.h ")
must have the perimeter posted with signs displaying the
radiation symbol and the words danger: high radiation area.
The perimeter of areas with radiation levels in excess of
50 µSv-h-1 (5 mR-h-1) must be posted with a sign displaying
the radiation symbol and the words caution: radiation area.
Areas within a perimeter with radiation levels in excess of

RADIATION PRINCIPLES AND SOURCES I 11 3

20 µSv-h-1 (2 mR-h-1) or 50 mSv·yr-1 (5 R-yr-1) are restricted, low energy X-rays. Likewise, the size of the Geiger-Muller
with access limited to radiographic personnel. Access to tube or chamber must also be considered. A large detector
these areas must be controlled by the radiographer. will give erroneous values when applied to small radiation
leaks.
Surveys
Federal regulations require quarterly calibration of the
Radiation surveys are an integral part of the safe opera- survey instrument whereas regulations of some agreement
tion of X-ray machines. These surveys are conducted to states, such as Texas, require six-month calibration. Use of a
determine the extent of radiation hazard in any given area. calibration source similar in energy to the measured X-rays
The survey meter is a rate instrument which indicates the is preferable. The conscientious use of a calibrated survey
exposure that will be received per unit time. Most com- meter is the most reliable way to ensure the safe use of
monly used instruments are the Geiger-Muller (GM) and X-ray equipment. Failure to use the survey meter is a factor
ionization chamber meters. Energy response of the meter in most occupational overexposures. This function may be
· should be consistent with energy of the X-radiation. A meter performed either by those performing the X-ray work at the
suitable for field isotope use may be relatively insensitive to facility or by an outside service at specified intervals (annu-
ally or semiannually).

114 I NONDESTRUCTIVETESTING OVERVIEW

PART 5

ISOTOPES FOR RADIOGRAPHY

Radioactivity I (~J (Eq.12)

Historical Background JN (Eq.13)
= (~
A year after the discovery of X-rays in 1895 by Rontgen
in Germany, the French physicist Becquerel discovered that A more convenient expression in terms of time t elapsed
uranium emitted penetrating radiation. Under his direction, would be:
the Curies isolated and identified several natural elements
such as radium and polonium, which were less stable than I e--0.6931/T (Eq.14)
uranium. The unstable elements were called radioactive. Io
or the logarithmic form:
Rutherford in England and Villiard in France identified
the alpha, beta, and gamma radiations emitted by radioac- ln(t) = 0.693~ (Eq.15)
tive elements. By 1934, artificial radioactive elements had T
been prepared by charged particle (Cockcroft and Walton)
or neutron (Fermi) bombardment of stable elements and
their properties, such as half­life, were well known.

Half-Life Thus, if intensity of radiation from a quantity of radioac-
tivity is plotted as a function of time on semilogarithmic
Half-life Tis the time required for half the original num- coordinates, a straight line results, as shown in Fig. 24.
ber of atoms to decay or change to the daughter atoms.2
Half-life describes the probability of decay for large num- For the convenience of users, suppliers of radiographic
bers of atoms and is much more commonly used than the radioactive isotopes have converted these half-life or decay
actual probability A (decay constant) of an atom disintegrat- charts, as shown in Fig. 24, into digital printouts of activity
ing per unit time. The number of atoms disintegrating per values for specific dates.
unit time can be expressed as A times the total number N of
parent atoms: Selection of Radiographic Sources

disintegrations (Eq. 10) Of the several hundred known radioactive isotopes, a
time handful have become widely used for radiography. The
remainder are unsuitable for a variety of reasons, including
The curie (Ci) is the unit formerly used to describe decay short half-life, unacceptable radiation or energy of radiation,
rate as 3.7 x 1010 disintegrations per second (dps). The SI low available intensity and/or high cost.
system uses the becquerel (Bq), which is one disintegration
per second. The half-life Tis related to the decay constant: The following discusses the production and radiation
characteristics of the four most popular radiographic
T= 0.693 (Eq. 11) sources (see Table 3 and Figs. 25 and 26).
')...
Radium-226 is no longer used for radiography because
where 0.693 is the natural logarithm of 2. The number of of the hazards presented by its alpha decay, its gaseous
radioactive atoms or the number of atoms decaying per unit radioactive daughter (radon) and the fact that it is a bone
time changes exponentially with time. seeking element.

