US2900555A - Bombardment conducting target - Google Patents

Description

United St s. ten Q .BGMBARDMENTCONDUCTIN G TARGET Robert JJSchneeberger, Pittsburgh, Pa., assignor-to'Westinghouse Electric Corporation, East Pittsburgh, Pa., 2 .corporation of Pennsylvania 1 Application August 1,1955, 1SerialNo. 1525,'594

6 Claims. 01. sis-ass This invention relates -.to bombardment-induced conductivityin insulators and to applications withinelectrical 'defices.

The principles ofmy invention maybe employed in-an Xeray intensifying device, such as disclosed in .my copending application entitled X-raylmage IntensifyingDevice,'-.SerialNo. 416,879, filed .March 17, 1954. In X -ray image .intensifying devices of this ,type, the X-ray image is focused upon a fluorescent screen which excites the ,phosphor therein producing alight image which-in causes .a closely adjacent photocathodeelectrode to pro- .vide .photoelectron emission at arate proportional to the brightness of each element. The photoelectrons are actcelcratcd toa velocity .of the ,orderof ,lOto 2O kilovolts and focused to a reduced .size upon .atarget electrode in- (binding a semi-insulating layer of material exhibiting the property of electronflbombardment induced conductivity. The electron image from the photocathode penetrates through a thin electron permeable aluminum backing 'layerlon the target to produce on the insulating layer what :may be-thought of as a conductivity inverted imagedupli- .cating the space distribution of the electron image from the photocathode, and so of the X-ray image.

In thegscanning section of the tube, an electron beam is provided on the oppositeside of the target with respect to the photocathode and scans the face of the semi-insulating layer to' bring the entire surface to :a similar. potential as .thecathode of the electron beam generating gun. With .noX-rays projected ontotheinput screen, the face of the semi-insulating layer facing the electron gun is maintained at cathode potential normally ground while the other surface by means of the aluminum backing layer may be retained at a positive potential with respect to the gun cathode of the order of 50 volts. When the'X-ray image is projected onto the input screemphotoelectrons from the photocathode bombard the bombardmentsurface of the target and induce conductivity-in the semiinsulating layer and causes the scanned surface of the insulator'to act as a leaky capacitor and change from electrongun cathode potential to some positive poteutiallcss than S.0 volts. "This effect on the .semi insulatorwmaybe referred to as a conductive image. "The change in .potential of elements of the scanned surface is roughlyproportional to intensitycf' bombardment. When .the electron-scanning beam scans the .surface,.each .element will be restored. to gun cathode potential. A signal may-be derived .from the backing plate in :conventional manner for transmission or direct connectiento a conventional display device.

.ln,tar -gets of'thetype described above, it has been found by utilizing'materials. such. as antimony trisulphide and arsenic iztrisulphide -which :exhibit electron-bombardment r 2,900,555 Pat nted Aug- 18, 95.9

2 V ,in the Proceedings of the Physical Society, London, April 1951, v.01. 64, sec. A, pages 362 to379. The range ofthcsc .carriers iszthought to be of theorder of 0.1 micron. The limitation of the short-range carriers within these materials requires thatin order toinsureconductivity through the entirelayer the incident bombarding electrons from the photocathode must substantially penetrate the entire layer. It has been found that .a semi-insulating layer prepared byevaporation in a vacuum having the thickness of twonmicrons must be bombarded with electrons of voltages in excess of 25 kv. in order to penetrate th bulk of thematerial and thereby induce conductivity. If the entire target is not penetrated, vthen the target hasa very low sensitivity due to the short-range charge carriers. It is desirable to attempt to limit the-required accelerating voltages of the electrons from the photocathodeto below 25 kilovolts. However, even though it is practical to evaporate a thin semi-insulatinglayer, theta getnowsuffers from high capacity between the scanned surfaceand the back plate surface or bombardment surface. Ifhe high capacity of the target results in that theelectron scanning beamcannot fully discharge the storedssignals during a single scan, and an objectionable timelagfinthe transmitted pictureis obtained. a g

It is,-therefore, an object of myinventiontoprovidean improved bombardment-induced conductivity target.

