US3887807A

US3887807A - Elements and process for recording direct image neutron radiographs

Info

Publication number: US3887807A
Application number: US381106A
Authority: US
Inventors: Jr Robert V Poignant; Edwin P Przybylowicz
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1973-07-20
Filing date: 1973-07-20
Publication date: 1975-06-03
Anticipated expiration: 1992-06-03
Also published as: FR2238160B1; FR2238160A1

Abstract

An element is provided for recording a direct image neutron radiograph, thus eliminating the need for a transfer step (i.e. the use of a transfer screen). The element is capable of holding an electrostatic charge and comprises a first layer for absorbing neutrons and generating a current by dissipation of said electrostatic charge in proportion to the number of neutrons absorbed, and a second layer for conducting the current generated by the absorbed neutrons, said neutron absorbing layer comprising an insulative layer comprising neutron absorbing agents in a concentration of at least 1017 atoms per cubic centimeter. An element for enhancing the effect of the neutron beam by utilizing the secondary emanations of neutron absorbing materials is also disclosed along with a process for using said device.

Description

Poignant, Jr. et a].

June 3, 1975 1 i ELEMENTS AND PROCESS FOR RECORDING DIRECT IMAGE NEUTRON RADIOGRAPHS [75] Inventors: Robert V. Poignant, Jr.; Edwin P.

Przybylowicz, both of Rochester, NY.

[73] Assignee: Eastman Kodak Company,

Rochester, NY.

[22] Filed: July 20, 1973 [21] Appl. No.: 381,106

{52] US. Cl. 250/315; 250/327; 250/390 (51] Int. Cl. G012 3/00 [58] Field of Search 250/315 A, 327, 390, 391, 250/392 156] References Cited UNITED STATES PATENTS 2,825,814 3/1958 Walkup 250/3l5 A 3,390,270 6/1968 Treinen et all 250/390 Primary Examiner.lames W. Lawrence Assistant Examiner-Davis L. Willis Attorney, Agent, or Firm-R. P. Hilst [57] ABSTRACT An element is provided for recording a direct image neutron radiograph, thus eliminating the need for a transfer step (Le. the use of a transfer screen). The element is capable of holding an electrostatic charge and comprises a first layer for absorbing neutrons and generating a current by dissipation of said electrostatic charge in proportion to the number of neutrons absorbed, and a second layer for conducting the current generated by the absorbed neutrons, said neutron absorbing layer comprising an insulative layer comprising neutron absorbing agents in a concentration of at least 10 atoms per cubic centimeter. An element for enhancing the effect of the neutron beam by utilizing the secondary emanations of neutron absorbing materials is also disclosed along with a process for using said device.

22 Claims, 3 Drawing Figures ELEMENTS AND PROCESS FOR RECORDING DIRECT IMAGE NEUTRON RADIOGRAPHS FIELD OF THE INVENTION This invention relates to neutron radiography and. more particularly. to elements for making direct-print neutron radiographic recordings.

BACKGROUND OF THE INVENTION Neutron radiography. i.e.. the production of photographic images formed as a result of the differential attenuation of neutrons by an object. is in many respects similar to X-radiography. Both techniques provide an image of the inside of an object. The two techniques complement each other. in that many materials opaque to one are transparent to the other. The field of nondestructive testing offers a particularly valuable area for the use of neutron radiography. It is known that x-rays interact with orbiting atomic electrons, and x-ray attenuation is therefore proportional to material density and atomic number. Neutrons, however. interact with the atomic nucleus. and their attenuation is proportional to nuclear capture cross sections and the density of atoms per unit volume but independent of atomic number. Accordingly. neutron radiography permits examination of many material combinations that cannot be effectively differentiated by x-rays. For example. neutron radiography is capable of detecting the presence and location of light elements such as hydrogen. beryllium. lithium. and boron within a block of lead.

Neutrons are produced in three ways: from nuclear reactions induced in an accelerator. a radioisotope. or a nuclear reactor. In each case neutrons are removed from an atom during a nuclear transmutation process. Neutrons are available in the enormous energy range of 1O electron volts. specifically. from 10 to [0" eV.

Accelerator is a general name given to devices that accelerate a beam of charged particles and direct them onto a target. An interaction takes place between the bombarding particles and target atoms. and this results in the expulsion of other particles. With particular combinations of incident particle and target material the ejected particles are neutrons. Typical of such a system is a device which ionizes the atoms of deuterium gas and uses a CockcrofLWalton voltage generator 100-400kV) to accelerate the ionized atom onto a tritium target. The reaction releases an l4MeV neutron.

Radioisotopes are produced by bombarding nuclei with charged particles in an accelerator or nuclear reactor. A nucleus becomes radioactive when it changes from a stable unexcited state to an unstable excited condition. The extra energy imparted to the nucleus to change it to the unstable excited condition is eventually emitted in the form of gamma rays or other particles as the nucleus decays back to its stable unexcitcd state. Unfortunately there are few radioisotopes which emit neutrons. Neutron production involving radioisotopes is generally achieved by bombarding a target with gamma rays or alpha particles emitted from the radioisotope. There are a few radioisotopes which decay by spontaneous fission to produce neutrons. but of these only californium-ZSZ has sufficient output to be considered.

The nuclear reactor is a device that produces fast and slow neutrons. gamma rays and charged particles. all in prolific quantities. Generally those reactor sources which have a relatively high thermal neutron intensity appear most useful because great relative differences in neutron absorption cross-section exist for thermal neatrons. The nuclear reactor is the most intense source of neutrons and therefore has been the most widely used for neutron radiography.