The rate of decay for a number of radioactive atoms (or Cobalt-60
intensity I of radiation from them) and the number of atoms
at any time can be expressed (in terms of time) as the num- Cobalt is a magnetic metal, · having a melting point of
ber n of elapsed half-lives: 1,495 °C (2,723 °F) and a density of 8.9 g·cm-3. It is some-
what similar in physical properties to iron. It occurs in
nature as a single isotope, cobalt-59, which changes into

RADIATION PRINCIPLES AND SOURCES I 11 5

TABLE3. Characteristicsof four widely used radiographicisotopesources
Element

Characteristic Cobalt Cesium Iridium Thulium

Isotope 60 137 192 0.60 170
Half life 5.27 years 30. 1 years 74.3 days 129 days
Co metal CsCI ceramic Ir metal Tm203
Chemical form 8.9 1.9 22.4 9.3
Density (g·cm-3) 1.33; 1.17 0.66 0.31; 0.47; 0.084; 0.052

Gamma rays (megaelectronvolts) 1.0; 1.0 0.92 1.47; 0.67; 0.27 0.03; 0.05
0.31 0.5 0.6 1.0
Abundance of gamma rays 351 (1.30) 92 (0.34) 130 (0.48) 0.81 (0.0030)
1,850 (50) 925 (25) 13,000 (350) 37,000 ( 1,000)
(gamma rays per disintegration) 17,000 (450) 3,300 (90) 300,000 (8,000) 150,000 (4,000)
6,000 (600) 330 (33) 44,000 (4,400) 100 (10)
Beta rays (megaelectronvolts)
µSv·h-1 at 1 m-Gsq-1 (R·h-1 at 1 m-Ci '] 1,850 (50) 2,800 (75) 7,400 (200) 1,850 (50)
650 (65) 300 (30) 960 (96) 1 (0.1)
Practical specific activity in GBq·g-1 (Ci·g-1)

Practical GBq-cm-3 (Ci-cm-3)

Practical mGy-h-1 at 1 mcrrr? (R·h-1 at 1 rncrrr ']

Common radiographic source activities

gigabecquerels (curies)
mSv·h-1 at 1 m (R·h-1 at 1 m)

FIGURE 24. Half-lifeplot radioactive cobalt-60 after capturing a neutron (59Co + n =

JOO 0 100 60Co + capture gamma rays). This isotope decays with a half-
90 life of 5.27 years by emission of a beta particle and two high
80 I so energy gamma rays (Fig. 25) resulting in the 1.2 MeV aver-
70
60 2 25 age gamma ray energy.
3 12.5 Calculation of the specific activity to be expected in 1.6 x
so 4 6.25
5 3.12 1.6 mm (0.06 x 0.06 in.) pellets of cobalt after irradiation at
40 a flux of 1014 neutrons per square centimeter per second
··t··· (neutrons cm-2-s-1) for one 16-day cycle results in a value of
30 2.2 GBq (60 mCi) per pellet, or about 70 GBq (2 Ci) per
- ---- - -..---. gram specific activity.By leaving the cobalt in the reactor for
s~ 20 many cycles of irradiation, such as a year ( 17 cycles), at
t5 1014 neutrons cm-2-s-1 will result in the above pellets having

<(

LL JO FIGURE 25. Disintegrationschemes of
9 cobalt-60,thulium-170and cesium-137
0 (diagonalarrowsrepresent beta rays; vertical
arrowsrepresent gamma rays; energies given
zf- 8 7 in megaelectronvoltsJ
UuJ 6
D:'.'.
UJ
0....

5.3 y 130 d 33 Y
6oc0 17orm
mcs

0 J .17
ELAPSED TIME IN HALF LIVES J.33

116 I NONDESTRUCTIVE TESTING OVERVIEW

TABLE 4. lridium-192radiations Because of its penetrating radiation, cobalt-60 is a diffi-
cult material to shield; the average half value layer of lead is
Voltage Gamma Rays Percent Percent 12.7 mm (0.5 in.). Figure 27 shows the shielding necessary
Roentgens to meet the 2,000 µSv-h-1 (200 mR-h-1) at the surface of a
(kilovolts) per Disintegration Gamma package, required for shipment on common carriers. The
various curves refer to the exterior box size, within which is
310 l .47 61 47 located the spherical lead shield of stated diameter.

470 0.67 28 35 Additional requirements for packaging and shipment of
radioactive isotopes are detailed in the United States Code
600 0.27 11 JS of Federal Regulations, Title 49 (Part 173), Title 10
(Part 71), and the International Atomic Energy Agency Reg­
about 1 Ci of activity each. Such small pellets can be used ulations for Safe Transport of Radioactive Materials, Safety
either singly or in groups to supply the desired total activity. Series No. 6 (1979) and Safety Series No. 37 (1982).
The pellets must be encapsulated in stainless steel to facili-
tate handling and to prevent small, radioactive cobalt oxida- lridium-192
tion particles from entering the environment.
The 74.3 day half-life isotope iridium-192 is produced by
Radiographers employ cobalt-60 chiefly for inspection of neutron irradiation of the element, a white metal of the plat-
iron, brass, copper, and other medium weight metals with inum family that melts at 2,410 °C (4,370 °F) and has a den-
thicknesses greater than 25 mm (1 in.). Cobalt-60 is radio- sity of 22.4 g-cm-3. Natural iridium occurs as two isotopes,
graphically equivalent to a 3 MeV X-ray generator, though it 38 percent iridium-Ijrl and 62 percent iridium-193. The
is not so intense a source. It can be used to make good radio- lighter isotope yields the desired radioactivity.
graphs through at most 200 mm (8 in.) of steel.