IIt is another object to provide a target of-low capacity and thereby remove objectionable time lag within the transmittedpicture,

Itis another object to provide an improved ,imagepickup device of high sensitivity.

These and other objects are effected byrmy vention as will be apparent from the following descriptiontaken in accordance with the accompanying drawing throughout which like reference charactersindicate like parts, and in which:

Figurejl .is a schematic view inlongitudinal section of a prior art devicethat may be used to embody thfipljiptciples of my'invention;

Fig. 2 is a view in section of enlarged scale ofa of a target embodying my invention that may @beemployed in'Figrl;

:Fig. 3 is a view in section of enlarged scale of a modified target .embodying my invention; and

Fig. 4 is a view in section of enlarged scale of another modified form ofa target embodying my invention.

'Referring in detail to Fig. l, a vacuum-tight enclosure or envelope 10, which may be of any suitable material such. as glass,'has an input screen 12 positioned at one end. of the envelope 10 which is comprised of a thin glass curved plate 14 coatedon its inner concave surface with a thin layer 16 of transparent conductive material, such as stannic oxide, and the latter coated a photoemissive. material 20, such as cesiated antimony. Thecc lv ix or outer surface is coated with alayerof fluorescentmw terial 18, such as zinc cadmium sulphide. It is obvi us that in. applications where the photoemissive materiajljs sensitive to the radiations such as ordinary light, that.the fluorescent layer may be dispensed with.

The X-ray imageis projected into the input screen 12 through the wall of the enclosure 10 and lightis generated within the X-ray excited fluorescent layer;18and istransmitted through the glass support member 14 and the conductive layeri16 to the photoemissive layer 20. The light from the fluorescent layer 18 generates at the surfaceof the photoemissive layer 20 .an electron/image which is a replica-of the X-ray image incident on the input .screen 12. A suitable electron lens system, such resillustrated by the electrodes 22 and 24 provided with suitable voltages, focuses and accelerates a; i a ed replica of .theielectron image onto; target :30. target 30 comprises essentially a screen 32 of metal wire semi-insulator. the layer must also exhibit the property of high electronwhich serves as a support member for the target 30 and has a large ratio of open space to a solid area. If a suitable backing layer is utilized, this support mesh 32 may be dispensed with. The support mesh 32 has an aluminum backing layer 34, thin enoughto be permeable to electrons, deposited on the surface of the mesh 32 opposite to theinputscreen 12. The aluminum layer .34 serves as the backing plate of the target 30 and is con- ,nected to the exterior of the tube for application of a suitable voltage and from which an output signal may be derived from a resistor 35. The exposed surface of the aluminum layer 34 is coated with a thin layer 36 of a simi-insulating material or dielectric which exhibits the .property of electron-bombardment induced conductivity.

The exposedface of the dielectric layer 36 of the The mode of operation of the device has previously generally been described within the statement of the invention and a more thorough description may be found in the above-mentioned copending application.

Referring in detail to the structure of the target 30 and specifically to the semi-insulating or dielectric coating 36,

