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MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS - 1963
CONť-670605--1
MASTER

MAY 8 1967
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HC 5.360; Mix 65
Low-Voltage Radiography and Microradiography of Graphite (1)
By R. W. McClung(2)
LEGAL NOTICE
This raport m. properod u an account of Government sponsored work. Noither the United
Sinton, nor the Commission, nor any person acting on baball of the Commission:
A. Makes any warruaty or representation, expressed or implied, with respert to the accu-
racy, com, letenast, or wrotulotus of the loformation contained in this report, or that the use
of any informativa, appartus, method, or procesi di cloud to the report may not lofringo
privately owned righta; or
B. Aspmar may Habilities with respect to the use of, or for damages rutuldas from the
une of any information, appuntu, method, or procon decloud in this report.
As und in the abova, "pollod acting on behalf of the Commission" Lacludes way on-
ployee or contractor of the Comodinion, or employee of mch contractor, to the extent that
such employ or contractor of the Commission, or employw of such contractor prepares,
dienominator, or provides acco, to, zay information pursuant to his employment or contract
with the Commission, or his employment with such contractor.
(1) Research sponsored by the U. S. Atomic Energy Commission under
contract with the Union Carbide Corporation.
(2) Metals and Ceramics Division, Oak Ridge National Laboratory,
Oak Ridge, Tennessee.
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LOW-VOLTAGE RADIOGRAPHY AND MICRORADIOGRAPHY
OF GRAPHITE
R. W. McClung
ABSTRACT
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Low-voltage radiography was studied to provide optimum techniques
for graphite thicknesses less than 2 in. Significant improvements were
made in image quality and sensitivity by use of an intermediate atmosphere
of hellum, bare film and a thin beryllium-window X ray tube. Use of a
high resolution photographic emulsion allows contact microradiography
to be performed on miniature specimens with a resolutio?i of one micron.
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In recent years in the nuclear and other industries, there have been
increasing requirements for improved properties in graphite which, in
turn, created a greater need for better nondestructive testing techniques
to evaluate the material. At the Oak Ridge National Laboratory we have
found radiography to be a valuable tool and developed both low-voltage
and microradiographic techniques to solve many of the graphite evaluation
problems with which we were faced. This work included parametric studies
of specimen thickness, X ray energy and exposure, and attainable sensi-
tivity, as well as many special detailed techniques for unusual problems.
Parametric Studies
General Consideration
Graphite is a form of carbon, the sixth element in the periodic
table. In general, the lower the atomic number of an element, the smaller
will be the value of the mass attenuation coefficient, Me for any given
radiation energy. The effect of the attenuation coefficient on the inten-
sity of the rallation beam es it passes through a specimen' is given by
the relationship,
where
I = radiation transmitted through the specimen, r/hr,
I = radiation incident upon the entry surface of the specimen, r/nr,
: H = attenuation coefficient, cm²/8,
o = density, g/cm", and
* = specimen thickness, cm.
For useful radiography there must be sufficient change in the radiation
transmitted through the specimen, I, to achieve an interpretable contrast
of density on the processed radiograph. As evident from the equation,
this is governed by the exponential terms, H, O, and . For most practical
cases, o and x are variables but are restricted within fairly narrow
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limits. For instance, graphite 18 a very low-density solid as compared
with most structural materials normally considered for radiography. Speci.
men thickness is normally determined by design requirements, and for most
of the requirements discussed in this paper was less than 2 in. thick.
This leaves only u as a true variable with which the radiation transmission
can be controlled. As with density, graphite has a low value of u when
compared with many other materials. However, the attenuation coefficient
18 energy dependent and is generally larger et lower energy levels. Thus,
for examination of thin sections of low-density graphite, It ha. been
necessary to use low-energy (low-kilovoltage) radlation. This provided
a large enough change in transmission to permit the detection of the
minute changes associated with the discontinuities of interest.
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Equipment
We conducted similar studies earlier for the deve.lopment of techniques
for the radiographic evaluation of beryllium, aluminum, and stainless
steel (2). Much of the equipment for the radiography of graphite is iden-
tical to that described in the report on metals. For that reason only a
summary is incorporated here.
The equipment was capable or generating continuously variable X ray
energies from 0 to 50 kvp. With the lower X ray energies which are readily
absorbed 1.n the thin graphite sections, it was necessary to eliminate all
extraneous material between the target of the X ray tube and the specimen.
These materials not only reduce the intensity of the X ray beam, but also
preferentially absorb the softer components of the spectrum which are so
important in producing high-contrast radiographs. The first step in mini-
mizing excess absorbers was the use of an X ray tube with a low-filtration
beryllium window only 0.010 in, thick. Air is an effective absorber for
the soft X rays, so helium was substituted as the atmosphere between the
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(3) "Techniques for Low-Voltage Radiography," Nondestructive Testing
20(4), 248-53 (July-August 1962). Many more figures, technique charts and
other plotted data for the three metals are found in a report with the same
title, ORNL-3252, Feb. 14, 1962.
