US 3609359 A
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Description (OCR text may contain errors)
United States Patent inventors Eugene Wainer;
Richard A. Fotland, both of 2905 East 79th St., Cleveland, Ohio 44104 Jan. 8, 1969 Sept. 28, 1971 Appl. No. Filed Patented X-RAY IMAGE INTENSIFIER WITH ELECTRON MICHROCHANNELS AND ELECTRON MULTIPLYING MEANS 38 Claims, 5 Drawing Figs.
US. Cl 250/65 R, 204/35, 250/213 VT Int. Cl. G031) 41/16 Field of Search ..250/65, 213 VT qlncidenLb Rays?  References Cited UNITED STATES PATENTS 3,045,1 17 7/ 1962 Beatty 250/65 3,282,697 11/1966 Blank 250/65 3,394,261 7/1968 Manley 250/213 Primary Examiner-James W. Lawrence Assistant Examiner-C. E. Church Attorney-Lawrence 1. Field X-RAY IMAGE INTENSIFIER WITH ELECTRON MICHROCHANNELS AND ELECTRON MULTIPLYING MEANS This invention relates to. the amplification of images produced in photosensitive film as a result of imagewise exposure to X-radiation.
In the biomedical field, X-ray image intensification is desired in order to diminish the dose of radiation to which a subject is exposed while at the same time permitting sufficient information to be'placed on the X-ray photoreceptor so as to allow adequate analysis of the results by the physician or surgeon orother individual seeking information to be disclosed by theexposure to such X-radiation. In addition, in view of the increasing use of X-ray examination techniques in industry for quality control purposes, reduction of exposure time in industrial radiology is also desired so as to increase the efficiency and lower the cost of such quality control techniques.
Presently known X-ray amplifiers or intensifiers are bulky, expensive pieces of equipment based on vacuum tube or Geiger-Muller counters in conjunction with complex circuitry. The present invention is a simple, compact unit which can be fitted into the usual radiological cassette normally used for biomedical exposures and which can be used with no significant change in the present practice for taking X-rays except that smaller amounts of X-radiation may be used in the exposure as a consequence of the amplification available from the present invention.
It is a principal object of this invention to supply a thinlayered solid state device capable of intensifying the photographic effects of exposure to X-rays and capable of operating in nonvacuum situations. More explicitly, it is not necessary to enclose the solid state X-ray intensifying device of this invention in a vacuum chamber, while the exposure is being made.
lt is a further object of this invention to supply a solid state thin-layered device suitable for intensification of X-rayexposure by taking advantage of the photoelectrons produced from such X-ray exposure, utilizing such photoelectrons to produce secondary electrons by secondary emission phenomena, accelerating these secondary emitted electrons through a short distance by means of an appropriately applied potential, and impacting such electrons on an electron beam, photosensitive receptor.
It is a further object of this invention to provide a thin- Iayered solid state structure for X-ray intensification purposes in which gas of chosen nature located in a suitably structured microannular section is ionized as a consequence of the impact of the X-radiation, the ions produced therefrom are then accelerated through said structure by suitable application of potential, and the X-ray image is reco'rded on an ion-sensitive photoreceptor.
It is a further object of this invention to locate and restrict the structure of the multiplication layer so as to retain the resolution inherent in the incident X-ray beam.
For ease of description, the mode of operation which utilizes secondary electron-emitting structures, in which the electron beam sensitivity of the film is important, may properly be designated as the X-ray secondary electron emission mode" and the multiplying structure which utilizes the formation of accelerated ions or other charged particles as the means for recording the image on ion-sensitive photoreceptor may be designated as the Geiger-Muller mode." As will be seen from the description to follow in this specification, structures to be used for-electron multiplication purposes are identical with structures which use the Geiger-Muller" mode, but their mode of use is different.
in order to achieve the foregoing generally stated objectives, the solid state layered X-ray-intensifying structure which is the basis for this invention must provide a variety of functions. First, the intensifier structure should contain a sufficient amount of high atomic number elements in order to absorb a significant portion of the incident X-ray beam. Through the medium of the present invention, this absorption of X-rays in the medical and low-energy industrial X-ray regionis accompanied with the production, with a high efficiency, of highenergy photoelectrons. Suitable voltage ranges for such initial X-rays are generally within 10 and 300kilovolts. A second function of the intensifying structure involves the means for providing secondary electrons through utilization of secondary emission principles from the high-energy photoelectrons usually be actions occurring in or on the surface of the solid. A third function of the intensifying structure involves a means for providing lower energy charged particles, (generally ions ,4
or charged atoms, or molecules) from the high-energy photoelectrons through actions occurring either in a solid or a gas. The second and third functions described briefly in the foregoing sentences may not be used simultaneously and in a practical sense it is best to have different types of modes of use to supply respectively the second and third functions. While the second function is accomplished by means for increasing the number of electrons available from the initial primary photoelectrons produced by each absorbed X-ray photon, the third function is accomplished by means of increasing the number of charged particles available from each absorbed X- ray photon. A fourth and important function performed by the intensifier structure of the present invention isthe acceleration of the electrons and/or the charged particles through an electrical field so that they are able to impinge on the photoreceptor with increased energy, wherein they form either a latent image capable of amplification by development, or a visible printout image depending on the nature of the photoreceptor films. The image rendition in final form may be comprised of dye molecules, a polymer, or metal atoms such as silver.
In summary, therefore, the intensifier is required to provide significant absorption of the incident X-ray beam and in the present invention this is accomplished by means which convert the absorbed X-rays to high-energy photoelectrons. In the present device, means are also provided to convert the highenergy photoelectrons to low-energy electrons, positive ions, negative ions or charged particles. Positive ions, negative ions and charged particles are all included in the category of charged particles and represent the Geiger-Muller mode of amplification. When conversion of the high-energy photoelectrons is from primary electrons to lower energy electrons, and no further change occurs, this represents the secondary emission electron amplification mode and procedures to be described as part of the incident invention are significantly different depending on which mode is utilized. Whether electrons or charged particles are produced, these are accelerated by means of a properly connected DC electric field and are caused to impinge on a film which is sensitive to electrons and/or charged particles whereby a latent and/or visible image is formed capable of further amplification in the majority of cases by specified development procedures.
The invention will be better understood from the description which follows taken into conjunction with the attached drawings, in which:
FIG. 1 is a schematic showing in section of one means for practicing the invention; and
H65. 2 through 5 are similar views of modifications of the device of FIG. 1.
As shown in FIG. 1, the apparatus comprises two electrodes l0, 12 which provide an electric field when connected to a battery 20 or other source of potential, the field extending across the intensifying structure and the image receptor. An insulating film base 14 is provided as a support for a chargesensitive coating 16. A multiplying layer 18 is provided for the purpose of converting the higher energy photoelectrons to a larger number of electrons and/or to a larger number of charged particles. In FIG. 1, the electrode 10 may be the X- ray absorber and may comprise a thin sheet of metallic lead. When metallic lead is not used aselectrode l0, electrode 10 is then advantageously a thin sheet of Al.