FIGURE26. Disintegrationschemeof iridium-192;energylevels in kilovolts;numbersin arrowsare
numbersof gamma rays per 1 00 disintegrations

1,064 1,456

690 1,359
484 1,201
383 I, 155

0 920.9
784.5
612.5

316.5

RADIATION PRINCIPLES AND SOURCES I 11 7

Decay of iridium-192 proceeds chiefly by beta ray emis- reactor for varying numbers of three week cycles. These
curves reach maxima both because the isotope decays dur-
sion to platinum-192 but also by electron capture to ing irradiation and because the target material is being grad-
ually used up. The curies shown are effective curies (the
osmium-192, both of which are stable. At least 24 gamma values obtained if the gamma ray output of the wafer in
rays are known; the currently accepted decay scheme3 is R-h-1 at 1 mis divided by 0.550). A maximum specific activ-
shown in Fig. 26. For radiographic purposes, iridium-192 ity of about 18.5 TBq (500 Ci) per gram of iridium is the
radiations may be approximated by the three gamma rays most that can be generated in 1.6 x 1.6 mm (0.06 x 0.06 in)
shown in Table 4. wafers (or pellets) that eliminate some neutron self-absorp-
tion loss of larger wafers.
Production of iridium-192 is shown in Fig. 28, which
presents the results of computations for 3.2 x 3.2 mm Iridium-192 is used widely for the radiography of steel in
(0.13 x 0.13 in.) metal wafers (or pellets) inserted into the sections 3.2 to 76 mm (0.13 to 3.0 in.) thick, where it pro-
duces results similar to those from a 1 MeV X-ray generator.
FIGURE27. Shieldingrequirementsfor However, the percent sensitivity required by some specifi-
shipmentof cobalt-60in interstatecommerce cations is very difficult to achieve in steel sections less than
(lead shieldingmustbe encasedin steel to 19 mm (0.75 in.) thick. Its relatively low energy permits the
preventshieldinglossin a fire); curvesrefer to use of uranium shields weighing under 18 kg (40 lb) for
lead spheresin various size boxes;becausethe source strengths of 2 to 5 TBq (50 to 125 Ci), making the
curvesassumenegligiblesourcesize, source isotope ideal for field work where portability and small size
diametermustbe added to pig diameterto
obtain requiredoutsidediameter

100 //// / 1,000 FIGURE28. Productionof iridium-192in
7 100 3.2 x 3.2 mm (0.12 x 0.12 in.) metal wafers
4 // 10
for variousnumbers of three-week irradiation
atJ 10 //­ 01 cycles;curies (gigabecquerels)are as measured
/f by gamma ray output; one fourth the activity
0,: 7 will result from irradiationof 1.6 x 1.6 mm
4 ii' ,, (0.06 x 0.06 in.) wafers

1.0 x/ / ~ // ~ :'.'.] 100 ~ -:_".;.."~'·
7 8
/ 'ff ---= l..LJ ~v/7/". vlV..Ll J A ~ ./'/
00 / V/;' C!::::
7 y // V/ -= /
/ //I 8l:..JLJ ~/ :'.'.]
V) >I// ///~ __; 0cl..LoJ /
l..LJ
a~ 2 -h1 .1/ ///// i3 / C!::::

C!:::: '/ r-vvu:J ~0,: ~ -1 ,000 :l..JLJ
10 0u
/ lc..LoJ
::J 7 r / / II 0:§lc..iL5J 10
8 /
_J
B5 6
~
W/Vcol
:// V /1 /~ i3
/ // I
V/ II' I // 2' :§lc_..JLoJ
34 5 6
/, vV/ v1' / C!::::
I vv/0 V/ /
- l..LJ
0 50 I 00 150 200 250 300 350
(0) (2) (4) (6) (8) (10) (12) (14) 100 Vcol
0
LEAD PIG DIAMETER
millimeters 2 34 68 3 4 68
(inches) 1013 1014 101s

LEGEND UNPERTURBED FLUX
l. 45 cm ( I 8 in.) OR LARGER BOX EDGE
2. 30 cm (12 in.) BOX LEGEND
3. 22 cm (9 in.) BOX I. CYCLE l
2. CYCLE 2
4. 16 cm (6.4 in.) BOX 3 CYCLE 3
4. CYCLE 4
5. 13 cm (5 in.) BOX
6. BARE PIG