Two forms of neutron radiography have been practiced using photographic film for recording purposes. They are transfer neutron radiography and direct neu tron radiography. Transfer neutron radiography may also be practiced with conventional electrographic elements. This process. which will be referred to herein as transfer neutron electroradiography or more simply. the transfer process. is a combination of two known processes. namely transfer neutron radiography and electrophotography. and consists of:

l. exposing a potentially radioactive metal foil (transfer screen") in an imagewise fashion to a neu tron source to induce artificial radioactivity in the new tron-exposed regions. which radioactivity is proportional to the intensity of the absorbed neutron flux;

2. removing (transfer step) the metal foil (which is now radioactive in an imagewise fashion) and placing it in close proximity to a conventional charged electrographic element such as a photoconductive element having a uniform surface charge. this operation being performed in a way not to expose the electrographic element prematurely to light. or a. B and 7 radiation;

3. allowing the radioactive emanations from the transfer screen to strike the electrographic element for a period of time sufficient to create an imagewise discharge; and

4. removing the transfer screen and developing the radioactively-induced electrostatic image by any one of several electrographic development techniques.

It would be advantageous if a direct neutron electroradiographic" process were available. This is because. among other reasons. fewer steps would be in volved with a direct neutron electroradiographic process than with the above-described "transfer process". Besides being cumbersome. the transfer process suffers from additional problems including possible image degradation, the half-life of the transfer screen. and disposal of the transfer screen. The principal cause of the degradation is the small. but finite. separation which should be maintained between the transfer screen and the recording element. This separation is necessary to prevent another possible source of degradation in image quality. that arising from an electrical interaction between the transfer screen and the recording ele ment.

SUMMARY OF THE INVENTION According to the invention an element is provided for recording a direct image neutron radiograph. the element being capable of holding an electrostatic charge and comprising means. preferably in the form of a first layer. for absorbing neutrons and generating a current by dissipation of the electrostatic charge in proportion to the number of neutrons absorbed. the neutron absorbing means comprising an insulative layer comprising neutron absorbing agents in a concentration of at least 10" atoms per cubic centimeter.

The concentration of neutron absorbing agent (C) is evaluated by the formula ('=nl ,,,"N". where n is the number of neutron interacting atoms per molecule. l',,

is the molor volume. and N" is Avogadros number. A general criteria for choosing neutron absorbing agents include the following considerations: l for a particular neutron energy range of the source being used. the total neutron cross-section per atom should be at least one barn; (2) for most applications. neutron interac tion should not produce long-lived radioisotopes; and (3) the ratio of neutron to gamma ray absorption coefl'tcients should be one or larger.

There is further provided an element for recording a direct image neutron radiograph. said element enhancing the effect of the neutron beam by emanating secon' dary radiation simultaneously while absorbing neutrons, and capturing said secondary emanations to form an imagewise electrostatic charge pattern.

In the direct neutron electroradiographic process of the present invention, the elements to be exposed is directly exposed by virtue of neutron induced interac tions within the electrographic medium of the element. Thus the danger arising from handling radioactive transfer screens is eliminated. Many practical neutron sources, however, have a high gamma ray flux in addition to neutron flux. This gamma ray flux could deleteriously tend to expose certain of the electroradiographic elements of the invention in addition to the desired exposure effected by the neutron flux. thereby producing a hazed image. Therefore, in accord with certain preferred embodiments of the invention there are provided novel electrographic elements which are relatively insensitive to the gamma radiation associated with most practical nuetron sources.

lt is the object of the present invention to provide novel radiographic elements which can be used for direct neutron electroradiography.

It is another object of the present invention to provide novel electrographic compositions which are sensitive to neutron radiation.

It is still another object of the present invention to provide novel neutron sensitive electrographic elements which may be conveniently handled. exposed, and developed in the present of light.

It is still a further object of the present invention to provide novel electrographic elements which are relatively insensitive to the gamma radiation associated with most practical neutron sources.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the invention will become apparent to those skilled in the art upon consideration of the accompanying disclosure and drawing. in which:

FlG. illustrates in a simplified form, the essential features of a preferred neutron radiographic element in accordance with the invention;

PK]. 2 is a side elevational view of a neutron radio graphic element in accordance with a preferred em bodiment of the invention; and

FIG. 3 is an enlarged view of the neutron radio graphic element of FIG. 2 and illustrates the mechanism of the process for certain preferred materials.

Referring to FIG. 1, a neutron recording element 11 in accordance with this invention basically comprises two layers. via. a neutron absorbing layer 18 and a con ductive layer 19. The conductive layer 19 may serve as mechanical support for the neutron absorbing layer 18 if the mechanical properties of the latter are insufficient to provide self-support. Typically a test object 16 Ill is placed between the neutron recording device I l and a neutron source 12. The neutron source 12 emits a beam of neutrons l4 which expose the test object to the neutron recording device.

The neutron absorbing layer 18 is comprised of neutron absorbing agents. Neutron absorbing agents found useful in the practice of the present invention generally have relatively large neutron absorption coefficients. and. in addition. ratios of neutron to gamma ray ab sorption coefficients equal to or greater than unity. Table I shows a comparison of the thermal neutron and gamma ray mass absorption coefficients of chemical elements suitable for use in thermal neutron applications. Similar data may be obtained for epithermal neutron applications. and the like.

TABLE I ELEMENTS SHOWING GOOD THERMAL NEUTRON ABSORPTION CHARACTERISTICS Mass Absorption ('oefficient The concentration of neutron absorbing agent in the neutron absorbing layer is evaluated for a pure sub stance from the formula nV,,, N. where n is the number of neutron interacting atoms per molecule. V,,, is the molar volume, and N" is Avogadro's number. The concentration of said neutron interacting atoms should be at least l0 atoms per cubic centimeter. It is preferred. however. that the concentration be in the range of from about 10 to about l0 atoms per cubic centimeter.