Electrode 12 may be laminated directly on the insulating layer 14 and may consist of a thin aluminum foil or the electrode 12 may be painted or vacuum deposited on the insulating layer base which is generally comprised of a dimensionally stable transparent plastic film such as a polyester, polycarbonate, polystyrene, cellulose acetate, or the like. Furthermore, the electrode 12 may be formed by the metallic surface in a cassette which holds the remainder of the laminate of FIG. I under proper conditions of pressure and proximity.
As indicated, film base 14 may consist of any insulating transparent plastic material. The thickness of the film base is not critical; 3 to 7 mils being convenient working thicknesses. The preferred sensitive coating 16 is hereinafter described. In the case of the polyesters at a micron thickness, 20 kilovolt electrons are completely absorbed in such a layer as are ions accelerated through several kilovolts.
Electrode may, under certain circumstances, act as the X-ray absorber layer and will then contain a high proportion of elements having an atomic number higher than silverx Lead, bismuth, uranium, and thorium and their compounds being the preferred elements for the X-ray absorber layer. An absorber formed of one of these high atomic number elements in metallic form absorbs 70 percent or more of the incident X- rays having energies of 50 to 75 kilovolts if the layer is approximately 5 mils thick. A 2 mil layer absorbs 10 to percent of the incident X-rays. Unfortunately, the range of high-energy photoelectrons in metals formed of high atomic number elements is only 0.1 to 0.4 mils (2.5 to 10 microns). It is thus preferably that, in some cases, rather than using a single metallic X-ray-absorbing layer, the absorber should be formed of a high atomic number compound or particles of the metal of such compound structured in such a manner that it would provide both hard absorption for incident X-rays and a short path distance for the photoelectrons to emerge from the absorbing media. In the embodiment of the intensifier structure shown in FIG. 1, the absorber may function as the electrode 10 and consist of a sheet of lead foil 0.5 to 2 mils in thickness, as indicated previously, or the absorber may consist of particles of compounds of lead, bismuth, uranium, or thorium, or metals or oxides of these metals dispersed in a volume percentage of at least 20 percent and not greater than 50 percent in a partially conducting plastic film former such as polyvinyl alcohol, gelatin, methyl cellulose, and the like containing a sufficient amount of glycols or other high-boiling-point alcohols with or without the presence of small amounts of boric acid to increase the conductivity of such oxygen-containing plastic binders in a manner well known to those skilled in the art so that under the high potential field applied across the package this surface containing dispersed high atomic number particles is sufficiently conductive to act as an electrode. When made in this just-described form, the electrode 10 is the multiplying structure and layer 18 is not required, in certain cases, though the presence of layer 18 always represents an advantage.
In a more or less pure Geiger-Muller mode, representing the least-preferred embodiment of this invention, the multiplication layer 18 may consist of a gas-containing region, and the spacing between the X-ray absorber layer 10 and the chargesensitive film layer 16 may range from about 2 to 10 mils. This spacing is maintained by insulating spacers 22 at the edges of the device which separate the electrode 10 and the chargesensitive layer 16 by the appropriate distance. it is not necessary that the spacer 22 constitute a hermetic seal. In the device of FIG. 1, this region is filled with a gas or gases; and at the operating potential of the device, a gas discharge occurs which is excited by an X-ray-generated photoelectron. In order for this device to act in a practical sense, it needs to be enclosed in a cassette which can be gas filled with an appropriate gas at the appropriate pressure and is capable of being hermetically sealed readily through the use of suitable gaskets.
The gas pressure and potentials are selected so that the gas discharge operates in the Geiger-Muller region, thus providing a high level of charged particle multiplication. The gas may be composed of argon or nitrogen or mixtures of the two together with small quantities of alcohol, methane or halogen which serves as a quenching gas. This quenching gas eliminates the possibility of a continuous discharge in the multiplying region. As one alternative, the quenching gas may be eliminated since the intensifier is in effect self-quenching by virtue of the charge buildup across the insulating base 14. In the selfquenching condition, however, the potential must only be applied for short periods of time during excitation with X-rays since stray radiation will also initiate the discharge. The gas pressure ranges from a few millimeters of mercury up to atmospheric pressure; the higher pressure levels being useful for small electrode spacings.
in a preferred embodiment of the Geiger-Muller mode of operation, the multiplying layer 18 is composed of materials which may or may not be high atomic number X-ray absorbers and are not required to be X-ray absorbers particularly if electrode 10 is in the form of such an X-ray absorber as hitherto described. It is a preferred embodiment of the Geiger-muller mode that the region 18 may be in the form of either a porous film, open mesh, a grid, or a layer containing a large number of channels, such channels leading through the layer 18 from electrode 10 to sensitive layer 16.
Suitable porous structures may be utilized for layer 18 in a variety of ways. For example, ultrafine lead oxide powder is mixed with microglass spheres in a solution of a thermoplastic resin and is spread on the underside of electrode 10 in wet film thicknesses so that after drying a dried layer thickness of l to 2 mils is obtained. This is accomplished by mixing 50 volume parts of lead oxide with 50 volume parts of the thin-walled microglass spheres. These dry powders are thoroughly mixed in a solution of l0 parts by volume of polystyrene in parts by volume of benzene and after mixing, the mixture is spread on the underside of electrode 10 with a doctor blade and the solvent allowed to evaporate at room temperature. When the underside of electrode 10 is made of aluminum, adhesion of this dried thermoplastic layer is good in spite of its low resin content, even without specialized treatment of the aluminum surface.
Altemately, and preferably, the structure 18 may be produced in the form of lead oxide grid, which may be either a sintered form of lead oxide which is then processed to provide a number of open holes through the structure or preferably may comprise a dispersion of 30 to 50 parts by volume of ultrafinely divided lead oxide in 70 to 50 parts by volume of polystyrene or polypropylene. In order to enhance the emission of secondary electrons, the lead oxide advantageously may contain 5 to 20 percent of a compound of the alkali metal and alkaline earth metals and particularly their halides. The desired porous structure is then developed by using conventional photoresist techniques, followed by suitable etching and controlled back washing. For film thicknesses of the order of l to 2 mils or less, the combination of the use of photoresist techniques and etching is easily capable of yielding pore sizes of the order of 0.5 to 2 microns with center-to-center distances between pores of not more than 50 percent of the diameter of the individual pore, thus yielding an open-toclosed area of about 50 percent or more.