118 I NONDESTRUCTIVETESTING OVERVIEW

are at a premium. Figure 29 gives data on container sizes the nucleus is left in an excited state which is stabilized
and lead sphere diameters necessary to shield iridium-192 either by emission of an 84 keV gamma ray or by ejection of
in interstate commerce. an orbital electron (internal conversion). It has been
shown4•5 that 3.1 percent of the disintegrations result in
Thulium-170 84 keV gamma ray emission and 5 percent in 52 keV X-rays
characteristic of ytterbium. In addition to these two soft
The element thulium is one of the rare earth metals hav- quanta, a proportion of continuous X-radiation is also gener-
ing a density of about 9 g·cm-3. It exists in nature as the sin- ated by deceleration of the 1 MeV beta rays in the body of
gle isotope thulium-169. Because the metal is extremely the source. The spectra produced by a concentrated and a
difficult to produce, the material is generally handled as the diluted source of Tm203 are shown in Fig. 30.
oxide Tm203, either as an encapsulated powder of density
approximately 4 g·cm-3 or sintered into pellets of density The strengths of radiographic sources are reported to vary
7 g·cm--3. Single neutron capture produces thulium-170. from 0.5 to 3 µGy-h-1 at 1 m per GBq (2.5 to 13 mR-h-1 at 1 m
per Ci) for sources ranging from 50 to 200 mg of Tm203, and
Thulium-170 decays with a 129 day half-life by emission 5 µGy-h-1 at 1 m per GBq (20 mR-h-1 at 1 m per Ci) from a
of 1 MeV beta particles. In 24 percent of the disintegrations, 400 mg metallic source.6

FIGURE 29. Shieldingrequirementsfor Because thulium-170 sources can be produced only in
shipmentof iridium-192in interstate limited intensity (1.5 mGy-h-1 at 1 m [150 mR-h-1 at 1 m] of
commerce; curves refer to lead spheres in soft radiation is about the maximum), the isotope has not yet
varioussize boxes and assume negligible found widespread application in industrial radiography. Its
source size main virtues are small size and portability; a 25 mm (1 in.)
thick lead shield is , sufficient to reduce radiations from a
v /I1,000 1.85 TBq (50 Ci) low density source to tolerance levels. It can.
7 be used for radiography of thicknesses as low as 0.8 mm
/ / /. (0.03 in.) of steel or 13 mm (0.5 in.) of aluminum with 2 per-
cent radiographic sensitivity. However, some codes prohibit
/ radiographing aluminum with a radiographic isotope.
VIV/
I /IV Cesium-137
100 / VIJ / I
7 I Cesium-137 (30.1 year half-life) has been used as a
Y/ radiographic source. Originally used as cesium chloride, less
I
II FIGURE 30. Radiationemergentfrom 50 mg
I/ '/ II1.0 thulium-170 source (compressed2 x 2 mm
[0.08 x 0.08 in.J pellet} in 2,000 mm3 solution
// //7
I / I/ // THUUUM-170

I

I---

'l;' J"/ J
I l/11 /JI1.0
7
Jv

I I/ / I

v /. vLV!.l.J

o2
4I

2 / 4/
1// VIu::J

:J
100 I I I
__J ,s

VI I I/~ 4

JO I 00 150 (7) 200
50 (3) (4) (5) (6) (8)

(I) (2) LEAD PIG DIAMETER
millimeters
LEGEND (inches)
I. 450 mm (18 in.) BOX
2. 300 mm (12 in.) BOX 40 I 00 150 200 250 300
3. 200 mm (8 in.) BOX
4. 150 mm (5 in.) BOX KILOVOLTS
5. BARE SHIELD

RADIATION PRINCIPLES AND SOURCES I 11 9

corrosive materials (for the stainless steel capsule) presently nuclear reactors. The difficulty lies in separating the element
from the uranium fuel and other fission fragments. Once this
used include ceramics and glass. These materials also is done, sources of almost any strength can be produced. Its
specific activity is limited by the presence of other isotopes
reduce loss of cesium from a capsule with a defective seal. of cesium, principally cesium-133 and cesium-135, resulting
in a maximum of 1,000 GBq (25 Ci) per gram of CsCl.
Ninety-two percent of the nuclear disintegrations are by Sources can be made to 3,500 GBq (90 Ci) per cubic mil-
limeter of CsCl. Because cesium compounds are corrosive,
beta particle emission (forming barium-137, with a half-life sources are usually doubly encapsulated with inner and outer
containers made of heliarc welded stainless steel.6•7 Cesium-
of 2.6 minutes), which emits a 0.66 MeV gamma photon). 137 is frequently used to calibrate survey meters.
Radiation intensity from cesium-137 is 80 µGy-h-1 at 1 m
per GBq (0.34 R-h-1 at 1 m per Ci).

