Note: Descriptions are shown in the official language in which they were submitted.
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RADIOGRAPHIC INSPECTION ME~NS AND METHOD
The present invention relates to radioyraphlc imaging
systems and, more particularly, to radiographic apparatus
having means for producing images derived from radiation
shadowgraphs derived with the use of neutron or
X-radiation sources~
The forming and processing of radiographically
produced shadowgraphs or radiatlon transmission patterns
to produce visual images of a specimen or workpiece is of
interest in various applications, such as the radiographic
inspectlon of various structural components. Previously,
such inspection techniques entailed the for~ing of
photoshadowgraphs. A photographic film plate was
positioned adjacent an object to be inspected by the
neutron or X-ray source, the object being positioned
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between the film and the source of radiation. When
neutron or X-radiation is transmitted through any
hetrogeneous object, it is differentially absorbed,
depending upon the varying thickness, density, and
chemical composition of the object. The image registered
by the emergent rays on a f ilm adjacent to the specimem
under examination constitutes a shadowgraph, or
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ra~iograph, i.e. an intensity pattern of the rays
transmitted, of the interior of the speclmen.
X-radiation may used in industrial applications
wherein, for example, it is desired to evaluate a metal
- casting suspecting of having internal cracks, ~eperations,
voids, or other defects7 and it is, of course, employed
widely in medical applicatlons. X-rays are, in general,
substantially more Renetrating than neutron radiation with
respect to "low-z" materials such as aluminums, plastic,
boron, carbon, and the like. Radiographs produced from
neutron radiation are employed, for example, when it is
desired to form an image of hydrogenous, or organic
materials which may be present within metallic
structures. Neutrons penetrate low-thermal-cross-section
20 `";
materials such as lead, aluminum, steel, and titanium, but
are absorbed by organic, hydrogenous materials. With
respect to metallic structural members, an indication of
such hydrogenous materials within the structure ~ay reveal
the presence of water, hydroxides, and other corrosion
products. Such corroslon may be ln the form of
intergranular corrosion, with accompanying exfoliation, of
materials such as aluminum, and certain other metals.
Stresses in aluminum aircraft components, for example,
produce internal, intergranular corrosion which ls
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lnvisible and not accurately lmaged by conventlonal,
non-destructive inspection tachniques; such corrosion may
result in critical failure of major structural elements'if
lt continues undetected. As in the design of load-bearinq
or structural members for various industrlal applications,
the conventional design philosophy for aerospace
components entails a substantlal degree of "over design"
for ensuring structural integrity of the components. As
will be understood by those in the art, such an excess of
material results in correspondlngly higher weight and
cost, and in lower performance and fuel efficiency than
would be obtained if compensation for potential,
undetectable internal deterioration was not necessary.
Similarly, the permissible useful life o~ such components
' is also based upon safety margins which can be
substantially reduced if positive assurance were
obtainable that internal, or hidden deterioration had not
occurred to a significant degree.
Further difficulties with respect to non-destructive
~esting of aerospace components relate to the
possibilities of surface corrosion on internal components
hidden from visual inspection. Corrosion which may occur
within honeycomb cell structures or panels may result in
the separation of honeycomb cores from outer skin
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surfaces, and the like.
In the past it has been attempted to produce process
images produced from low level radiation such as neutron,
or low level X-radiation, by exposing photographic films,
to the radiation for an appropriate perlod of time, and
developing the film for inspection. The use of
photographic film provides the advantage that, through
exposure over an extended period of time, very low levels
of radiation may form a satisfactory photoradiograph.
Exposure times, film speed, radiation levels and film
types may be varied. It will be understood, however, that
the delays entailed in set-up film processing imparts
limitations in inspection efficiency, particularly, when
it is desired to inspect, and reinspect, large components,
or large numbers of components. For this reason, modern
- radiographic inspection systems have employed
low-light-level television cameras for producing
television images derived from the radiation of a apezimen,
whereby a television display corresponding to a
2V radiophotograph is formed. The television monitor may be
located in a facility remote from the radiation source,
which may afford advantages when hazardous radiation is
present. Additionally, television monitoring permits
continuous monitoring of a component for real, or "near
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123~5'7~
real ~imel' examination. Such low-light-level television
cameras may be of the image orthicon type or of other
types, and often employ multiple stages of image
intensification or amplification. Modern, low-light-level
cameras include various refinements and intensification
techniques, such as silicon intensified -targets (SIT),
secondary electron conduction (SEC), charge-storing, and
amplifying.