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X ray tube and the specimen. This was accomplished with a simple 34-in.-
long, 8-in.-diameter cylindrical chamber with 0.0005-in.-thick polyethylene
diaphragms placed over each end to retain the helium within the drum-like
chambez. The very thin plastic membranes were almost transparent to the
X rays being used. This system is shown in Fig. 1. The helium chamber
extended from the X ray tube to within approximately 2 in. of the working
surface. Thus, both the specimen and film were in the normal room atmo-
sphere and were completely accessible for change without violating the .
bellum atmosphere.
Another important step in reducing unnecessary absorption was the
elimination of the cassette or film holder and the placement of the speci-
men directly on the bare film. This, of course, required us to use dark-
room exposure techniques.
Experimental Procedure
An extensive program was followed to study the effects of variation
in X ray energy and specimen thickness and in the use ci intermediate
atmospheres of both helium and air. We prepared a step wedge of graphite
with thicknesses varying from 0.100 to 1.650 in. The graphite wedge was
then radiographed using both helium and air atmospheres at discrete energy
levels from approximately 5 to 45 kvp. At each energy level, enough
radiographs were made with different exposure times to provide a complete
range of readable densities for each thickness. Thus, for each thickness,
curves of film density versus exposure time could be plotted as a function
of energy level and atmosphere. Maximum exposure times were approximately
10 min but most of the exposures were less than 5 min. Eastman type "M"
film was used throughout the project.
Film Processing
Control of the film and fllni-processing variables was mandatory to
attain reproducible results and to allow cross referencing and correlation
between the different radiographic conditions. All exposures were made
from the same box of film. All films exposed under a given atmosphere
(hellum or air) were processed as a batch. With these precautions, good
reproducibility was achieved and very smooth curves with little data scatter
were possible.
All film densities were measured on dry processed film with a Macbeth-
Ansco Model 12A Densitometer (4) which is capable of making measurements
over a density range from 0 to 8.0 with an accuracy of 20.02.
Results:
All of the data were transferred to basic graphs to display the
milliampere-second exposures versus the film density for each specimen
thickness. For each chart the X ray energy and intermediate atmosphere
were held constant. Figure 2 18 a typical chart showing the data for an
air atmosphere at an energy level of 45 kvp. These curves then served as
the basis for subsequent data evaluation. Useful exposure charts for
radiography of graphite were generated as a function of energy, intermediate
atmosphere, and specimen thickness.
Figure 3 18 a typical exposure chart for an air atmosphere. Super-
imposed on this chart is the exposure curve for 0.100 in. of graphite 14
a helium atmosphere were substituted between the X ray tube and specimen.
As described earlier, several precautionary steps were taken to elimi-
nate extraneous absorbing material between the tube and specimen. This
was most beneficial at lower specimen thicknesses and X ray energies.
Experimental exposures and calculations were made to determine the condi-
tions at which there is little advantage in the special precautions. We
arbitrarily decided that any decrease in exposure time of 10% or less was
insignificant. The film holder which was selected for comparison was a
flexible plastic cassette which interposed a single layer of plastic
between the specimen and film. Typical results are shown in Fig. 4 to define
threshold conditions at which a 10% decrease in exposure is realized by
removing or replacing the absorber. For X ray energies and thicknesses
below the curves, greater than 10% benefits are obtained. Removal of the
cassette produced at least 10% inprovement throughout the thickness range
studied. :
(4) Macbeth Instrument Corporation, Newburgh, New York.
The graphite used in this study had a specific gravity of 1.7.
Therefore, for graphites having a different density, a proportional cor-
rection would be necessary for all data involving thickness.
Applications
The low-voltage radiographic techniques have been successfully applied
to a variety of graphite inspection proolems. One such application was
for the evaluation of 6-cm-diameter graphite spheres which were subsequently
to be machined into shells for nuclear fuel elements. Because of the
spherical configuration, the specimens provided a large range of thickness
for the X rays. This range coupled with the inherent radiation scatter
allowed only a small cyiindrical section which was coaxial with the X ray
beam to be evaluated in a single exposure. The radiographic quality was
inferior. However, introduction of the unique masking tray shown in Fig. 5.
allowed a great improvement to radiographic sensitivity. One-inch Lucite
was machined to provide hemispherical cavities meitching the curvature of
the spheres. The Lucite mask allowed a selective absorption of scattered
X rays as well as providing a more nearly constant total X ray absorption
over the entire projected sphere diameter. With this technique the entire
spherical volume could be evaluated with a single exposure for three-
dimensional discontinuities such as porosity. The sensitivity and
resolution were shown to be capable of detecting flaws about 0.040 in.
in diameter. This is equivalent to approximately 2-11 radiography as
defined by ASTM E-142. We felt that smaller flaws could have been detecto......
able, but no such standards were made. Three-view radicgraphy (in X, Y,
and Z directions) was performed to provide added assurance for the detec.
tion of internal cracks or laminations.