When oxides of metals of high atomic number, e.g. oxides of lead, bismuth, uranium or thorium, are utilized as the multiplying layer 18, they act not only as the multiplying layer particularly when salts of alkalis and alkaline earths are incorporated, but also as the X-ray absorber and it is not always necessary to utilize in addition an X-ray absorber such as electrode 10. In this case, the electrode 10 may be a sheet of aluminum or a similar low atomic number material. For very hard X-rays, the electrode 10 is a 2 to 4 mil sheet of lead.
While the use of the oxides of these high atomic numbered metals may be preferred as described in previous sentences, finely divided particles of the metals themselves may be utilized in this same manner though they are somewhat leas convenient for preparing the structure than the oxides. Thin sheet metallic lead may be utilized appropriately through the medium of developing a pore structure initially with a combination of photoresist and etching techniques. The mesh thus produced is then electrochemically oxidized after which it is suitable for use as the multiplying layer 18.
Alternately, multiplying layer 18 may be a high lead glass or similar material structured in the form of channels running perpendicularly between layer and layer 16, these channels being produced by techniques well known to those skilled in the art. Such high lead glasses serve the dual purpose of X-ray absorber and secondary electron emitter. Such lead glass channel structures may be obtained with straight sidewalls with diameters as small as 1 to 2 microns with open-to-closed ratios of the order of 50 percent. When used for Geiger- Muller mode, the height of the channels should be of 2 to 10 mils. When used for the secondary electron emission mode, the height of the channels should be approximately 60 to 100 times that of the diameter of the individual channel. Again, when such a lead glass is used as the multiplying layer 18, it is not necessary to add an X-ray absorber to electrode 10, except when very hard X-rays are used.
When used in the Geiger-Muller mode, the advantage of the structured, porous, or channeled layer 18, is that maintenance of the proper composition of the gas at suitable pressure is facilitated without the use of special devices during exposure with assurance that stray radiation will not be capable of initiating the discharge in view of the insulating protection of the mesh structure. Further assurance is had by predefining the thickness of this mesh channel-type structure of maintaining the proper position and proximity between the electrode 10 and the charge-sensitive layer 16.
When operating the Geiger-Muller mode in accordance with the device shown in FIG. 1, electrode 10 is positive and electrode 12 is negative.
A most useful multiplying layer 18, and a preferred embodirnent'of this invention, is comprised of porous aluminum oxide which may or may not be used in the partially hydrated condition. Films such as this are readily prepared by anodizing aluminum foil in an oxalic acid bath at anodizing potentials in the region of 100 to 300 volts and at 5m 15 amperes per square decimeter, e.g. as described in Wernick or any of the standard texts on anodizing. After a period of 90 minutes, the porous aluminum oxide films containing some water of hydration and ionic residues in small percentages from the oxalic acid bath are formed having thicknesses in a region of 2 to 3 mils. The porosity arises due to the formation of a large number of channels running through the film. The porosity of an anodized aluminum oxide film, i.e. the channel diameter, may be increased by etching the aluminum oxide in suitable etchants, such as 5 percent oxalic acid at 60 to 7 C. or other known etchants.
In the general anodizing procedure as defined above, pores are formed continuously in the anodizing layer as the layer grows in the electrochemical process, these pores having a diameter of approximately 0.03 microns. These pores are relatively straight walled and usually comprise an open-to-closed ratio of about percent. In other words, 20 percent of the surface of the anodized layer is comprised of these 0.03 micron diameter pores and channels. The bottom of the pore next to the metal is covered with a barrier layer comprised of aluminum oxide having a thickness of approximately 50 to 75 angstrom units. By controlled etching, this barrier layer is removed and whether removed or not it is thin enough so that it represents no significant barrier to the passage of electrons.
By suitably controlled etching procedures, the pores can be opened up substantially without such etching procedures having any major effect on the reduction of the overall thickness of the anodized layer. With such controlled etching procedures of known nature, the pores may be opened up to a diameter of 0.1 to 0.8 microns, such pores having substantially straight-sided walls. Further, as a consequence of the etching, the open-to-closed ratio may be defined at will depending on the severity and time of the etching from an open-to-closed ratio of approximately 30 percent to, in some cases, as high as 70 percent. An optimum range of values for pore size and open-to-closed ratio is a pore size in the region of 0.2 to 0.5 microns in diameter with an open-to-closed ratio of 50 percent, when the anodized layer is 25 to 50 microns thick.
A very valuable property of these anodized layers whether utilized in the unetched condition or etched condition is their surprising ability to absorb and adsorb chemicals from solution on the inner walls of these pores. These solutions pass into the pores with extreme rapidity by capillary action and the solution can proceed completely to the bottom of the pore in a time-soaking period not exceeding 3 minutes. Thus, the lining action by abstraction of chemicals from the properly prepared solution is complete and uniform.
The anodized layer may be treated in several difierent ways depending not only on the mode of amplification it is expected to undergo (i.e. the Geiger-Muller mode or the secondary electron multiplication mode) but also on the voltage level at which it is expected to be utilized. For example, an anodized layer which is 2 mils thick has been etched to a pore size of approximately 0.3 microns with an open-to-closed ratio of approximately 50 percent may be utilized in the soft X-ray region and in the Geiger-Muller mode without the use of an X- ray absorber either on the surface of the aluminum electrode 10 or on the walls of the pores. Under these conditions, an amplification of a factor of approximately 3 is obtained for X- rays below 50 kilovolts and this type of mode for the multiplying layer 18 is most effective in the l to 10 kilovolt X-ray range.
If, after etching to the geometry defined in the previous paragraph, the anodized aluminum layer and its supporting aluminum backing is heat treated in hydrogen at 550 C. for 1 hour and then allowed to cool to room temperature after which the assembly is then stored in a flushing atmosphere comprised of 98 percent nitrogen and 2 percent methane, and repeatedly flushed to remove air from the pores and replace such air with the nitrogen-methane atmosphere, and then placed into the assembly as shown in FIG. 1, without the addition of an specialized X-ray absorbers, an amplification factor of 25 to 50 is then obtained in the Geiger-Muller mode, particularly for X-rays at 10 kilovolts and below. It appears that the capability of these pores for retaining gas is extremely high and lamination of the sensitive surface 16 against such open gas-containing pores with mechanical pressure is sufficient to hold the gas in the pores until the exposure is completed.
The preferred technique is to line the pores with a compound of a high atomic number which may act not only as an X-ray absorber for the production of primary photoelectrons, but may also contain, preferably, a secondary emitter of electrons in thin layers in and on the surface of such lining of X- ray absorber. Suitable materials which may serve as such secondary emitters are compounds of the alkalis and the alkaline earths. Under these conditions, the amplifier operates well in the secondary electron emission mode (i.e. where electrode 10 is negative) and is capable of amplifying the effects of the exposure to X-rays over the entire X-ray region whether soft or hard X-rays are being considered. Amplification factors of at least 100 are usually obtained and under carefully selected circumstances factors of 1,000 or more may be achieved.