Two general approaches to the formation of television
images of irradiated specimens are illustrated in U.S.
Patent Nos. 3,280,253 and 3,668,396, to R.C. McMaster et
al and J.A. Asars et al. The system of the McMaster
Patent employs a single stage camera tube which is
sensitive to X-radiation. In use, a radiation source is
positioned to direct X-radiation directly toward the
television camera tube after transmission through a
workpiece to be inspected, and an image is formed on the
camera tube target by electrons derived from the
X-radiation directed toward the camera; the image is
intensified by the use of periodic beam scanning, in which
the radiation builds up adequate image potential (an image
pattern comprising a loss of positive charges at portions
of a semiconductor target) between raster scanning
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cycles. A satisfactory TV image is produced by
intermittent scanning of the target by the electron beam
raster scanner~ The McMaster camera includes no
intermediate intensifying stages. Such single stage
camera tubes provide relatively moderate gain in
comparison with hlghly sensitive tubes such as that
disclosed in the recent Asars patent. The Mc~aster system
may thus be considered to have a relatively high level of
input radiation (radiation directly from the X-ray source,
which is of generally higher intensity and penetrating
potential than portable neutron sources) and a relatively
low level of internal intensification or amplification in
comparison with multi-stage cameras such as that disclosed
in the Afiar patent. Such systems are advantageous for
- "; certain applications, and such single stage television
cameras are less expensive and complex than multi-stage,
very low-light-level cameras.
The Asars system employs a phosphor screen to provide
a large field of view of appropriate resolution and
detail, the phosphor screen serving to generate
scintillations of light as the screen receives gama
radiation derived from a neutron source. The light
scintillations on the screen are detected and intensified
through the sensitive, multi-stage SEC camera tube. To
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provide adequate light ampliEication, the camera tube
employs several stages of image intensiflcation, including
- an initial image intensifier tube section and an
intermediate image intensifying section. As will be
understood by those in the art, sophisticated low-light
level cameras such as that employed in the Asars system
are highly complex and expensive.
Tne present system is intended to provide a
radiographic television display with a relatively lower
cost and less comple~ camera system, while at the same
time providing very high sensitivity to low radiation
levels~ In particular, it is intended to provide a
radiographic system sensitive to "soft" or thermal neutron
radiation, i.e., radiation from which the higher energy
neutron and, gama rays have been removed, as may be
obtained from portable radiographic generator systems such
as that disclosed in U.S. Patent No. 4r300~054~ issued
November 10, 1981, to W.E. Dance et al. The system of the
4,300,054 Patent employs a modulator fluid and filter for
attenuating the hard, gama radiation from energy produced
by a radiation generator tube. There is a need,
particularly in the inspection of aircraft and other
components by low power, non-isotopic radiation sources,
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for an efficient television radiographic display means
wherein a high resolution image is produced for convenient
viewing. In such systems, radioactive isotopes as
radiographic sources are not employed, eliminating the
hazards and inconveniences entailed in the transportation
and storage of such materials.
A problem entailed in prior radiographic systems has
difficulty in producing a high resolution, finely
detailed image in the presence of varying levels of
radiation. High radiation peaks may tend to overload and
blur the camera and may even damage the camera. Another
problem has been that very low levels of radiation, such
as those obtained from thermal neutron sources and from
low level X-rays, have been difficult to record because of
inherent system noise. The obtaining of detailed images
required to show fissures and details of internal
deterioration of metals with sufficient resolution to
ensure that no critical faults exist in a piece under
inspection is of importance in many applications. A
further deficiency in prior inspection systems has been
their limitation to undesirably narrow ranges of energy
levels. That is, those instruments sensitive to high
level radiation such as that produced by X-radiation have been
insensitive and not usable with lower levels of radiation
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commonly received as neutron radiation. Prior systems
were not usable with low-level neutron radiation.
It is, accordingly, a major object of the present
invention to provide a new and improved radiographic
5 imaging system.