A general criteria for choosing neutron absorbing agents to be compounded into useful materials for the neutron absorbing layer should be based on three principle considerations: l for the particular neutron energy range of the source being used. the total neutron cross-section per atom should be at least one barn. and preferably more than ten barns; (2) for most applications, neutron interaction should not produce longlived radioisotopes; and (3t in order to minimize the influence of direct gamma ray interaction the ratio of neutron to x-ray absorption coefficients should be one or larger. especially when the source emits a mixture of neutrons and gamma rays. The element boron is a good example of an element which satisfies all three requirements. It has a thermal neutron cross-section of 3.770 barns. does not produce long-lived radioisotopcs upon neutron interaction. and has a l7(lto-l ratio of thermal neutron absorption coefficient to the lZS Ke\' gamma ray absorption coefficient.

The sensitivity of the neutron radiographic element is dependent on the half-life of the neutron absorbing agent used therein. The length of half-life of the neutron absorbing agent is directly proportional to the sen sitivity of the element. Neutron absorbing agents which produced longlived radioisotopes. i.e. isotopes with relatively long half-li\es. are not generally suitable for rcuseable neutron radiographic elements. For reuseable elements. therefore. it is essential that isotopes of the neutron absorbing agent have a relatively short half-life. i.e. the half-life is short enough to decay radioactivity before the next exposure of the element. For these reusable elements, it is preferred that the isotopes of the neutron absorbing agent possess half-lives of less than one second.

The thickness ofthe neutron absorbing layer 18 used in the electrographic neutron recording process is based upon several factors. It is desirable to have as much neutron interaction as possible. subject to the constraint that image quality not suffer via excessive lateral charge diffusion. The thickness is governed by the concentration of neutron absorbers per cubic centimeter, the neutron cross-section of the absorber. the dielectric breakdown (maximum electrical field) strength. and the value of the applied voltage.

The neutron absorbing layer may be comprised of any materials that meet the above requirements. provided that the electrical resistivity of the layer is sufficiently high so that an electrical charge is retained on the surface of the film. Generally, a resistance of 10'" ohm-cm or more is required for satisfactory electro graphic performance. In some special uses. however. the resistance of the neutron absorbing layer can be as low as [O ohm-cm. Typically, these neutron absorbing layers can be l films of solid organic materials having a high hydrogen content, i.e. a high concentration of neutron interacting hydrogen atoms per unit volume. (2) composites of binder with neutron absorbing pig ments, and (3) vacuum deposited films comprised of electrically insulating neutron absorbing compounds.

Useful solid organic materials having a high hydrogen content can be determined by calculating the concentration of neutron interacting hydrogen atoms with the formula nV f N". An example of a high hydrogen content material is solid methane, which has 6 X hydrogen atoms per cubic centimeter.

Organometallic compounds are also useful as neutron absorbing agents. Particularly useful organometallic compounds are those containing isotopes of boron, especially "5, and lithium. especially Li. Those organometallic compounds containing natural isotopes of cadmium, Samarium, europium, gadolinium. gold, indium, silver, rhodium. and dysprosium are also useful in thermal neutron applications. Specific examples include tri B-naphthyl borate; tri a-naphthyl borane; mono. bis, and tritmethylamine) borane; gadolinium hexaantipyrine chloride; hexaantipyrene gadolinium (Ill) perchlorate; triphenylborine amine; copper triphenylphosphineborane having the formula [P(C.,-H ),-,]-,CuBH and tetraarylborates as described in Photog. Sci. Eng. 16, 300-312. 1972.

Compounds. particularly the oxides. nitrides and chalcogenides of neutron interacting elements can be formulated into films appropriate for use as the neutron absorbing layer. Especially useful inorganic materials include. for example: B 03. BN. BP. BC. CdS. CdF- (M 0 and gadolinium gallium garnet.

Films of these neutron absorbing materials can be formulated in many ways. For example. the agents can be dispersed in a binder. Materials can be utilized in the form of coated or sintered layers as well as hot-pressed sheets. Films can also be \acuum deposited on an electrically conductive support layer.

Binders useful in the preparation of neutron absorbing layers containing neutron absorbing pigments can be either organic or inorganic material. Typical exam ples of organic binders include. for example: natural and synthetic plastic resins. waxes. colloids, gels and the like including gelatins. desirably photographicgrade gelatin, various polysaccharides including dextran. dextrin, hydrophyllic cellulose ethers and esters. acylated starches. natural and synthetic waves including paraffin. beeswax. polyvinylacetals. polymers of acrylic and methacrylic esters and amines. hydrolyzed interpolymers of vinyl acetate and unsaturated addition polymerizable compounds such as maleic anhydride, acrylic and methacrylic esters and styrene, vinyl acetate polymers and copolymers and their derivatives. in cluding completely and partially hydrolyzed products thereof. polyvinyl acetate, polyvinyl alcohol. polyethylene oxide polymers, polyvinylpyrrolidene, polyvinyl acetals including polyvinyl acetaldehyde acetal. polyvinyl butyraldehyde acetal. polyvinyl sodium-osulfoben- Zaldehyde acetal. polyvinyl formaldehyde acetal and numerous other insulating. film-forming binder materials known in the electrographic art.

As is well known in the art. in the preparation of smooth uniform continuous coatings of binder materials. small amounts of conventional coating aids may be employed therewith as viscosity controlling agents. leveling agents, dispersing agents, and the like.