it is conventional to form these coatings by evaporation within a vacuum. The semi-insulating materials utilized within image intensifier application must have characteristics such that the product of its resistivity and specific dielectric constant is equal to or greater than sistivity is given in ohm centimeters and specific dielectric constant with air as a reference equal to unity. This figure is necessary in order that the charge placed on the insulator surface will not leak through appreciably to ,the opposite side or back plate in the absence of any electron bombardment in the standard secondTV frame interval. This material may be referred to as a The semi-insulator material utilized in bombardment induced conductivity which is exemplified in that on the average for each bombarding electron of the energy of 10 kv. to 25 kv., ten or more electronic charges flow by means of carriers from one surface to the other surface of the target with a field impressed thereon of the order of 5,000 volts/cm. or greater. This in effect decreases the resistance between surfaces of the target. Suitable materials which possess these properties are antimony trisulphide, arsenic trisulphide and amor- 3.43 grameslcmfi. If the insulating material is evaporated in a few millimeters of mecury pressure of an inert gas, such as argon, it will deposit as a smoke-like layer whose average density is much less than that of the bulk material. By this latter method of evaporation, it is, therefore, possible to deposit a layer of insulating material with lower effective dielectric constant resulting in lower capacity due to space between particles and high penetrability to electron bombardment. Although this structure exhibits the properties of low capacitance, it is not satisfactory as a target due to the high penetrability of the electrons without loss of energy. It is found that an electron having a voltage of 25 kv. goes completely through a low density deposit of 2 to 3 microns in thickness with little loss of energy. It is necessary that an appreciable amount of the bombarding electrons energies must be given up in the insulating material in order to induce conductivity. It is thus seen that a target comprised of a smokedike or low density deposit of semiinsulating material on an aluminum backing plate alone would have low sensitivity.

In order to fully appreciate the principles of my invention, it is necessary to study the movement of bombarding electrons within a material. For example, when electrons bombard a semi-insulating target, it is found that some of the electrons are completely absorbed in the material, others are altered in direction by successive scattering events and traverse relatively long paths while others may emerge without loss of energy. In materials of relatively low atomic number, such as arsenic trisulphide which has an equivalent atomic number 23, very little scattering of electrons takes place. In the case of a vacuum deposited layer of insulating material of low atomic number, the absorption of energy per unit path length is moderately high, while in the case of the smokelike layer deposited in the inert atmosphere, absorption is very low. High atomic-numbered materials, such as gold, have a very high absorption per unit length, as well as a high scattering coeflicient with respect to electron bombardment. Therefore, if the path of a primary electron is kept short by using a thin film of high atomicnumbered material, the total absorption will be small. However, there will be a considerable amount of scattering of the electrons incident on the gold film, and electrons are scattered over a wide range of angles, both forwards and backwards.

In Figs. 2, 3 and 4, enlarged sections of a very small portion of three different type target structures are shown embodying my invention. The effect of the aluminum back plates on the incident electrons is negligible as far as scattering or absorption. The actual mechanisms of absorption and scattering within asolid are very complex. The dotted lines within Figs. 2, 3 and 4 indicate crude representation of the most probable type of path under existing conditions. As the electron coming from the input screen is absorbed and scattered within the insulating material, it initiates many secondary effects,

one of which is the excitation of carriers which drift through the material under the influence of the electric field applied between the back plate and the scanned surface. A representation of the secondary effects within the insulating layer is not shown. It is these carriers that establish conductivity Within the insulating layer and, therefore, between the faces of the insulator. It is found that one bombarding electron may excite enough carriers so that electronic charges are transported between the faces of the insulator.

In the case of the vacuum-evaporating coating of given thickness, the electron of a given energy is absorbed by the insulating layer before total penetration occurs, and

bombardment-induced conductivity cannot take place under these conditions in materials with short-range car.- riers, such as arsenic trisulphide and antimony trisulphide. In the case of the smoke-like deposit of similar thickness on the aluminum backing plate, the bombarding electron of similar energy would totally penetrate the layer with small energy loss, and, therefore, only slight bombardment-induced conductivity in the material would take place.

In the specific embodiment shown in Fig. 2, the target comprises a layer of gold 52 of the order of 50 to 100 angstroms in thickness deposited on the aluminum backing plate-'34 prior to the evaporation of a smoke-like deposited layer 54. A bombarding electron striking the In Fig. 3, there is shown another embodiment of my invention in which the target 60 comprises a continuous gold film 62 relatively thick with respect to the first gold film 52 of the order of at least 500 A. in thickness evaporated on the scanned side of the insulator 54. Due to the high reflectivity of the gold layer 62, the incident electrons after passing through the insulator 54 will be back scattered into the insulator 54. The back scattered electrons passing through the. insulator 54 will give a further increase in yield of carriers and thus increased induced conductivity. The layer 62 should be of sufl'lcient thickness so that no electrons completely penetrate the layer 62.