Another object is to provide such an imaging system
which is sensitive to relatively low levels of radiation,
including ~hermal neutron radiation free of any
substantial gamma radiation. A still further object is to
10 provide such a system which is operable to produce images
of high resolution derived from thermal neutron radiation
of, for example at least about 100 neutrons per square
cm. per second and X-rays of low and very high levels.
Yet another object is to provide such an imaging
15 system which is usable to provide high resolution
television images, and which can provide such images
derived from shadowgraphs produced by both neutron and
X-radiation.
A further object is to provide such an imaging system
20 in which highly complex, low-light-level television
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cameras with mul~iple stages o~ lntensification are not
required, yet whlch provides an overall radiographic
sensitivlty comparable to or greater than such prlor
systems.
Yet another object is to provide such a system in
which the television image produced i5 free of any
substantial noise and distortlon, whereln highly detailed
images may be displayed permitting accurate inspection of
components for small cracks, voids, fis~ures, and other
similar faults.
Yet another object is to provide such an imaging
system whlch is of practicable, relatively straightforward
and inexpensive construction, permitting convenient
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portability.
Other objects and advantages will be apparent from the
speclicatlon and claims and rom the accompanying drawing
illustrative of the invention.
In the drawing:
FIGURE l is a partially diagrammatic, sectional view
of the radiographic imaging system;
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FIGVRE 2 ls a diagrammatic representation of the video
tube, in combination with the beam blanking, video
processing, and video monitoxing circuitry.
With initial reference to FIGURE 1, radiographic
system 10 comprises a housing consisting of flrst and
second segments llA and llB. As will be understood more
fully from the discussion, the first housing llA is closed
to prevent the entry of light, and the second housing, in
addition to being closed, defines a substantially gas
impervious chamber 12. The second housing segment llB is
of elongated configuration, and is of a sufficiently large
diameter to accommodate a low-light-sensitive
television camera 13. Camera 13 is mounted (by internal
: spiders or other mounting means, not shown) coaxially
within the housing segment llB, and with its optical input
46 directioned toward the first housing segment llA~ The
camera 13 is a beam scanning camera having a
: semi-conductor target screen, and is suitably of the
silicon intensified camera type (SIT); alternatively,
: other electronic scanning cameras may be employed such as
the secondary emission charged coupled device (CCD), or
charge injected device (CID) types. An example of a
commercially available silicon intensified camera is
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manufactured by Arvin Diamond Corporation company as Model
No. 6073B.
The first and second housing segments llA, llB are
connected by means such as flanges 14A, 14B affixed within
abutting end portions of the respective housings, bolted
or otherwise connected. A transparent, suitable glass
plate 15 is sealingly mounted across an open end of the
second housing segment llB in front of the camera 13
permitting the passage of light to the camera input screen
~6, as will be described in more detail hereinbelow. The
first housing segment llA is preferably generally
L-shaped, having a perpendicularly facing window 16 at its
end opposite the second housing segment llB.
A flange 17 is mounted peripherally within the opening
16 and a radioluminescent screen structure 20 is fitted
within the opening 16 against the internally mounted
flange 17. ~n outer molding frame member 21 is detachably
affixed to the housing segment llA around the opening 16,
the housing being suitably provided with latch mechanisms
22 permitting convenient fastening of the framemember 21
against the radioluminescent or phosphorescent imaging
screen 20 which in turn is in intimate contact with the
internal molding 17, whereby extraneous light is kept from
r
12
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the housing segment llA. At the en~ of the housing
segment llA above the opening 16, a mirror 23 is mounted
extending along a plane skewed by 45 degrees from the
longitudinal axis of the housing and from an axis
perpendicular to the plane of the screen structure 20,
whereby images produced on the radioluminescent screen 20
are reflected by the mirror along the longitudinal axis of
the housings llA, llB, and toward the camera 13. A
workpiece 24 is positioned closely adjacent the
radioluminescent screen 20 between the screen 20 and a
source of radiation 25, the source 25 suitably being a
source of or thermal neutron radiation, or low level or
high level X-radiation, preferably emitted from essentially
a point source, for providing as sharp a shadowradiograph
as possible upon the radioluminescent screen 20. Preferably,
as suggested above, the source 25 may be a non~isotopic,
portable neutron generator as disclosed in U. S. Patent
4,300,054, which produces a collimated baam of thermal
neutrons directioned toward the screen.