Cesium-137 is one of the most probable products of
nuclear fission, resulting from about 6 percent of the fissions.
This fission fragment is generated in great abundance in all

120 I NONDESTRUCTIVE TESTING OVERVIEW

PART 6

SOURCE HANDLING EQUIPMENT

Requirements Remote Handling Equipment

Radiographic sources must be handled so that the radio- Commercial field radiography currently requires the use
graphic exposure may be made without appreciable expo- of sources as much as 100 times larger than can be manually
sure to the radiographer. As the required distance between handled. Radiography in laboratory or shop facilities may
radiographer and exposure device increases, the cost of the use sources up to 37 TBq (1,000 Ci). Operator exposures
equipment increases rapidly. Only low intensity sources are from such sources would be intolerable if manual handling
handled with the simplest devices. High intensity sources were attempted. Remote handling devices permit the radio-
(such as multicurie iridium-192 and cobalt-60 sources) are grapher to be two to ten times farther from the exposed
best handled with equipment that permits the radiographer source. With modern equipment (Figs. 31 to 39), these
to remain several meters from the exposure device when the larger sources may be handled without exceeding exposure
source is manipulated. limits (which are one third of earlier limits) while making
many radiographic exposures per day.
Classification
Type 1 (ANSI Type 2) Devices
Various schemes have been devised to provide remote
handling of radiographic sources. Most of the remote expo- These devices move the source capsule from the shielded
sure devices presently in use fit into one of the following storage position to an unshielded position (Figs. 31 and 32).
categories: (1) those that move the source radially from the Although less commonly used than Type 2 (ANSI Type 1)
center to the surface of the shielding container or axially to devices, Type 1 (ANSI Type 2) devices are preferred for
an opening; and (2) those that move the source out of the radiography of pipelines and for spot radiography of welds
shielding container to an exposure site some distance away on plate structures and as of 1996 have not been required to
from the shield. meet regulations for Type 2 (ANSI Type 1) crankout devices.

The first type (Type 1 or ANSI Type 2) generally yields a FIGURE31. Diagramof Type 1 (ANSIType 2)
beam of radiation somewhat restricted in size and direction, sourcehandling device: (a) stored; (b) exposed
whereas the second type (Type 2 or ANSI Type 1) approxi-
mates a free source of radiation unless additional collima- (a)
tion is provided at the point of exposure.

Mechanical, electrical, hydraulic or pneumatic source
movement methods have been used. Experience shows the
direct mechanical linkage to the source to be inherently
more fail-safe.

Manual Manipulationof Sources (bJ ~ ..

The simplest equipment for making a radiographic expo-
sure employs a source capsule tied to a rod by a string. The
rod or string is attached while the source is in its shield. Large
exposures to operators are likely with such devices so such
use is presently prohibited for routine radiographic work.

RADIATION PRINCIPLES AND SOURCES I 1 21

FIGURE32. Type 1 (ANSIType 2) source handling device for up to 3.7 TBq (100 Ci) of iridium-192:
(aJ crosssection; (bJ photograph

fa) HANDLE

DEPLETED

URANIUM

STAINLESS SHIELD

STEEL

HOUSING STOP

PIN

OPTIONAL ON/OFF KNOB
EXTENSION ALUMINUM BOTTOM PLATE

HANDLE

D
ON/OFF KNOB
ROTATE 180 DEGREES
TO EXPOSE SOURCE

fbJ

FROM SOURCE PRODUCTION & EQUIPMENT COMPANY. PRINTED WITH PERMISSION.

122 I NONDESTRUCTIVETESTING OVERVIEW

Type 2 (ANSI Type l) Devices A guide tube conducts the source capsule and drive
cable from the shield to the exposure site. The guide tube
These devices permit the source capsule to be moved
from the storage position inside the shield to an exposure may be made of flexible braided metal tubing or of heavy
location that may be many meters (yards) from the shield.
Figure 33 is a diagram of such a system. The source is man- walled plastic tubing. The guide tube ends with a metal stop
ufactured so that the capsule is attached to one end of a
short piece of flexible metal cable, often called a pig tail. A to position the source exactly where it needs to be to make
special connector is attached to the end of the pig tail. The the radiograph. A collimator (Fig. 34) may be added to the
connector end of the pig tail extends outside of the shield
when the source capsule is centered in the shield. A drive stop at the end of the guide tube to provide a beam or circu-
cable is attached to the source connector by its mate on the
end of a flexible metal drive cable. A locking mechanism is lar band of radiation appropriate for the exposure. Such a
placed at the junction of the shield and the drive cable collimator may reduce radiation intensity ( outside of that
assembly to prevent unauthorized or accidental removal of
the source capsule from its shielded position. A positive needed for the radiograph) to less than one percent of the
connection between the source and drive cable is now
required before the source lock can be released. The source radiation without the collimator. Figures 33 to 37 show
capsule moves through the shield (depleted uranium) in a
smooth tube shaped to resemble a flattened letter S. The examples of some crankout devices.
tube is zirconium or similar metal to prevent contact Regulatory requirements for crankout devices, begin-
between the source and the uranium shield.
ning in 1996, include a reduced dose rate at the surface of
the device and an automatic mechanism for securil).g the