In carrying out the preparation of the anodized aluminum layer for these purposes the film is first etched as heretofore described so as to produce a pore size in the region of 0.5 to 0.6 microns in diameter with an open-to-closed ratio in the range of 50 to 70 percent. After etching, Gieger-Muller pores are then impregnated with a 5 percent solution of one of the acetates of a high atomic number metal taken from the class of lead, bismuth, thorium or uranium and such solution may or may not be complexed with an amount of potassium acetate not exceeding 20 percent by weight of the lead acetate. The acetates are retained in clear solution by the addition of glacial acetic acid in the amount of 10 percent by volume of the total volume of the solution. The etched anodized aluminum layer is then soaked in such solution for 5 minutes, the surface of the layer swabbed with water damp cotton, after which it is then dried at room temperature, after which the assembly is heated to C. and maintained at this temperature for 30 minutes. The assembly is then heated to 500 C. for 30 minutes. A single treatment of this type is sufficient to provide a uniform layer on the walls of the pores approximately 0.05 microns in thickness.
The amplification factor for secondary emission mode of this alkali and heat-treated layer is in the range of 300 to 1,000 and for the Geiger-Muller mode about 200 to 800. A variation of this procedure for deposition of high atomic number metal compounds is particularly applicable to compounds of lead. In this case, a solution of lead acetate in the amount of percent, thiourea in the amount of 5 percent and sodium hydroxide in the amount of percent is prepared in methyl alcohol. The absence of water in this solution ensures that the solution is not an etchant for the aluminum oxide surface. The pores are soaked in such a solution for approximately 2 minutes after which the surface is wiped off with a cloth and about 3 or 4 minutes thereafter a thin shiny film of lead sulfide starts to deposit on the inside of the pores and such deposition is complete within 2 or 3 minutes thereafter. The film in this form is first washed with two washings of methyl alcohol, followed by washing in water after which the film is then dried and made ready for use by final firing at 550 C. for a period of 30 minutes in an atmosphere comprising 90 percent nitrogen and I0 percent hydrogen sulfide.
This type of lining showed good response to the secondary electron mission mode possibly due to entrapped alkali ions in the lead sulfide surface exhibiting an amplification for X-rays of 300 to 1,000, and about 150 to 600 for the Geiger-Muller mode.
The determination as to whether the plate is to be used in the Geiger-Muller mode involving the formation of positive ions or the secondary electron mode is a function of the nature of the gas that is present in the pores. If, after firing, the pores are saturated with the gas comprising a mixture of nitrogen and methane as defined previously and the sensitive surface 16 laminated to the surface of such pores immediately thereafter, then the device is most suitable for the Geiger- Muller mode in which the electrode 10 is charged positively. If, however, sensitive surface 16 is contacted to the open surface of the electron-multiplying area 18 after it has been treated with a mixture of high atomic number compounds, plus minor amounts of an alkali compound such as potassium salts under vacuum conditions so that little or no gas remains, then the layer 16 is an effective seal and will maintain the vacuum chamber, these periods of maintenance of such vacuum extending for several hours. On this basis, electrode 10 is negative and in both instances amplification factors of a factor of 1,000 or more are achieved throughout the X-ray region whether soft or hard X-rays are being considered.
Thus, a preferred embodiment of the invention is the use of an etched, anodized layer, retaining a sufficient thickness of the aluminum on the back of the anodized layer to maintain strength, impregnating the pores which have been opened up by etching to a diameter above 0.5 microns with compounds of lead, bismuth, thorium or uranium, and adding to the impregnating solution minor percentages of the acetate salts of the alkalis or the alkaline earths, heat treating at temperatures up to 550 C. for periods of time sufficient to produce a stable compound of the impregnating solution on the walls of the pores, using a heat-treating atmosphere which maintains the desired compound in the desired stable condition, and utilizing such a device either in the Geiger-Muller gas-filled mode which emphasizes the production of positive ions, or positively charged particles, or in the secondary electron emission mode which emphasizes the formation of electrons only with little or no formation of ions, the Geiger-Muller mode being obtained if the pores are filled with a gas comprising a mixture of a major amount of argon or nitrogen with a minor amount of methane, and the secondary electron emission mode being maintained if the pores are essentially gas-free.
A second preferred modification and embodiment of the invention but somewhat less preferred than the one utilizing the anodized aluminum layer is to use a channel structure 18 comprised chiefly of lead oxide which may be made through the combination of photoresist, followed by controlled etching. In this case, an electrode 10 is then laminated to the surface of this lead oxide mesh in which the electrode 10 may be a thin sheet of aluminum or metallic lead. Again, as before, in this embodiment, the surface of the lead oxide mesh may be impregnated with slats of alkalis or alkaline earths to enhance the secondary electron emission in case this mode is desired.
FIG. 1 is a diagram of one physical embodiment of the invention irrespective of the mode used and defining one type of electrical connection. In this particular case, the electrode may be 1 to 5 mils in thickness, the multiplying layer 18 may be between 2 and 10 mils in thickness, the photosensitive layer 16 is generally 3 to 10 microns in thickness, the insulating layer 14 may be between 3 and 7 mils in thickness and electrode l2, 2 to 5 mils in thickness. The separators 22 are utilized only when the pure Geiger-Muller mode is used and when area 18 is filled with the Geiger-Muller type of gas. These separators 22 are not necessary when either lead oxide mesh or an anodized layer of aluminum is utilized. Under conditions of FIG. 1, the potential across the entire package should be at least 10 kilovolts and may be as high as 20 kilovolts, 15 kilovolts being considered the optimum.
FIG. 2 defines a different means for making a connection which in effect represents a voltage divider circuit. In FIG. 2, one circuit 26 goes across the entire package in the same manner as utilized for FIG. 1 and a second circuit 28 is between electrode 10 and electrode 12' and the voltage across these latter areas will vary in the range between 500 and 2,000 volts. This type of circuit is somewhat more controllable than that given in FIG. 1 and is most effective when the insulating layer 14 is a good insulator such as is available from a polyester film, for example.
In the embodiment shown in FIG. 3, the connections are a first connection 30 between the source of potential and electrodes 10 and 12 and a second circuit 32 connecting electrode 12 and layer 12. This modification yields a degree of flexibility and better control of operation when insulating layer 14 is somewhat more polar (i.e. less insulating) than a polyester. In this particular case, the voltage across the two connections are comparable in value and will operate in the range of 3 to 8 kilovolts.