A test pattern projector 30 is mounted above the
screen within the first housing segment llA and is
directioned to form a projected image upon the inner side
of the radioluminescent screen 20. As will be understood
13
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from the description of the operation of the system, the
system preferably includes such a projection system
because at low flux levels, the very low levels of
scintillations produced on the screen 20 are not
sufficient to permit adjustment of the focus current and
target bias voltages during operation of the camera. The
test pattern projected on the screen is of sufficient
intensity to permit convenient adjustment of both focus
current and bias voltage, for subsequent use with the
radiation source (without the projected image). The test
pattern projector 30 includes a test pattern transparency
31, a condensing lens 32 being positioned between the
transparency and a projection lamp 33 positioned in front
of a projection mirror 34. Projection lens system 35 is
directioned toward the radioluminescent screen 20. The
housing of the test pattern projector 30 is removably
affixed through an opening formed in the upper portion of
the first housing segment llA by means of a flange
structure 36, which may be bolted to the housing.
The television camera 13, in the preferred
embodiment of the system, is cooled to reduce to the greatest
extent possible any noise. As shown diagrammatically in
- FIGURE 1, the tube 13 is preferably fitted within a
cooling ring 40, the cooling ring 40 being mounted
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335~3
circumrerentially of the target section of the camera, as
will be more fully described h~reinbelow with reference to
FIGURE 2, for maintaining the targat preferably at
temperatures in the range -15 to -40 C. The cooling
ring 40 may comprise an annulus through which cryogenic,
liquid nitrogen is circulated from a source,represented at
41, external of the housing segment llB. Alternatively,
the cooling ring 40 may comprise a Peltier junctlon device
powered electrically. The second housing segment llB is
preferably insulated by insulation 42 formed on its inner
wall surfaces. The interior 12 of the second housing
segment llB is preferably maintained as a moisture-free
environment to prevent condensation upon the televislon
tube 13r and a lens structure 39 positioned between the
television tube input and the glass plate 15, as wili be
20`
- more fully described hereinbelow. ~or preventing
condensation on the camera 13 or lens system 39~ the
interior of the second housing segment llB may be
evacuated, or preferably, charged with a drying agent such
as nitrogen from source indicated at 43. Suitably, the
nitrogen source 43 or other dry non flammable,
electrically insulating gas communicates through valve 44
through tubing conducted through fitting 45 mounted within
a suitable opening formed in the wall of the housing
segment, and an outlet 48 is incorporated into the
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opposite end of the housing segment llB. Prior to use of
the system 10, outlet 48 is opened and nitrogen or o-ther
gas is permitted to flow through the inlet formed through
fitting 45 for a period of time sufficient to remove most
of the air and moisture from within the housing chamber 12
and charge the chamber with nitrogen. Subsequently,
the chamber 12 is pressurized to approximately 6 psi,
after which valve 44 is closed. It has been found in our
experiments that such a charge is sufficient for preventing
condensation on the camera tube 13 and optics 39 over an
extended period of time of, for example,several months,
with no need for further charge.
With reference now to FIGURE 2, the optics and
circuitry of the system are shown diagrammatically in
somewhat greater detail. An important feature of the
invention is the combination of a high output,
radioluminescent screen structure 20 with a sensitive,
low-noise camera 13 upon which light scintillations are
gathered and integrated internally by means of target
blanking. It should be understood that, in contrast with
prior systems, target blanking is not employed for
increasing image intensity, but is instead employed for
statistical purposes, i.e., for accumulating sufficient
scintillation information to form a radiographic image
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bett~een electron beam raster scannlng of the target to
provide an adequate image. The llght emlttlng,
radioluminescent screen structure 20 includes an outar
plate 20A of a material as transparent as possible to the
radiation employed, but whlch is opaque to light. It ls
typically of aluminum, or of another material of a low ~Z~
number and low thermal neutron absorption
characteristics. For convenience, a seallng layer 20B, of
alumlnum foll, is fiuitably employed adjacent the aluminum
plate 20A to protect the phosphorous layer 20C. The
phosphor layer 20C is suitably coated or deposited on a
substrate 20D, which is of a transparent material such as
glass. Alternatively, the phosphorous layer 20C can be
deposited on the interior surface of the lnltial, outer
plate 20A, with or without a protecti~e glass plate 20D.