An example of an inorganic binder which may be found particularly useful consists of the vitreous form of H 0 The amount of neutron absorbing agents incorporated as a pigment in the binder may range from (Ll to 99.9 percent by weight. The particular amount depends on the neutron absorbing characteristics of both the pigment and the binder. The binder itself may or may not have significant neutron absorbing capacity. In the above example B. .O is a good neutron absorber. As long as the film or layer formed contains the required concentration of neutron interacting atoms, it does not matter how they are proportioned between the binder and the pigment.

An insulative neutron absorbing glass, e.g. CdS-CdO- B 0 can also be prepared and utilized as the neutron absorbing layer. Cadmium sulfide precipitation provides two useful glass compositions which expressed in terms of weight percent are as follows: 41% CdS- 48.8'7r CdO 47. V7: 8203([0 ohm/cm) and 4.9% CdS- 46.6% CdO-48.7% B 0; (3 X l0 ohm/cm). Both glasses are insensitive to visible light and are insulators (see. for example. Mem. Sci. Eng... Okayama Univ. Vol. 6, pp. 47-52 (l97l The conductive layer 19 can be a conductive coating on a suitable film support ofa conductive material with useful mechanical properties. A suitable support mate rial should possess useful mechanical properties and should not exhibit a significant absorption crosssection for neutrons when the result of the absorptions are induced radioactive specie with long half-lives. Any conductive support may be used as long as this test is met. For example. aluminum sheet metal is a suitable support element which interacts little with thermal neutrons. When a hydrogen rich polymer is used as the neutron absorbing layer. no additional support may be needed and the conductive layer may be coated or vacuum deposited on one side of the neutron absorbing layer.

It should be noted that although a conductive layer is a preferred feature of the neutron elcctroradio graphic element it is not an essential feature. A conductive layer is not used in a process where the top surface of the electrographic element is charged to one polarity while simultaneously charging the bottom surface to the opposite polarity. Upon exposure. the charge in the image area dissipates Using the neutron recording element described above a process of direct neutron radiography" can be practiced because this invention provides a novel class ofelectrographic materials which exhibit neutroninduced electrical conductivity. A process of direct neutron electroradiography comprises:

1. uniformly corona charging the surface ofa neutron sensitive electrographic element.

2. placing the neutron sensitive elcctrographic element behind the object to be examined and providing a neutron exposure sufficient to discharge the electro- '1 graphic element in those areas where the UbJCCK is transparent to neutrons; and

3. developing the neutron induced electrostatic images by any of the several electrographic development techniques.

Many procedures can be utilized to obtain an electrostatic charge pattcrn and to obtain a developed image. Early work is described in Carlson U.S. Pat. No. 2.297.69l. issued Oct. 6. [942. wherein a charge pattern is formed and developed on a sensitive element. The charge pattern can be transferred to a receiver prior to development as in the various TEST processes (Transfer of Electrostatic Image] described in Walkup U.S. Pat. No. 2.833.648. issued May 6. 1958. Walkup U.S. Pat. No. 2.937.943. issued May 24. I960. Carlson et al. US. Pat. No. 2.982.647. issued May 2. l96l. Dreyfoos et al. US. Pat. No. 3.055.006. issued Sept. 18. i962 and Walkup US. Pat. No. 2325.814. issued Mar. 4. 1958. Grid controlled corona charging methods also can be used as described in Frank British Pat. Nos. l.l49.90l. issued Aug. 20. 1969. l.l52.308. issued Sept. lU. I969 and 1.52309. issued Sept. l0, I969. It is also possible to use a simultaneous charging and exposing mode of operation as described in U.S. Pat. No. 3.598.579 by G. H. Robinson, issued Aug. l0. l9? 1 The neutron clectroradiographic elements of this invention might conceivably be utilized in any known electrographic processes in which exposure by neutron is possible.

Conventional electroradiographic elements. such as those containing Se or PhD. etc. which are intended for X-ray application are not suitable for use in the process described above. since they usually have large gamma ray absorption coefficients. but poor neutron absorption properties. Table [I shows a comparison of the neutron and gamma ray mass absorption coefficients of chemical elements commonly used in materi' als for X-ray applications.

TABLE ll ELEMENTS SHOWTNG GOOD GAMMA-RAY ABSORPTION COEFFICIENTS Mass Absorption Coefficients Ratio TABLE ll-Continued ELI-.Ml-INIS SHOWING GOOD UAMMARAY ABSORPTION (Ol-FFICIENTS Mass Absorption ('oel'licients Note that the chemical elements commonly used for X-Ray applications generally have large gamma ray ab sorption coefficients. but low neutron absorption coefficients and ratios of neutron to gamma ray absorption coefficients less than unity.

Standard electroradiographic elements could be utilized in the direct process if used in conjunction with a conversion screen and a neutron beam with a low gamma radiation content. The conversion screen consists of a material which absorbs and reacts with ncutrons and in turn emanates a. [3 or 7' radiation in proportion to the intensity of neutron absorption. The secondary radiation then forms the imagewise pattern on the recording element. A problem is that the recording element is also exposed to the neutron source and would react with any gamma radiation which is associated with most practical neutron sources. This could produce high background intensity. It is also not possi ble to examine radioactive objects by the direct process using a standard electroradiographic element.

It is thought that neutron recording elements as herein described are useful for electrographic neutron radiography applications because the electrical conductivity of the neutron absorbing layer 18 increases when exposed to neutrons. This allows a charge on the surface of the neutron absorbing layer to be conducted through the layer to the conductive layer 19 in direct proportion to the number of neutrons absorbed during exposure.

In a further embodiment of the invention. some of the neutron interacting agents of Table l which emit or. B. or y rays upon neutron activation may be rendered conductive. The rays. if captured by a suitable material. can enhance the effect of the neutron beam. Radiographic sensitive materials which may contain chemical elements listed in Table II can be utilized to capture these secondary emanations. If these materials become conductive when exposed to the secondary emanations of the Table l materials. they will effectively enhance the neutron beam effects.