An interesting application was in support of liquid-salt impregnation
studies in CGB, AGOT, and other types of graphite. Low-voltage radio-
graphs were taken prior to impregnation to detect flaws, after initial
impregnation to determine the nature of the salt flow, and again after
heat treatment to observe changes in the graphite in the impregnated
condition. Figure 6 is a reproduction of a radiograph showing the salt
which has filled existing cracks in the graphite specimen
(density = 1.87 g/cmº) but has not penetrated into the body in any other
region.
Tenisile and compression specimens for destructive tests on graphite
are radiographed to detect internal flaws. Good correlation has been
shown between presence of flaws, site of fracture, and reduced breaking
strength. This has allowed a reduced scatter hand of data on graphite
strength.
Microradiography
Equipment
Substitution of extremely fine-grained emulsions for the X ray film
has allowed the performance of contact microradiography with good resolu-
tion. A. aumber of photographic detectors were tested but the optimua for
our purposes was shown to be Eastman Kodak High Resolution Plates. Although
several fine-grain developing solutions were tested, none were found to be
superior to the standard X ray film developer which was being used for
conventional radiography. All other aspects of the low-voltage radio-
Graphic system remained the same for our microradiography.
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A major application of our microradiography has been on nuclear fuel
particles coated with a thin layer of pyrolytic carbon. The X ray energy
levels used for the particle evaluation vary from 10 kvp down to about
4 kvp, depending upon the character and density of the coating and the
information desired. Because of the very thin emulsion and fine-grain
size of the high-resolution plates, the response to X rays is rather slow
and exposure times as long as 1 hr at the lower energies are not unusual.
Typical coated fuel particles include a core or kernel 0.008 in. in
diameter with a 0.004-in.-thick coating which may be composed of several
thinner layers with different densities. The microradiography is performed
to allow measurement of core and coating dimensions and evaluation of
integrity. The resolution and accuracy have been shown to be abo::t 1 4.
The principal limitations on observation of detail or resolution are the
resolving power of the optical systems (about 1 h) and the electron dif-
fusion in the emulsion which is rated at about 1 u at the energy level
being used. For this reason, the helium chamber vas shortened to abouü
19 in. for coated particle microradiography since this increased the geo-
metric unsharpness to only approximately 2/3 u, but decreased the required
exposure times by a factor of approximately 4.
Viewing
Several thousand coated particles can easily be radiographed in a
single exposure on the 2-in.-square plates. The contact microradiograph
which 18 produced may then be viewed on a microscope or metallograph
equipped for transmitted light. Examinations are made at magnifications
up to 500x with little difficulty being caused by the grain size. For
recordings of the magnified image, a photomicrograph can be made of a
representative area of the contact microradiograph. The film used for
this 18 - Eastman Royal Orthofilm exposed with a green (Wratten No. 54)
filter and developed in DK-50 according to recommended procedu::es...
Contact prints can now be made from this photomicrograph restoring the
same relationship of light and dark areas present on the original micro-
radi.ograph.
Results
Figure 7 is a print of a group of coated particles selected to
demonstrate the detail that may be detected in a multilayer variable
density coating by microrcicography. The technique is being used as an
evaluation tool for process control of the coating process, for routine
evaluation of the coated product, and for post-service examination after
heat treatment or reactor testing. The latter has required a degree of
technique modification but good quality microradiography is possible
despite a high level of radiation from the specimen.
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Summary
Low-voltage radiographic and microradiographic techniques were
demonstrated to be very useful for the evaluation of graphite and carbon.
As a result of parametric studies, detailed exposure charts for graphite
radiography have been prepared. We determined conditions under which
significant improvements could be made by removal of such absorbers as
the air atmosphere and film holder. Use of a high-resolution photo-
garaphic emulsion allows performance of microradiography with l-u
resolution on miniature specimens.
Acknowledgments
The author is pleased to acknow.ledge the significant contributions
of W. J. Mason toward both the technique devell ment and the data
accumulation.
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LIST OF FIGURES
Fig. 1.-(Y-32546) Low-voltage X Ray Equipment.
Fig. 2--(ORNL-DWG-67-3427) X Rey Exposure Versus Density Chart for
Radiography of Graphite at 45 kvp.
Fig. 3-- (ORNL-DWG-67-3428) Exposure Chart for Various Thicknesses of
Graphite with Air and Hellum.
Fig. 4-- (ORNL-DWG-67-3426) Conditions under which Absorber Removal will
Effect 10% Change in Exposure Requirement for Rediography of
Graphite.
Fig. 5--(Y-52695) Masking Tray for Radiography of Graphite Spheres.
Fig. 6--(Y-48956) Radiograph of Graphite Specimen with Complex Salt
Impregnating Cracks.
Fig. 7--(4-70858) Microradiograph of selected Coated Particles to Show
Multilayer, Variable Density Coating.

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X RAY EXPOSURE (mamp-sec)
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ORNL-DWG 67 - 3428
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DATE FILMED
6 / 7 /67