2~
The system differs from prior ~ystems ln its use of a
relatively high intensity radioluminescent layer 20C on
2S the imaging screen, which permits the use of a relatively
lower gain camera 13, and ln the integration of
sclntillations on the target durlng the blanking periods.
Preferably, the phosphor layer consists of a thin layer of
a non-radioactive isotope of lithium in lithium fluoride,
suitably combined with ~inc sulfide, deposited on the
substrateO In prior art systems, lithium-based phosphor
33~73
layers have been used for gross, low resolution imaging
purposes, but they inherently produce light scattering and
diffusion, when subjected to radiation, which has in the
past prevented their use in normal or high resolution
imaging. In the present system, the prior difficulties
are eliminated by the use of a very thin layer of the
material, of about .025 inches or less and preferably of
about .02-0 inches or less. The lithium powder is mixed
with a lithium binder material. Suitably the neutron-to-
light radiophosphorescent convert material,-consists of a
mixture of lithium fluoride and zinc sulfide powders
mixed with a binder material, preferably one which also
contains lithium,or the powder is otherwise held in
place on the substrate by a thin transparent coating
material. It is desirable to limit the quantity of
binder materials to approximately 10-15~ by weight for
providing maximum light output. Although various methods
are known and utilized for laying materials in a thin
film on a substrate, e.g. thin film chromotography,
colloidal suspension in a settling tank etc., a recommended
method is to form a suspension of the powder mixture in
liquid solution containing a small quantity of the binder
materials and then apply the resulting slurry to the
substrate by "painting" or leading the substrate with
the material, and then drawing a blade orknif~like edge
such as a "doctors bar" across the surface to spread the
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~23~S~73
material uniformly across the substrate. In order to
maintain uniform thickness, the substrate must be flat,
and rigidly attached to a machined fla~ surface. Coating
thickness can be controlled to within .001" using this
technique. As previously suggested, thin coatings of,
for example, about .010 to .020inches have been satisfactory,
and preferably .012" to .025" coatings are employed.
When used with neutron radiation, the neutron particles
react with the lithium to produce alpha particles by
nuclear conversion of the lithium molecule, and the alpha
particle reacts with the zinc sulfide to produce a
scintillation of light energy. The lithium flouride
component is thus a conversion element, for converting
radiation to alpha particles, and the zinc sulphide
component is a light producing element for producing
light from alpha energy, for laying the material in a
thin film on the substrate at a nominal thickness of
.01 inches to .02 inches.
The phosphor radioluminescent layer 20C is of
importance as a radiation conversion material for
converting the radiation received into low level, visible
light radiation.
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Lens structure 43 is an objec-tive lens structure
positioned to form an image of the phosphor layer 20C on the
input fiber optic screen 46 of the camera. Th-e camera
tube 13 itself includes a fiber optic based "lens" input
section 46 having a concave, inner photo imaging layer 47,
which converts light derived from the scintillations into
electron energy which is accelerated through an
intensifying section 50 to the camera target 51 by means
of an electric po*ential field. The target 51 comprises a
semiconductor, suitably silicon structure. Cooling ring
40 preferably maintains the target at a low temperature of
-15C to -40C, sufficient to minimize background noise
and distortion during blanking periods. As is generally
known by those in the art, such tubes 13 incorporate a
raster scanning section, represented diagrammatically
at 53, and typically employ a blanking grid 54 for
imparting a blanking bias preventing scanning of the
target 51 in the event of failure of the raster scanning
circuitry (for the purpose of preventing damage to the
target 51 by extended bombardment of a fixed location
on the target 51 by the electron beam. The camera
assembly typically includes internal circuitry for
effecting the raster scanning, including the raster
generator and scanning circuitry. Such internal raster
scanning circuitry is generally operated in response to
external synchronizing or triggering signals to provide
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12335~73
television scanning of the target by the electron beam. A
video or picture processing unit 56, suitably a unit of
the type manufactured by the Quantex Corporation as Model
No. DS-20, is employed for generating timing and
synchronization signals, and for storing, processing, and
providing playback of video displays. Its output is fed
to a video monitor unit 57 for permitting monitoring of
images accumulated through integration of one or more
successive frames of scanning. An internal timing and
control circuit in the beam blanking generator portion of
the picture procsssor 56 generates timing signals, which
are logic signals of a selected time period. The logic
signals are fed through a logic driver 58, suitably an
open collector, TTL driver employed to increase the power
of the blanking signal and invert it prior to its
application to the tube scan failure beam cutoff circuit
71 and subsequently to the camera 13. The beam blanking
generator portion 57 of the processing circuitry may be
adjusted to vary the blanking period; during application
of a blanking signal to the grid 54, the electron scanning
beam is biased off. During the blanking period electron
charge is stored in the silicon target in a pattern
corresponding to the image which is scintillating on the
radiofluorescent screen.