source when it is returned to the shielded position/ Some

action by the radiographer at the exposure device is

required to release tl1e source for another exposur~. Older

devices have been modified or replaced to meet these
requirements.

Figure 38a illustrates a source change-out shipping con-

tainer that is set up for source removal in Fig. 3&~. Use of
such equipment can greatly reduce personnel exposures

during exchange of an old source for a new source ( about
every six months for iridium-192). ·
FIGURE 33. Operationof type 2 {ANSI type 1)
source handling device: (a) stored position; Safety Considerations
(b) source in transit; [c] exposure positoi n
Radiographic exposure devices today are quite reliable
faJ LOCK and are shielded better than earlier devices. This is impor-
tant for the safety not only of the radiographer but of the
CRANKOUT SOURCE general public as well. Unreliable items such as pneumatic
- EXPOSURE DEVICE source drives, red-yellow-green lights to indicate position of
the source ( actually the position of the drive cable) and
JDRIVE CABLE meters that count turns of the hand crank have been prop-
erly abandoned.
(bJ
Some of the designs to meet the 1996 regulations should
COLLJMA.TOR ~ improve safety. If modifications to crankout devices are sub-
stituted for proper use of a working survey meter by a trained
j~\ radiographer, then regulations will be violated and the safety
for which the changes were made will not be achieved.

Proper attitude and understanding of the radiographic
process by the operator are essential to the safe perfor-
mance of radiography. These aspects are covered in
required training and certification of radiographers. Safety
of operation of an exposure device can only be ensured by
use of an appropriate, working survey meter. Failure to do
so is irresponsible and negligent on the part of the radiogra-
pher. Survey meters are discussed below.

Although training and certification of radiographers for
proper radiographic technique often cover safety, safety cer-
tification of radiographers is usually handled separately.

RADIATION PRINCIPLES AND SOURCES I 1 23

Radiation safety courses are required for radiographers to more than 500 µSv·h-1 (50 mRh-1) is permitted at 150 mm
be certified for work. (6 in.) from any exterior surface of a smaller shield.

Shielding Equipment When the radiographer is to remain in close proximity to
these shields for long periods of time, lower radiation levels
Because the radiographic source carinot be turned off (better shielding) are necessary. It may be necessary to use a
like an X-ray machine, shielding must be provided when- shield designed for a larger source. Again, collimators (Figs. 33
ever the source is not intentionally being exposed. This and 34) should be used when ever possible to reduce radiation:
shielding must conform to regulations written for specific exposures to personnel. Orily use of a survey meter will pro-
uses. In the United States, regulations state that (1) no more vide infor~ation about the adequacy of the shielding.
than 2,000 µSv-h-1 (200 mR-h-1) is permitted at any exterior
surface of the shield; (2) no more than 100 µSv-h-1 Survey Meter Equipment
(10 mklr") at one meter is permitted from any exterior sur-
face, for shields in which the source storage position is Use of a working, radiographic survey meter by a trained
100 mm (4 in.) or more from any exterior surface; or (3) no operator could reduce or prevent most radiographic overex-
posures. Only such an instrument can tell an operator what

FIGURE 34. Guide tube collimators for reducing personnel exposure: fa) crosssection; (b) photograph
fa) SETSCREW

II (Q)I I
II
I- - - - - - - - J. _I - - - _I,/
II I\
II I\

\I

I\ ',
I\
I\
I\

SOURCE COLLIMATION SOURCE PLACEMENT

(bJ

FROM AMERSHAM CORPORATION. REPRINTED WITH PERMISSION.

1 24 I NONDESTRUCTIVETESTING OVERVIEW

FIGURE35. Exposuredevice for up to 7.4 TBq {200 Ci) of iridium-192:fa} crosssection;
(bj photograph

faJ
HANDLE ASSEMBLY

ACRYLIC POTTING COMPOUND

FOAM FILL

I - 35 TO 45 kg·m-3 (- 2 TO 3 lb·fr3)
PROTECTIVE FLANGE

OUTLET END FLANGE
ATTACHMENT BOSS

LOCK CAf' ASSEMBLY OUTLET PANEL
ASSEMBLY

SAFETY PLUG ASSEMBLY

POSITIONING SHIM

fbJ

FROM SOURCE PRODUCTION AND EQUIPMENT COMPANY. REPRINTED WITH PERMJSSION.