In principle. any ionizing radiation sensitive. insulative compound can be used to capture the secondary emanations of the neutron absorbing compounds by placing it in close proximity or in contact with the neutron sensitive agent. This can be accomplished in a number of ways. for example: l by homogeneously dispersing an ionizing radiation sensitive insulative compound which may contain a chemical element of Table II in a binder along with a neutron-absorbing agent (a chemical element of Table l is suitable for thermal neutrons) (2) by depositing a layer comprised of an ionizing radiation sensitive insulative compound (which may contain a chemical element of Table II on top of a layer comprised of a neutron-absorbing agent containing. for example. chemical element of Table I (in this case the neutron-abosorbing layer need not be insulating); and (3) by placing an elcctrographic element comprised of an ionizating radiation sensitive layer in close proximity (separated by a narrow air gap for example) to an element comprised of a neutron absorbing layer containing. for example. a chemical element of Table I as illustrated by the electrographic clement in FIG. 2.

Typically, an clectrographic element comprising an ionizing radiation sensitive insulative layer and a neu tron-absorbing layer can be produced. for example. by coating lead oxide or amorphous selenium on top of a boron containing support. The insulative chargeaccepting layer is relatively transparent to thermal neutrons. However. the boron containing support will absorb thermal neutrons and convert them into ionizing radiation. The ionizing radiation. in turn. alters the conductivity of the insulative charge-accepting layer. A charge pattern is then produced on the insulator surface in accordance with variations in the neutron flux. In the above examples the boron containing support" can consist ofa conductive boron or a boride layer sup ported by a conductive base (such as sheet aluminum). Lead oxide and amorphous selenium exhibit considerable gamma ray interaction. Therefore materials like these would not be desirable in situations in which the neutron beam is contaminated with high gamma flux.

In a preferred embodiment. a neutron absorbing organic formulation is coated on top of a boron or gadolinium coated conductive support to enhance the neutron sensitivity of the electrographic element. A neu tron absorbing device as described above may be (a) used alone or (b) further enhanced by capturing the secondary emanations of the neutron-absorbing com pounds by coating thereon a layer containing an ionizing radiation sensitive insulative compound.

Referring to FIG. 2, a preferred embodiment of the invention is shown. FIG. 2 shows a side elevation view of a neutron sensitive device exposed to a neutron flux 21. The neutron sensitive device is comprised of five layers. A first layer 22 is an electrically conductive layer. A second layer 23 is an insulative charge accepting layer. A third layer 27 is an insulative layer which exhibits increased electrical conductivity when struck by ionizing radiation. A fourth layer 24 is a neutron sensitive layer comprising an element from Table I which will produce secondary emanations when struck by the neutron flux 2]. A fifth layer 25 is another electrically conductive layer. Connected across

conductive layer

22 and 25 is a high voltage source 26.

The neutron sensitive device illustrated in FIG. 2 can be used in a preferred process to record the image of test object exposed to a neutron flux. In this process. a voltage is placed across the neutron sensitive device of FIG. 2 by connecting the

conductive layers

22 and 25 to opposite terminals of a high voltage source 26. A test object is placed between the neutron sensitive device and a neutron source and exposed to a neutron flux. The neutrons are captured by the neutron absorbing layer 24 of the neutron sensitive device. When struck by neutrons. the neutron absorbing layer 24 produces secondary emanations. Some of this ionizing radiation is back scattered to layer 27. an insulative layer which exhibits increased electrical conductivity when struck by the ionizing radiation. A charge is thus conducted to the insulative layer 23 which holds the charge. The charge pattern on layer 23 is an imagewise pattern of the test object being exposed. This charge pattern on layer 23 is then developed by any conventional electrographic developing technique.

The

conductive layers

22 and 25 can be any conductive material. If

layers

23 and 24 are self-supporting then layers 22 and 25 need be only a conductive film. Generally, however. layers 22 and 25 contribute mechanical strength to the other layers. A typically useful conductive material is. for example. aluminum sheet.

The charge accepting layer 23 can comprise an insu lating material. the principle requirement being the capability of holding a charge pattern on its surface until development. Typically useful materials are those instilative. filmforming polymeric materials which are commonly used in the electrophotographie art as binders for photoconductors. Materials of this type include styrene-butadiene copolymers; silicone resins; styrenealkyd resins; siliconealkyd resins; soya-alkyd resins; poly(vinyl chloride); poly(vinylidene chloride); vinylidene chloride-acrylonitrile copolymers; poly( vinyl acetate); vinyl acetate-vinyl chloride copolymers; poly(vi nyl acetals). such as poly(vinyl butyral); polyacrylic and methacrylic esters. such as poly(methyl methacrylate). poly(n-butyl methacrylate). poly( isobutyl metha crylate), etc; polystyrene; nitrated polystyrene; polymethylstyrene; isobutylene polymers; polyesters, such as polyl ethylene-co-alkylenebis( alkyleneoxyaryl phenylenedicarboxylate]; phenolformaldehyde resins; ketone resins; poly-amides; polycarbonates; polythiocarbonates; poly[ethylene-co-isopropylidene-2.2- bis(ethyleneoxyphenylene)terephthalate]; copolymers of vinyl haloarylates and vinyl acetate such as poly(vinyl-m-bromobenzoateco-vinyl acetate); etc.