FIG. 4 shows still another modification in which there is a connection 34 between electrode 10 and electrode 12 with a potential in the amount of 500 to 2,000 volts and a connection 36 between electrode 12' and electrode 12, the voltage amount depending on the insulating value of layer 14. If the insulating value is high, the potential difference is of the order of 10 kilovolts or more. If the insulating value is low, the potential difference may be of the order of 0.5 to 3.0 kilovolts.
When layer 14 is a poor insulator, the electrical connection 40 shown in FIG. 5 is preferred. In this particular case, electrode 12" is produced by evaporating a quarter wavelength thickness of aluminum or chromium or a similar semitransparent material onto the somewhat conductive base 14. This kind of device is most effective when the base is white and opaque, such as is available from photographic baryta paper. The photosensitive layer 16 is then laid down on this metallized surface by the usual techniques and in view of the reflecting power of layer 14 relatively little effect on the details shown in the fully developed layer 16 after exposure to X-rays and development is seen. On this basis, the best electrical connection is between layer 10 and layer 12 and the charge buildup on layer 14 is removed by connecting this layer to ground by lead 19. In many cases, this is a preferred procedure in that it reduces the potential for electrical damage to the surroundings and possibly to the patient.
While a preferred photosensitive composition sensitive to charged particles, ions and electrons is given in the examples which follow, it will be understood that other films sensitive to charged ions and electrons could be used in place of the composition given in such examples. Thus, silver halide films could be used or other charge-sensitive films of known compositions may be used.
Preferred compositions set forth exhibit a sensitivity to the near-ultraviolet and ultraviolet illumination. The sensitivity to electron beams is such that 10 electrons/cm. or l0" coulombs/cm. exposure by 15 kilovolt electrons forms an image of unity density after development. After exposure, the film is developed by blanket exposure to an intense source of red light provided from a tungsten illuminator, filtered with a Corning No. 2408 red glass filter for a period of several minutes. The film is then fixed by heating in an oven for 1 minute at a temperature of 140 C. e
The capacity per unit area of a 5 mil polyester base such a Mylar is 21 picofarads/cmf. when exposed to charged particles, the Mylar polyester vase, being a nonconductor charges. This capacity of charging is such that at kilovolts, 2X10 coulombs/per square centimeter is the saturation charge density at the Mylar surface. This charge exposure is several orders of magnitude greater than the sensitivity level of the film as established through direct electron beam exposure. In addition, the charge sensitivity of the film was evaluated by charging the film with a corona discharge to a potential of several kilovolts. With a 10 kilovolt charging voltage on the corona wire, full density (3.5 to 4.0) was developed in the film. It was found, in addition, that in applying a corona, the sensitivity of the film was approximately 1 order of magnitude greater to positively charged particles, i.e. a positive corona wire voltage provided higher sensitivity, this being in accordance with the expectation of a Geiger-Muller mode region discharge which accentuates the formation of positive ions.
The following examples are illustrative of the practice of the present invention: Example 1:
A preferred electron-sensitive film composition comprising the following constituents was coated on a 5 mil Mylar polyester base under red light:
600 mg. leuco crystal violet 500 mg. triphenylstibine 800 mg. hexachlorethane 1,200 mg. carbon tetrabromide 50 mg. eicolane 3 drops of a 1 percent solution of m-dimethyl aminophenol in methylene chloride 28 cc]! of a 20 percent solution of polystyrene in benzene In preparing this film, the several constituents were dissolved in a 50:50 volume mixture of methylene chloride and benzene and the resulting composition was coated onto the polyester film using a Bird applicator bar to provide a 1.5 mil wet coating thickness, which, after drying, diminishes to a dry thickness of about 7 microns due to the dissipation of the solvent.
Other electron-sensitive film compositions suitable for use in the present invention are described in U.S. Pat. Nos. 3,147,117 and 3,275,443, the disclosures of which are incorporated in this application by reference.
in this example, electrode 12 was an aluminum plate. The intensifier structure was formed of a 5 mil aluminum sheet 10 which had been anodized on one side for 90 minutes in a 5 percent oxalic acid at a potential of 150 volts. The thickness of the porous aluminum oxide was 2 mils. After completion of anodizing and washing with water, followed by washing with methyl alcohol, the anodized layer was soaked in a 10 percent solution of potassium hydroxide in methyl alcohol for a period of 5 minutes, after which the nonetching alcoholic KOH solution, was washed out of the pores with a vigorous stream of water. The desired etching taking place during the waterwashing step. This yielded a pore structure in which the pores exhibited an approximate diameter of 0.3 microns and the ratio of open-to-closed area was approximately 50 percent. After drying at room temperature, the elements were laminated together in a printing frame and 15 kilovolts of direct current potential was applied between the back and the front electrodes. The aluminum metal and oxide serves as the electrode and multiplier and the metal was maintained positive with respect to the film-backing electrode. An exposure was made to X-rays generated by a tungsten tube operating at 10 kilovolts constant potential. In controlled experiments, it
was found that the response of the film of the incident X-ray beam was increased by a factor of 3 with potential across the laminate compared to identical conditions where the potential was zero across the sandwich.
Same as in example 1, except that after etching and washing out the etchant with water and drying, the anodized plate is heated in hydrogen at 550 C. for 1 hour, allowing 1 hour of time to raise the temperature from room temperature to 550 C., the plate was then cooled in hydrogen to room temperature, after which the anodized plate is placed in a vacuum chamber and pumped out at 10 mm. of mercury, after which a gas atmosphere comprising a mixture of 98 percent nitrogen and 2 percent methane is admitted to the vacuum chamber until atmospheric pressure conditions are achieved, and the assembly is again pumped down a vacuum of 10 mm. of mercury, and finally completing the operating after the pump is turned off by admitting the 98 percent nitrogen, 2 percent methane atmosphere to the chamber until atmospheric pressure is achieved and allowing the assembly to remain in such an atmosphere for about 10 minutes. The assembly is then removed from the vacuum chamber and then incorporated into the laminated layered structure as described in example 1, and exposure is made with 10 kilovolt X-rays with the 15 kilovolt potential difference across the layered structure under the same electrical sign conditions as defined in example 1.
Under these conditions, an amplification factor of 35 was obtained. The amplification factor was determined by measuring the time of exposure required under the X-ray exposure conditions described to achieve a density of 1.0 on the film, it requiring 1] 35th of the time in the case of the presence of the operating amplifying structure when compared with exposures in the absence of the amplifying structure. Example 3:
The 2-mil-thick layer of anodized surface, anodized as described in example 1, is etched twice in accordance with the procedure defined in example 1. This produces a pore size of 0.6 microns and open-to-closed area of 60 percent. After washing and drying, the pores are impregnated with a water solution containing 5 parts by weight of lead acetate, 1 part by weight of potassium acetate, 10 parts by weight of glacial acetic acid. The etched sheet was soaked in this solution for 10 minutes, the sheet removed from the impregnating bath and the surface solution wiped off with a cotton swab. Thereafter, the sheet was heated over a period of one-half hour to 150 C. retaining the plate at this temperature for 20 minutes and the temperature was then raised to 550 C. at which temperature the plate was retained for 30 minutes. This heating was carried out in air.