RADIATION PRINCIPLES AND SOURCES I 125

FIGURE36. Exposuredevicefor up to 3.7 TBq (100 Ci) of iridium-192: (aJ photograph;[b] diagram
(aJ

(bJ REMOVABLE
STORAGE COVER
REMOVABLE
SHIPPING PLUG CONNECTOR

SHIPPING PLUG ASSEMBLY
FROM AMERSHAM CORPORATION. REPRINTED WITH PERMISSION.

126 I NONDESTRUCTIVE TESTING OVERVIEW
FIGURE37. Exposuredevice with crankout and guide tube

FROM SOURCE PRODUCTION AND EQUIPMENT COMPANY. REPRINTED WITH PERMISSION.

FIGURE38. Exposuredevice with source exchanger: {a) exchanger"fleftJ closed, exposure device
fitted with short exchange tube; (bJ exchanger open and attached; ready for source transter.to the
exchanger (note source pigtail in left hand storage position) ~-

(aJ

(bJ

FROM SOURCE PRODUCTION AND EQUIPMENT COMPANY. REPRINTED WITH PERMISSION.

RADIATION PRINCIPLES AND SOURCES I 1 27

FIGURE 39. Portable exposure device for up to United States regulations require the use of an instru-
9.25 TBq (250 Ci) of <:obalt-60 ment that can operate in fields as low as 10 µSv· h-1
(1 mRh-1) and as high as 10 mSv·h-1 (1,000 mli-h "). Also,
the instrument must have been calibrated within 90 days (or
six months in some agreement states) of its use. Survey
meters designed specifically for the application must be
used for X-ray, neutron and isotope radiography.

DosimetJ Equipment

In addition to the survey meter, which provides immedi-
ate information on radiation exposure, personnel dosimetry
equipment must be used. Generally this will include at least
a pocket dosimeter, a film or thermoluminescent dosimeter
(TLD) and an alarming rate meter. These devices provide
an exposure warning for the operator.

FROM AMERSHAM CORPORATION. REPRINTED WITH PERMISSION. Procedures

the exposure rate is from the time the shielded source is Procedures related to safety include (1) checking the
obtained, through the working day, and until the source is radiographic exposure device with the survey meter at the
replaced into storage. Because a human cannot detect radi- beginning and end of the day; (2) using the survey meter
ation without such a device, performing radiography with- during exposure of the source and whenever approaching
out a survey meter, or ignoring the instrument when it is the exposure device after an exposure; (3) placement of bar-
available, is inexcusable. ricades to prevent public entry into exposure areas;
(4) checking radiation exposure rates at the barricade limits;
and (5) periodic leak tests of the source capsule. Should the
source capsule lose some of its radioactive material, serious
overexposures to the radiographer and/or the public could
occur. Leakage of radioactive material from a source capsule
must be determined every six months following manufac-
ture. Finally, the radiographer must be provided with emer-
gency procedures and access to experienced help in the
event of an accident.

128 I NONDESTRUCTIVETESTING OVERVIEW

REFERENCES

1. Nondestructive Testing Handbook, second edition. 5. Halmshaw, R. "Thulium-170 for Industrial Radiog-
Vol. 3, Radiography and Radiation Testing. Colum- raphy." British Journal of Applied Physics. Vol. 6
bus, OH: American Society for Nondestructive (1955): p 8.
Testing (1985).
6. Harrington, E.L., H.E Johns, A.P. Wiles and C.
2. Evans, R.D. The Atomic Nucleus. New York, NY: Garrett. "The Fundamental Action of Intensifying
McGraw-Hill Book Company (1955). Screens in Gamma Radiography." Canadian Jour­
nal of Research. Vol. 28. Ottawa, Canada: National
3. Johns, M.W. and S.V Noble. "Disintegration of Irid- Research Council (1948): p 540.
ium-192 and Iridium-194." Physics Review. Vol. 96.
7. "Protection against Radiations from Radium,
Woodbury, NY: American Institute of Physics Cobalt-60 and Cesium-137." National Bureau of
(1954): p 1,599. Standards (US) Handbook. No. 54. Gaithersburg,
4. West, R. "Low-Energy Gamma Ray Sources." MD: National Institute of Standards and Technol-
Nucleonics. Vol. 11, No. 2. New York, NY: McGraw- ogy (1954).
Hill Book Company (1953): p 20.