At the completion of the blanking time interval, the
electron beam is unbiased and allowed to scan the target
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surface 51as steered by the deflection driver 59, including
the raster generator, which is typically connected to
deflection yokes 60 external of the tube. The deflection
driver 59 is synchronizedwith an output signal from the
beam blanking generator 57 and with the storage of video
information derived from the target potential output 61.
The target output, derived from 61, is amplified by a video
preamp unit 62 and fed to the video processor 56. A load
resistor 63 connected between the target output 51 and
a target power supply imparts a bias to the target.
The video processor 56 serves to accumulate frames
generated over a period of time during the non-blanking
periods and provides an output to the video monitor 57
-~ which is of high resolution, sufficient to permit
evaluation of finely detailed internal faults in the
specimens under examination. The image processor thus
periodically activates the electron beam generator,
reads resulting images, and processes the images for
integrating sequential frames and averaging the frames,
for improving clarity, and then continually reads out
the processed image to the monitor. Typically, the
electron scanning beam is blanked for a large portion of
the inspection time. For example, the period of image
storage may be on the order of 100 times greater than the
scanning period; in some, low level radiation inspection,
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there may be even longer periods of storage relative to
the scanning cycle period, depending upon the scintillation
output.
The very low levels of energy produced by scintillations
on phosphor screens from thermal neutron radiation or low
level X-radiation have not been previously employed for
the accumulation of statistical information on a semi-
conductor target during blanking periods, and the low
level of energies presents problems with respect to the
biasing and focusing of the camera tube 13. For this
reason, the test pattern projector 30 (Fig. 1) is initially
employed for adjusting the bias.
Thus, an important feature of the invention is its
ability to store and accumulate statistically significant
scintillation information on the target 51 within the tube
13 wherein the electron charge storage pattern builds up
on the semiconductor target screen from individual
scintillation events over a period of time until a
statistically satisfactory image may be scanned, rather
than being derived from external circuitry. This permits
the use of a camera tube 13 which is of rela-tively
inexpensive, rugged construction in comparison with those
employing multiple intensification sections, and thereby
minimizes noise and distortion which typically results
23
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from the use of multiple stages of light intensification.
Thusl the combination of the high output phosphor screen
with the blanking of the target scanning produce high
resolution images with components of moderate cost and
complexity. An important advantage of the apparatus is
its ability to produce clear images from low levels of
radiation and its ability to produce images derived fromboth
X-ray and N-ray sources without changing the internal
configuration of the apparatus, that is, without changes
of the structure of the phosphor screen or the camera,
etc. Extremely clear images are obtained at very low
radiation levels. In our experiments, satisfactory high
resolution radiographic images have been produced derived
from thermal neutron radiation levels, for example,
about 100 neutrons per square cm. per second, and with
X-radiation of very low and very high levels (e.g., from
40 KEV, at 0.5 ma, at 30 inches, to 10 MEV). Moreover,
images are obtained from such various radiation sources
without mechanical modification of the camera lens system,
or screen.
The test pattern projector 30, in combination with the
radioluminescent screen 20 facing inwardly within a closed
housing segment llA, permits accurate, convenient
focusing, both mechanically, i.e., by positioning lens 43,
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relative to the camera 13, and by ad~ustment of the target
bias, and camera electronic focus, prior to actual
radiography operations.
While only one embodiment of the invention, together
with modifications thereof, has been described in detail
herein and shown in the accompanying drawing, it will be
evident that various further modifications are possible in
the arrangement and construction of its components without
departing from the scope of the invention.
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