When this plate was used in the Geiger-Muller mode for 10 kilovolt X-rays, in the manner described in example 2, the amplification factor was 200. In this case, electrode 10 is charged positively.
The same plate can also be used advantageously in the secondary electron emission mode, in which case, the photosensitive layer 16 with its attached insulating backing is laid against the surface of the impregnated pores and the as sembly is then placed in a vacuum chamber which is then pumped down to a vacuum of 10 mm. of mercury and retained under such vacuum conditions for about 10 minutes. On removal of the vacuum, the layer 16 is found to be tightly laminated to the surface of the pores and after placing in the assembly shown in FIG. 1, and charging electrode 10 negatively, an amplification factor of 1,000 is achieved. For both the Geiger-Muller mode and the secondary electron emission mode, the potential across the layered assembly is 15 kilovolts and the amplification referred to in previous sentences in this example was obtained as a consequence of use of kilovolt X-rays. This amplification is reduced as the voltage of the X- rays is reduced and the drop in amplification starts to become noticeable at 50 kilovolt X-rays and less. For example, in the soft X-ray region at 10 kilovolts, the amplification in the Geiger-Muller mode is approximately 200 and the amplification in the secondary electron emission mode is approximately 300.
Same as example 3, except that the solution of lead acetate was replaced successively with similar strength solutions of bismuth acetate, thorium acetate, and uranium acetate. In the case of bismuth, about the same amplification was obtained as described for the lead acetate, whereas in the case of thorium and uranium, about a 10 percent higher degree of amplification was achieved.
The 2 mil anodized layer was etched as described in example 3. After washing and drying, the pores were then impregnated with the solution comprising 5 percent lead acetate, percent thiourea and sufficient alcoholic sodium hydroxide to yield a pH of 9, all dissolved in methyl alcohol. This solution was allowed to remain in the pores for 7 minutes and on removal from the bath the exposed surface was wiped with a cotton swab. The assembly was allowed to dry at room temperature and further heat treated in an atmosphere containing 90 percent nitrogen and 10 percent hydrogen sulfide raising the temperature of heat treatment from room temperature to 550 C. over a period of 1 hour. The assembly was maintained in a flowing atmosphere of nitrogen (90) and hydrogen sulfide (10) at this temperature for 30 minutes, after which the assembly is cooled in such an atmosphere to room temperature.
When this assembly was used in the Geiger-Muller mode under the conditions described in example 2, the amplification factor for both soft and hard X-rays with l5 kilovolts across the assembly was approximately 400 and when used in the secondary electron emission mode, the amplification factor in the 10 to 100 kilovolt range was approximately 1,000 and at 200 kilovolt X-rays, the amplification factor was 1,500. Example 6:
A mixture of 90 parts by weight ultrafine yellow lead oxide and 10 parts by weight of potassium chloride was ground in a ball mill until an average particle size of 0.2 microns or less was obtained. 45 volume percent of this ground finely divided mixture of lead oxide and potassium chloride was mixed with 55 volume percent of high molecular weight polypropylene. There was added thereto, 200 volumes of toluene and the mixture was then heated to 100 C. with stirring at which temperature substantially all of the polypropylene passed into solution. The somewhat viscous dispersion was stirred vigorously until dispersion was complete, then poured into shallow vitreous enamel pans and maintained at 100 C. until the majority of the toluene had evaporated off. The residue was scraped off the pan and thoroughly mixed further in a hot sigma-type mixer at 150 C. The hot mass was removed from the mixer and while hot was formed with paddles into a rough sheet form.
Smooth surface 4-mil-thick sheets were then prepared by passage of the plastic mass through calendar rolls maintained at 200 C. An Eastman Kodak photoresist (KPR) was applied to one surface of this 2 mil composite sheet to yield a dried layer thickness in the range of l to 2 microns. After the pho toresist has dried, it was then exposed to a photomask in which the design on the mask consists of black circles 0.6 microns in diameter and of such geometrical distribution that these black circles comprise 50 percent of the area of the photomask. Exposure was carried out for 10 minutes to a mercury arc lamp of 100 watt capacity, the lamp being positioned a distance of 3 feet from the surface so as to ensure collimation and accurate rendition of the design on the photomask. After exposure was complete, the resist on the surface of the lead oxide material was developed by immersing the assembly in the solvent supplied by Eastman Kodak for development of the KPR photoresist material. After drying, the assembly was then heat treated at 90 C. for minutes to harden the resist. The front surface of the lead oxide-KCL-propylene assembly was then treated with vigorously flowing toluene containing 5 percent polystyrene in solution maintained just under its boiling point.
By use of the vigorous flowing technique, the developed-out and washed-out areas are etched through the 2 mil film of lead oxide plus propylene without any serious undercutting leaving a conical hole through the thickness of the lead oxide film. The presence of the polystyrene is required to prevent such undercutting. Examination with the microscope indicates that the film has an opening just under the resist of approximately 0.6 microns which tapers down to a hole of approximately 0.2 microns on the opposite side. The flow of hot toluene was now directed against the back side which exhibits the 0.2 micron pores until these are opened to about 0.6 microns and again the holes on the opposite side are about 7 microns while the overall sheet thickness has been reduced to about 2.0 mils. The open-to-closed area was about 55 percent. After the etching process was complete, the assembly was removed from the etching solution and dried thoroughly at C. This now comprises the multiplying layer 18 as shown in FIG. 1, where the entire body of the mesh utilized for multiplying purpose is comprised of an insulator consisting of a mixture of lead oxide and potassium chloride, thus permitting the device to be used both for the Geiger-Muller mode and for the secondary electron emission mode. A 3 mil sheet of aluminum was laminated to the lead oxide containing grid with (a 2.5 micron layer of) a pressure-sensitive adhesive.
In the Geiger-Muller mode and for soft X-rays of the order of 10 kilovolts, again utilizing 98 percent nitrogen and 2 percent methane gas, the amplification factor is approximately 100 and about 500 for 100 kilovolt X-rays. This device is most effective for hard X-rays in the secondary electron emission mode. For example, at 100 kilovolts and utilizing the secondary emission mode, an amplification factor of approximately 1,200 is obtained.