The insulative layer which exhibits increased electrical conductivity 27 can be an air gap. a gap filled with some other insulative gas with high absorption for ionizing radiation. or some other suitably insulating material which exhibits increased electrical conductivity when struck by ionizing radiation. Typical examples of suitable insulating materials which exhibit these characteristics are any layers comprising a chemical element of Table II in an insulative. film-forming polymeric binder.

The neutron absorbing layer 24 may. for example, comprise any chemical element of Table I which has a suitably large neutron-cross-section and which produces secondary emanations upon absorbing. for example, thermal neutrons. The neutron absorbing layer can be electrically conductive in this embodiment.

FIG. 3 illustrates a particular embodiment ofthe neutron recording device of FIG. 2 in which the neutron absorbing layer 24 comprises a hot-pressed boron sheet. The reaction between boron and a neutron is given by the following equation:

'11 "B Li 4 The

conductive layers

22 and 25 can be. for example. aluminum sheet and the insulative layer 27 (which is rendered conductive by secondary emanations) is an air gap for purposes of this particular embodiment. When a neutron strikes and interacts with a boron nucleus a reaction takes place according to the above equation. The secondary emanations (alpha particles in this instance) thus produced can travel forward in the direction ofthc neutron beam as indicated at point X or they can travel back into the air gap in a reverse direction to the neutron beam as indicated at point X la order to form an electrostatic charge image of the neutrons. a high voltage potential is applied to the con ductive elements during exposure to neutrons. Ionizing radiation from the neutronboron interaction creates ionization of air within the air gap. A Townsend discharge may be supported. The net result is transport of charge to the insulating charge-accepting layer 23 adjacent to the neutron-struck areas, but not elsewhere.

One special advantage of this mode of operation is the relative insensitivity of the charge accepting layer 23 to effects of gamma ionized air. particularly if the converter 24 is not a gamma absorber. (Ionized air is a consequence of the gamma radiation that often accompanies thermal neutrons.) This means that the above process is less sensitive to certain gamma ray effects. Another advantage of this mode of operation is the possible high electrical gain (charge transferred per incident neutron) resulting from the radiation induced electrical discharge in the insulative gap 27. Another is most secondary emanations are charged particles which are readily absorbed in small distances by simple gas such as nitrogen, oxygen. etc.

The following examples will illustrate the neutron induced conductivity of the neutronabsorbing layers of the present invention.

EXAMPLE l A 4 mil thick poly(ethylene terephthalate) film support having a 0.24 OD (optical density) evaporated nickel layer was placed (nickel side down) upon a vacuum platen and charged in a negative corona to 2 KV surface potential. The charged film and platen were located in the path of a 14 MeV neutron beam having a flux of 8 X l0 neutrons/seocm for two minutes. The charged and exposed element was toned in an electrographic liquid developer comprising a colorant and a cobalt naphthenate charge agent dispersed in a mixture of cyclohexane and lsopar G (an isoparaffinic hydrocarbon from Humble Oil Co.) and compared to a con trol film which had received an identical treatment ex cept that no neutron exposure was utilized. The neutron exposed film had acquired lower quantity of toner (OD 0.38) compared to the unexposed control sample (OD 0.47) because of the neutron induced discharge of the electrostatic charge.

EXAMPLE 2 Twelve grams of nitrogen-flushed gadolinium oxide in a platinum boat was heated in a quartz combustion tube for 60 minutes in a muffle furnace under dry nitrogen at 400C. The sample was cooled under nitrogen to room temperature. Ten grams of the gadolinium oxide was ball-milled 24 hours in a l20 cc. glass jar contain ing 6.7 grams of a Pliolite S5 toluene mixture composed of 30 weight percent Pliolite S-S (styrene butadiene copolymer purchased by Goodyear Rubber Co.) and 7.0 grams of toluene. The mixture was ball milled for 24 hours and then coated at 50 percent solids on nickel coated poly(ethylene terephthalate) support at a wet thickness of 0.010 inch using a doctor blade.

In order to measure the neutron induced current of the element made according to the above procedure. electrodes were connected to the gadolinium oxide film and a constant bias was applied. When the displacement current had decreased to a very small value 1.2 X l0 A) the film was subject to a 14 MeV neutron exposure. The current through the film rose by nearly two orders of magnitude (to 76 X 10 A). increasing as the neutron flux was increased to l X It)" neutronslcm lsec. The current decreased abruptly (to 12 X l(J"- A) when the neutron source was shut off.

The above examples illustrate the change in conductivity of neutron-absorbing layers of the present invention when said layers are subjected to a neutron flux.

The invention has been described in detail with particular reference to certain preferred embodiments thereof. but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

We claim:

1. An element for reducing a direct image neutron radiograph, said element being capable of holding an electrostatic charge and comprising means for absorbing neutrons and generating a current by dissipation of said electrostatic charge in proportion to the number of neutrons absorbed, said neutron absorbing means comprising an insulative layer comprising a neutron absorbing agent in a concentration of at least 10 atoms per cubic centimeter.

2. An element for recording a direct image neutron radiograph. said element being capable of holding an electrostatic charge and comprising means for absorbing neutrons and generating a current by dissipation of said electrostatic charge in proportion to the number of neutrons absorbed. and means contiguous with said neutron absorbing means for conducting said current. said neutron absorbing means comprising an insulative layer comprising a neutron absorbing agent in a concentration of at least l0 atoms per cubic centimeter.

3. An element according to claim 2 wherein the neu tron absorbing means comprises an insulative layer having a resistivity of at least 10'" ohm-cm.

4. An element according to claim 2 wherein the neutron absorbing agcnts have a ratio of neutron absorption coefficient to X-ray absorption coefficient of at least unity and are present in a concentration range from about 10 to about 10'' atoms per cubic centime ter.