RADIATION PRINCIPLES AND SOURCES I 1 29

BIBLIOGRAPHY

1. Nondestructive Testing Handbook, second edition: 7. Knoll, G.F. Radiation Detection and Measurement,
Vol. 3, Radiography and Radiation Testing. Colum- second edition. New York, NY: John Wiley & Sons
bus, OH: American Society for Nondestructive (1989).
Testing (1985).
8. McGuire, S.A. and C.A. Peabody. Working Safely in
2. Becker, G., ed. Radiographic NDT Wilmington, Gamma Radiography. NUREG/BR-0024. Wash-
DE: E.I. du Pont de Nemours & Company (1990). ington, DC: United States Nuclear Regulatory
Commission (reprinted 1986).
3. Davis, J. Mark Mathematical Formulas and Refer­
ences for Nondestructive Testing, second edition. 9. Munro, J.J., III and F.E. Roy, Jr. Gamma Radiogra­
Itasca, IL: Art Room Corporation (1994). phy Radiation Safety Handbook. Burlington, MA:
Amersharn Corporation (n.d.).
4. Halmshaw, R., ed. Industrial Radiography, revised
edition. Mortsel, Belgium: Agfa-Gavaert N.V 10. Quinn, R.A. and C.C. Sigl, eds. Radiography in
Modem Industry, fourth edition. Rochester, NY:
5. Halmshaw, R. Industrial Radiology: Theory and Eastman Kodak Company (1980).
Practice, second edition. London, United Kingdom:
Chapman & Hall (1995). 11. Schneeman, Justin G. Industrial X­Ray Interpreta­
tion. Columbus, OH: American Society for Nonde-
6. International Institute of Welding. Handbook of structive Testing (1985).
Radiographic Apparatus and Techniques, second
edition. Abington, United Kingdom: The Welding
Institute (1973).



FILM RADIOGRAPHY 5SECTION

FROM RADIOGRAPHY IN MODERN INDUSTRY. 4TH EDITION, e EASTMAN KODAK COMPANY.REPRINTEDWITH PERMISSION.

1 32 I NONDESTRUCTIVE TESTING OVERVIEW

PART 1

FILM EXPOSURE

Making a Radiograph parts to mammoth missile components, in product composi-
tion through virtually every known material, and in manu-
Radiography1•2 is one of the oldest and most widely used factured form over an enormously wide variety of castings,
nondestructive testing methods. Despite its established po- weldments and assemblies. Radiographic examination has
sition, new developments are constantly modifying the been applied to organic and inorganic materials, to solids,
radiographic techniques applied by industrial and scientific liquids, and even to gases. An industry's production of radio-
users, thereby producing technical and economic advan- graphs may vary from the occasional examination of one or
tages, over previous techniques. This progressive trend con- several pieces to the examination of hundreds of specimens
tinues with such special equipment and techniques as per hour. This wide range of applications has resulted in the
microfocus X-ray generators, portable linear accelerators, establishment of independent, professional radiographic
radioscopy, neutron radiography, imaging on paper, digital
image analysis and image enhancement. FIGURE 1 . Diagram of setup for making an
industrial radiograph with X-rays
A radiograph is a photographic record produced by the
passage of X-rays or gamma rays through an object onto a SHEET OF~ FRONT
film (Fig. 1). When film is exposed to X-rays, gamma rays or LEAD SCREEN
light, an invisible change called a latent image is produced
in the film emulsion. The areas so exposed become dark HIGH DENSITY IN /
when the film is immersed in a developing solution, the RADIOGRAPH
degree of darkening depending on the amount of exposure. .._FILM
After development, the film is rinsed, preferably in a special
bath, to stop development. The film is next put into a fixing LOW DENSITY IN
bath, which dissolves the undarkened portions of the emul- RADIOGRAPH
sion's sensitive salt. The film is washed to remove the fixer
and dried so that it may be handled, interpreted and filed.
The developing, fixing and washing of the exposed film may
be done manually or in automated processing equipment.

The diagram in Fig. 1 shows the essential features in the
exposure of a radiograph. The focal spot is a small area in
the X-ray tube from which the radiation emanates. In
gamma radiography, it is the capsule containing the radioac-
tive material that is the source of radiation (for example,
cobalt-60). In either case the radiation proceeds in straight
lines to the object; some of the rays pass through and others
are absorbed - the amount transmitted depending on the
nature of the material and its thickness. For example, if the
object is a steel casting having a void formed by a gas bub-
ble, the void produces a reduction of the total thickness of
steel to be penetrated. Hence, more radiation will pass
through the section containing the void than through the
surrounding metal. A dark spot, corresponding to the pro-
jected position of the void, will appear on the film when it is
developed. Thus, a radiograph is a kind of shadow picture
- the darker regions on the film representing the more
penetrable parts of the object, and the lighter regions, those
more opaque to gamma or X-radiation.

Industrial radiography is tremendously versatile. Radio-
graphed objects range in size from microscopic electronic