A multiplying layer 18 is prepared as shown in example 6, except that instead of using aluminum as electrode 10 for laminating on the surface, a layer of lead foil 2 mils in thickness is utilized as a replacement. In both the Geiger- Muller and secondary electron emission modes, this device shows little or no effectiveness for soft X-rays and begins to show effective amplification starting at about 25 kilovolts and increasing rapidly up to 100 kilovolts. In these relatively hard X-rays regions, amplifications of 500 to 1,000 are obtained both for the Geiger-Muller and for the secondary electron emission modes, the device having the advantage that it prevents soft X-rays from being recorded, this quite often being a defect in biomedical examination in that it provides too much detail for soft tissue.
Microspheres of glass are classified so as to yield a supply of such microspheres of glass having diameters of 0.5 to 1 micron and wall thicknesses of the order of 0.05 microns. 50 parts by volume of ultrafine lead oxide are mixed with 50 parts by volume of these microspheres of glass. 10 parts by volume of polystyrene are added and the mixture is placed in suspension with 100 parts by volume of toluene and stirred until the polystyrene has gone into solution. The viscous product is then cast onto a 3 mil aluminum sheet and drawn down to a thickness of approximately 3 mils with a doctor blade. After drying at room temperature for 3 hours, a dried thickness of 1.5 mils is obtained. The curing is completed by baking at 80 C. for 30 minutes. The sensitive layer 16 with its associate backing is then laminated against this multiplying layer 18 comprising a mixture of lead oxide, minor amounts of polystyrene, and microglass spheres.
This layer starts to become effective at approximately 20 kilovolt X-rays and shows amplification of the order of 100 to 200 for X-rays in the 50 to 100 kilovolt range when the secondary electron emission mode is used. For the Geiger-Muller mode, the amplification is about 25.
47 volume percent of yellow lead oxide in a particle size range of the order of 0.2 microns and 3 volume percent of potassium chloride in the same particle size range, these particle size ranges having been obtained by grinding the mixture in a ball mill for 10 hours, is mixed with 45 volume percent polyvinyl alcohol and volume percent of a polymerized glycol, such as Carbowax 4000. 0.5 percent by weight of boric acid is added to the mixture. The mixture is dispersed in 300 volumes of water and stirred until the polyvinyl alcohol and Carbowax are completely in solution. This solution is cast onto a sheet of polyethylene which is ridged around the edge to a depth of mils so as to yield a pool 8 mils in thickness and allowed to dry at room temperature until a solid film is formed, after which drying is completed by maintaining at 75 C. for 1 hour. A 2 mil sheet remains which strips easily from the polyethylene backing.
When used in the mode as shown in FIG. 1, this layer is capable of acting not only as electrode 10 but also as the multiplying structure 18. Amplification, chiefly in the secondary electron emission mode, for hard X-rays of the order of 50 to 100 kilovolts, is obtained in the range of 50 to 100. When this type of an electrode is used with the channel-type lead oxidepotassium chloride structure described in a previous example, the amplification is enhanced, when the polyvinyl alcohol lead oxide channel structure is used with the multiplying layer 18. Under these conditions, amplification for the secondary electron emission mode achieves the value of the order of 2,000 for 100 kilovolt X-rays.
A 3 mil sheet of aluminum may be laminated to the composite structure described in example 10 by moistening the surface of this composite structure with a wet cotton swab and applied to the aluminum sheet with light pressure. In this case, the aluminum sheet then acts as electrode 10 and the composite lead oxide alkali containing structure takes the form of multiplying structure 18. About the same degree of amplification is achieved with X-rays as indicated in Example 11 and the advantage of this procedure is the addition of rigidity to this member and the protection of its surface against attack by moist atmospheres.
An electrode plus its associated multiplying structure is made from anodized aluminum in the manner as described in example 2, in which the multiplying structure 18 contains channel-type pores whose pores are lined with a mixture of lead oxide and potassium chloride. In this case, however, the photosensitive or X-ray sensitive layer 16, is placed on a substrate which is comprised primarily of a mixture of a major amount of polyvinyl alcohol, with minor amounts of a glycol, such as Carbowax 4000, and about 1 percent of ammonium borate. This type of layer is a fairly good conductor. The electrical connection mode in accordance with FIG. 4, is utilized in which a connection is made between electrode 10 and electrode 12' at a potential difference of 1,500 volts, when utilizing preferred anodized aluminum type of multiplying structure and a connection in turn is made between electrode 12 and electrode 12 at 4 kilovolts.
The partially conducting substrate (layer 14) as described in example l2 is utilized in a thickness of 5 mils. A quarter was thickness of aluminum metal is evaporated on the surface of such partially conducting substrate which enables a light transmission of about 60 to 70 percent to be obtained in spite of the metallic character of the evaporated aluminum. Thereafter, the sensitive layer 16, as defined in example 1, is laid down on a layer 12 and the assembly involving anodized aluminum with its associated aluminum backing as described in example 2 is placed on top of layer 16. An electrode connection is then made from layer 10 to layer 12 with a potential difference of approximately 1,200 volts as shown in FIG. 5. Layer [0 is charged positively or negatively depending on whether the Geiger-Muller or the secondary electron emission mode is being utilized. Layer [4 is connected to ground.
About the same degree of amplification is obtained in accordance with the connecting mode defined in FIG. 5, as achieved under the best conditions shown in FIG. 1, where very much higher voltages are placed across the entire layered structure.
1. An X-ray intensifier adapted to operate in absence of vacuum comprising:
a. a first means comprising a thin layer of electrically conductive material which absorbs X-rays impinged thereon from a source of radiation and which produces high-energy photoelectrons as a result thereof,
b. a second means comprising a thin photosensitive layer to transform the photoelectrons produced by said first means into electrons and charged particles;
c. A third means comprising a thin layer compound of material-producing multiplication of the number of said electrons and charged particles;
d. a fourth means comprising an imposed difference of potential relative to said first means for accelerating said electrons and charged particles; and
e. a photoreceptor disposed so as to receive the electrons and charged particles released by said second means; each of said first, second and third means and said photoreceptor being in the form of thin layers disposed as lamina adjacent to one another.
2. The X-ray image intensifier of claim 1 including a source of potential connected to said first means so as to provide an electric field extending across the X-ray image intensifying device.
3. The X-ray image intensifier of claim 1 including in addition, an insulating film base supporting said photoreceptor, and an electrically conductive layer on the side of said base opposite to said photoreceptor.
4. The X-ray image intensifier of claim 1 wherein the first means is a thin sheet of metal.
5. The X-ray image intensifier of claim 1 wherein the first means is a thin layer in which particles selected from the group consisting of lead, bismuth, uranium, thorium and their oxides are dispersed in a film-forming plastic.
6. The X-ray image intensifier of claim 5 in which the dispersed particles constitute between 20 percent by volume and 50 percent by volume of said layer.
7. The device of claim 1 wherein said second means is a gascontaining region disposed between said first means and said photoreceptor and said first means is spaced from said photoreceptor.
8. The device of claim 1 wherein there are microchannels extending through the second means and to the photoreceptor layer.
9. The device of claim 8 wherein the second means is a channelled layer having pores 0.5 to 2 microns in diameter and the distance between pores is not more than 50 percent of diameter of individual pores, the layer being between 25 and 50 microns thick.
10. The device of claim 1 wherein the second means is a layer including up to 50 percent by volume of lead oxide particles.
11. The device of claim 1 wherein the second means comprises a mixture of lead oxide particles and glass microspheres dispersed in polystyrene.
12. The device of claim 1 wherein the second means comprises a dispersion of lead oxide particles dispersed in synthetic polymer.
13. The device of claim 12 wherein the second means also contains 5-20 percent by weight of a halide selected from the group consisting of alkali metal halides and alkaline earth metal halides.
14. The device of claim 8 wherein the second means is a high lead glass layer with microchannels extending between first means and photoreceptor.
15. The device of claim 8 wherein said second means is a matrix of aluminum oxide containing a plurality of straightsided microchannels.
16. The device of claim 1 wherein the first means is a thin sheet of metal, the second means is a thin layer with straightsided microchannels extending through said layer from said first means to said photoreceptive layer and the second means is sandwiched between said first means and said photoreceptive layer.
17. The device of claim 8 wherein said second means is an anodized layer wherein the pores of the anodized layer have been increased in size by etchingafter initial formation by anodizing to give a pore size 0.1 to 0.8 microns and an opento-closed ratio between 30 percent and 70 percent.
18. The device of claim 17 wherein the pore size is from 0.2 to 0.5 microns, the layer is between about 25 to about 50 microns thick and the open-to-closed ratio is about 50 percent.
19. The device of claim 17 including a layer of an X-ray absorbent material coated on the walls of said pores.
20. The device of claim 19 wherein the Xray absorbent material is an oxide of a metal selected from the group consisting of Pb, Bi, Th and U and the layer is about 0.05 microns thick.
21. The device of claim 1 including an electrode disposed between said second means and said photoreceptor.
22. The device of claim 2 including an electrode disposed between said second means and said photoreceptor and a second source of potential connected across only a portion of the device.
23. The device of claim 22 wherein the second source of potential is connected between said electrode and said first means.
24. The device of claim 21 wherein a first source of potential is connected to the electrode and to one outer face of said device and a second source of potential is connected to the electrode and to the opposite outer face of said device.
25. The device of claim 1 comprising the following arrangement of layers:
a. electrode layer to absorb radiation b. layer to transform and to multiply the transformed radiation c. photoreceptive layer d. electrode layer e. poorly insulating base layer and a source of potential connects layer (a) with layer (d) and layer (e) is connected to ground.
26. The device of claim 1 comprising the following arrangement of layers:
a. electrode layer to absorb radiation b. layer to transform and to multiply the transformed radiation c. photoreceptive layer d. poorly insulating base layer e. electrode layer and a source of potential connects layer (a) with layer (d) and layer (e) is connected to ground.
27. In a method of forming a multiplying layer for an X-ray image intensifying device, the improvements which comprise:
providing a clean aluminum foil;
anodically etching said foil in an oxalic acid bath at anodizing potentials between 100 and 300 volts and at to amperes per square decimeter to produce a porous oxide surface on said foil, said surface being between about 2 and 3 mils thick and having a large number of channels extending through said layer from the outer surface to the foil base; and
increasing the porosity of said surface by further etching same to increase the diameter of said channels to between about 0.1 and 0.8 microns; the resulting product being a layer with straight sided microchannels on a solid base of sufficient thickness to maintain the structural integrity of the product.
28. The method of claim 27 including, in addition, the step of lining the channels in the layer resulting from said further etching with a compound of a member of the group consisting of lead, bismuth, thorium and uranium.
29. The method of claim 27 including, in addition, the step of lining the channels in the layer resulting from said further etching with a mixture containing an X-ray absorber and a secondary emitter of electrons.
30. The method including, in addition, the following subsequent treatment of the product of claim 27:
1. heat treating in a hydrogen atmosphere at about 550 C.
for about 1 hour;
2. cooling to room temperature; and
3. replacing the gas in said channels with an atmosphere consisting of about 98 percent nitrogen and 2 percent methane.
31. The method including, in addition, the following subsequent treatment of the product of claim 27 1. heat treating in a hydrogen atmosphere at about 550 C.
for about 1 hour;
2. cooling to room temperature; and
3. removing the gas from said channels by exposing said pores to a vacuum.
32. The method of claim 28 wherein the channel-lining material contains a small amount of material selected from the group consisting of alkali metal compounds and alkaline earth metal compounds.
33. The method of claim 28 wherein, after etching to provide a microchannel size of between about 0.5 and 0.6 microns in diameter and an open-to-closed ratio of between about 50 and 70 percent, the microchannels are impregnated with a solution containing an acetate of a metal selected from the group consisting of lead, bismuth, thorium, and uranium and material selected from the group consisting of alkali metal compounds and alkaline earth metal compounds, and then heated to provide a uniform layer of lining said microchannels, the heating of the impregnated layer being at about C. for 30 minutes and then at 500 C. for 30 minutes.
34. The method of claim 32 wherein the heating is at about 150 C. for 30 minutes and then at 500 C. for 30 minutes.
35. The method of claim 28 wherein, after etching to provide a microchannel size of between about 0.5 and 0.6 microns in diameter and an open-to-closed ratio of between about 50 and 70 percent, the microchannels are impregnated with a solution of acetate of a metal selected from the group consisting of lead, bismuth, thorium and uranium and then heated to provide a uniform layer of oxide lining said microchannels.
36. The method of claim 35 wherein the solution is a nonaqueous solution containing an acetate of a metal selected from the group consisting of lead, bismuth, thorium and uranium, thiourea and an alkali metal hydroxide dissolved in an alcohol, and wherein the solution deposits a film of heavy metal sulfide inside the microchannels.
37. The method of claim 36 wherein the product is washed and then dried and then fired in an atmosphere containing hydrogen sulfide.
38.'A method of making an X-ray image intensifier which comprises:
forming a thin sheet consisting of a mixture of an oxide of a metal selected from the group consisting of lead, bismuth, thorium and uranium and a halide selected from the group consisting of alkali metal halides and alkaline earth metal halides, uniformly dispersed in a synthetic polymer matrix; applying a resist to said sheet; photoetching said sheet to develop microchannels extending through said sheet; washing off the resist and enlarging the holes by applying a solvent to said sheet, so as to produce a sheet about 2 mils thick through which pores about 0.6 microns in diameter extend, the amount of pores being such that the open-toclosed area is about 50-70 percent; and laminating a thin metal sheet to one side of the resulting porous sheet and a photosensitive layer and another thin metal sheet to the other side of said porous sheet.