5. An element for recording a direct image neutron radiograph. said element being capable of holding an electrostatic charge and comprising means for absorbing neutrons and generating a current by dissipation of said electrostatic charge in proportion to the number of neutrons absorbed. and means contiguous with said neutron absorbing means for conducting said current. said neutron absorbing means comprising an insulative layer comprising a neutron absorbing agent in a concentration of at least [0 atoms per cubic centimeter. said neutron absorbing agent having a neutron crosssection per atom of at least one barn, and having a ratio of neutron mass absorption coefficient to gamma ray absorption coefficient of at least unity.

6. An element according to claim 5 wherein the neutron absorbing means comprises an insulative layer having a resistivity of at least l0 ohnrcm.

7. An element according to claim 5 wherein the neutron absorbing agents are present in a concentration range from about l0 to 10 atoms per cubic centimeter.

8. An element according to claim 5 wherein the neutron absorbing agent is an organic material having a high hydrogen content.

9. An element according to claim 5 wherein the neutron absorbing agent is an organometallic compound containing an isotope of an element selected from the group consisting of boron, cadmium, d) sprosium. europium. gadolinium. gold. indium. lithium. rhodium. sa marium and silver.

10. An element according to claim 9 wherein the neutron absorbing agent is an organometallie compound containing an isotope selected from the group consisting of boron, "B. and lithium. "Li.

11. An element according to claim 9 wherein the neutron absorbing agent is an organometallic compound selected from the group consisting of tri B-naphthyl borate.

tri a-naphthyl borane.

methylamine borane,

bis(methylamine)borane.

tritmethylamine)borane,

gadoliniumhexaantipyrine chloride.

hexaantipyrene gadolinium (Ill) perchlorate.

triphenylborine amine, and

copper triphenylphosphineborane.

12. An element according to claim 9 wherein the organometallic compound comprises a tetraarylborate.

13. An element according to claim 5 wherein the neutron absorbing agent comprises an element selected from the group consisting of berylium, boron, cadmium, carbon. chlorine. eupopium, gadolinium. hydrogen, lithium. nitrogen, oxygen and samurium.

[4. An element according to claim 13 wherein the neutron absorbing agent is selected from the group consisting of B BN, BP, BC. CdS, CdF Gd O and gadolinium gallium garnet.

15. An element according to claim 5 wherein the neutron absorbing agent is a composition comprising CdS, CdO and B 0 16. An element according to claim wherein the neutron absorbing agent is a composition comprising about 4.l percent by weight CdS, about 48.8 percent by weight CdO, and about 47.1 percent by weight B 0 17. An element according to claim 15 wherein the neutron absorbing agent is a composition comprising about 4.9 percent by weight CdS. about 46.4 percent by weight CdO, and about 48.7 percent by weight B 0 18. An element for recording a direct image neutron radiograph, said element being capable of holding an electrostatic charge and comprising means for absorbing neutrons said neutron absorbing means simultaneously emanating secondary radiation, means for enhancing the effect of the neutron beam by capturing said secondary emanations and generating a current by dissipation of said electrostatic charge in proportion to the number of neutrons absorbed, and means for conducting said current. said neutron absorbing means comprising neutron absorbing agents in a concentration of at least l0 atoms per cubic centimeter, said neutron absorbing agents having a neutron crosssection of at least one barn, having a ratio of neutron absorption coefficient to gamma ray absorption eoeffieient of at least unity. said neutron enhancing means comprising an ionizing radiation sensitive insulative material.

19. An element according to claim 18 wherein the neutron absorbing means comprises the element boron.

20. An element according to claim 19 wherein the neutron enhancing means comprises an ionizing radiation sensitive insulative material selected from the group consisting of lead oxide and amorphous selenium.

21. A process for using a neutron flux to make a direct image neutron radiograph of a test object on an element, said element being capable of holding an elec trostatic charge and comprising means for absorbing neutrons and generating a current by dissipation of said electrostatic charge in proportion to the number of neutrons absorbed, said neutron absorbing means comprising an insulative layer comprising a neutron absorbing agent in a concentration of at least l0 atoms per cubic centimeter,

said process comprising:

l. applying an electrostatic charge to the surface of said element;

2. exposing said test object to said neutron flux in the presence of said element to dissipate the electro' static charge in those areas of the surface of said element which correspond to the areas of said test object which are transparent to said neutron flux, thereby forming an electrostatic charge image of said test object on the surface of said element; and

3. developing said electrostatic charge image.

22. A process using a neutron flux to make a direct image neutron radiograph of a test object on an image recording means, the image recording means having means for absorbing neutrons and simultaneously producing secondary emanations. means for accepting and holding an electrostatic charge. means for insulating the neutron absorbing means from the chargeaccepting means, the insulating means being interposed between the neutron absorbing means and the chargeaccepting means and exhibiting increased electrical conductivity when struck by the secondary emanations, means for conducting charge to or from the charge accepting means, and means for conducting charge to or from the neutron absorbing means,

said process comprising:

l. applying a voltage across the image recording means;

2. exposing the test object to the neutron flux in the presence of the image recording means. thereby forming an electrostatic charge image of the test object on the charge-accepting means of the image recording means; and

3. developing the electrostatic charge image.

Claims

1. applying a voltage across the image recording means;

1. applying an electrostatic charge to the surface of said element;

1. An element for reducing a direct image neutron radiograph, said element being capable of holding an electrostatic charge and comprising means for absorbing neutrons and generating a current by dissipation of said electrostatic charge in proportion to the number of neutrons absorbed, said neutron absorbing means comprising an insulative layer comprising a neutron absorbing agent in a concentration of at least 1017 atoms per cubic centimeter.