In Fig. 4, the target 70 is comprised of a coating 72 applied on the scanned side of the insulator 54 applied in the form of a mosaic which is a large number of separate discrete layer elements 74 laid down side by side and separated from each other of similar thickness to layer 62. The mosaic coating 72 is necessary in order to forma varying charge pattern on the scanned surface which is required in image intensifier devices. One method of making a mosaic-like layer of gold as shown in Fig. 4 is to evaporate within a vacuum from a concentrated source through a fine mesh screen. The coating or layer 72 may be separated from the semi-insulator 54 limited by the resolution desired.

In the specific embodiments shown and described, the materials listed in the construction of the target are merely representative of a class which have similar properties. Gold is utilized as an example here due to its high atomicnumber. It is desirable to use a solid material having a high atomic number of greater than 50, such as tungsten, platinum and lead, which all exhibit high scattering coefiicients for electrons.

The back scattering effect is more fully described in a paper entitled Back-Scattering of 5 to 20K e.v. electrons from Metals and Insulators by J. E. Holiday and E. J. Sternglass and given at the 15th annual conference on Physical Electronics at M.I.T. on March 25, 1955.

While I have shown my invention in several forms, it will be obvious to those skilled in the art that it is not so limited, but is susceptible of various other changes and modifications without departing from the spirit and scope thereof.

I claim as my invention:

1. The method of producing a highly sensitive electron target, comprising the stepsof evaporating a thin amorphous film of semi-insulating material, exhibiting the property of becoming conductive when bombarded with electrons, within an inert atmosphere to produce a film having low bulk density, and applying a layer of electron scattering material of an atomic number of at least 50 to said film of semi-insulating material.

2. A target electrode for a pick-up tube comprising a thin film of a semi-insulating material selected from the 6 group consisting of antimony trisulphide, arsenic trisulphide and amorphous selenium which exhibit the property of decreased resistivity when bombarded with electrons, said film of semi-insulating material having an amorphous structure and low bulk density and inclusions consisting of a high atomic number electron scattering material within said film of semi-insulating material.

3. A target electrode for a pick-up tube comprising a thin film of semi-insulating material having a thickness of the order of three microns exhibiting the property of decreased resistivity upon electron bombardment, said film having an amorphous structure and of a density less than normal bulk state, said semi-insulating film having a coating of high atomic numbered material of a thickness of about one hundred angstroms upon which the bombarding electrons impinge to deflect said bombarding electrons within said semi-insulating film and a second coating on the opposite surface of said semi-insulating film of a high atomic numbered material of a thickness of about one thousand angstroms to backscatter said bombarding electrons back into the semi-insulator material.

4. The method of producing a highly sensitive electron target of semi-insulating material exhibiting the property of decreased resistivity upon electron bombardment comprising the steps of placing an electron defiect ing means on. the bombarded front surface of said target to increase the path of said electrons through said semi-insulating material and the positioning of an electron back scattering means on the rear surface of said target to back scatter the bombarding electrons back into said semi-insulating material.

5. A target electrode for a pickup tube comprising a thin film of semi-insulating material exhibiting the property of electron bombardment induced conductivity, said film of material having a density less than that in its normal bulk state and means contiguous with said layer of semi-insulating material, said means comprising a. material having an atomic number of at least 50 for deflecting electrons within said layer of semi-insulating material.

6. A target electrode comprising an amorphous layer of semi-insulating material that exhibits the property of decreased resistivity when bombarded with electrons, said layer having a density lower than its normal density, an electron scattering layer disposed on one surface of said semi-insulating layer, and a back scattering material disposed on the opposite surface of said semi-insulating layer, said electron scattering layer and said back scattering layer having an atomic number greater than 50.

References Cited in the file of this patent UNITED STATES PATENTS 2,527,632 Graham Oct. 31, 1950 2,544,754 Townes Mar. 13, 1951 2,645,721 Williams July 14, 1953

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