1. AN ELEMENT FOR REDUCING A DIRECT IMAGE NUETRON RADIOGRAPH, SAID ELEMENT BEING CAPABLE OF HOLDING AN ELECTROSTATIC CHARGE AND COMPRISING MEANS FOR ABSORBING NEUTRONS AND GENERATING A CURRENT BY DISSIPATION OF SAID ELECTROSTATIC CHARGE IN PROPORTION TO THE NUMBER OF NEUTRONS ABSORBED, SAID NEUTRON ABSORBING MEANS COMPRISING AN INSULATIVE LAYER COMPRISING A NEUTRON ABSORBING AGENT IN A CONCENTRATION OF AT LEAST 10**17 ATOMS PER CUBIC CENTIMETER.

2. An element for recording a direct image neutron radiograph, said element being capable of holding an electrostatic charge and comprising means for absorbing neutrons and generating a current by dissipation of said electrostatic charge in proportion to the number of neutrons absorbed, and means contiguous with said neutron absorbing means for conducting said current, said neutron absorbing means comprising an insulative layer comprising a neutron absorbing agent in a concentration of at least 1017 atoms per cubic centimeter.

2. exposing said test object to said neutron flux in the presence of said element to dissipate the electrostatic charge in those areas of the surface of said element which correspond to the areas of said test object which are transparent to said neutron flux, thereby forming an electrostatic charge image of said test object on the surface of said element; and

2. exposing the test object to the neutron flux in the presence of the imaGe recording means, thereby forming an electrostatic charge image of the test object on the charge-accepting means of the image recording means; and

3. developing the electrostatic charge image.

3. developing said electrostatic charge image.

3. An element according to claim 2 wherein the neutron absorbing means comprises an insulative layer having a resistivity of at least 1010 ohm-cm.

4. An element according to claim 2 wherein the neutron absorbing agents have a ratio of neutron absorption coefficient to X-ray absorption coefficient of at least unity and are present in a concentration range from about 1021 to about 1023 atoms per cubic centimeter.

5. An element for recording a direct image neutron radiograph, said element being capable of holding an electrostatic charge and comprising means for absorbing neutrons and generating a current by dissipation of said electrostatic charge in proportion to the number of neutrons absorbed, and means contiguous with said neutron absorbing means for conducting said current, said neutron absorbing means comprising an insulative layer comprising a neutron absorbing agent in a concentration of at least 1017 atoms per cubic centimeter, said neutron absorbing agent having a neutron cross-section per atom of at least one barn, and having a ratio of neutron mass absorption coefficient to gamma ray absorption coefficient of at least unity.

6. An element according to claim 5 wherein the neutron absorbing means comprises an insulative layer having a resistivity of at least 1010 ohm-cm.

7. An element according to claim 5 wherein the neutron absorbing agents are present in a concentration range from about 1021 to 1023 atoms per cubic centimeter.

9. An element according to claim 5 wherein the neutron absorbing agent is an organometallic compound containing an isotope of an element selected from the group consisting of boron, cadmium, dysprosium, europium, gadolinium, gold, indium, lithium, rhodium, samarium and silver.

10. An element according to claim 9 wherein the neutron absorbing agent is an organometallic compound containing an isotope selected from the group consisting of boron, 10B, and lithium, 6Li.

11. An element according to claim 9 wherein the neutron absorbing agent is an organometallic compound selected from the group consisting of tri Beta -naphthyl borate, tri Alpha -naphthyl borane, methylamine borane, bis(methylamine)borane, tri(methylamine)borane, gadoliniumhexaantipyrine chloride, hexaantipyrene gadolinium (III) perchlorate, triphenylborine amine, and copper triphenylphosphineborane.

13. An element according to claim 5 wherein the neutron absorbing agent comprises an element selected from the group consisting of berylium, boron, cadmium, carbon, chlorine, eupopium, gadolinium, hydrogen, lithium, nitrogen, oxygen and samurium.

14. An Element according to claim 13 wherein the neutron absorbing agent is selected from the group consisting of B2O3, BN, BP, BC, CdS, CdF2, Gd2O3 and gadolinium gallium garnet.

15. An element according to claim 5 wherein the neutron absorbing agent is a composition comprising CdS, CdO and B2O3.

16. An element according to claim 15 wherein the neutron absorbing agent is a composition comprising about 4.1 percent by weight CdS, about 48.8 percent by weight CdO, and about 47.1 percent by weight B2O3.

17. An element according to claim 15 wherein the neutron absorbing agent is a composition comprising about 4.9 percent by weight CdS, about 46.4 percent by weight CdO, and about 48.7 percent by weight B2O3.

18. An element for recording a direct image neutron radiograph, said element being capable of holding an electrostatic charge and comprising means for absorbing neutrons said neutron absorbing means simultaneously emanating secondary radiation, means for enhancing the effect of the neutron beam by capturing said secondary emanations and generating a current by dissipation of said electrostatic charge in proportion to the number of neutrons absorbed, and means for conducting said current, said neutron absorbing means comprising neutron absorbing agents in a concentration of at least 1017 atoms per cubic centimeter, said neutron absorbing agents having a neutron cross-section of at least one barn, having a ratio of neutron absorption coefficient to gamma ray absorption coefficient of at least unity, said neutron enhancing means comprising an ionizing radiation sensitive insulative material.

21. A process for using a neutron flux to make a direct image neutron radiograph of a test object on an element, said element being capable of holding an electrostatic charge and comprising means for absorbing neutrons and generating a current by dissipation of said electrostatic charge in proportion to the number of neutrons absorbed, said neutron absorbing means comprising an insulative layer comprising a neutron absorbing agent in a concentration of at least 1017 atoms per cubic centimeter, said process comprising: