|Publication number||US2928969 A|
|Publication date||15 Mar 1960|
|Filing date||11 May 1956|
|Priority date||11 May 1956|
|Publication number||US 2928969 A, US 2928969A, US-A-2928969, US2928969 A, US2928969A|
|Inventors||Robert J Schneeberger|
|Original Assignee||Westinghouse Electric Corp|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (11), Referenced by (11), Classifications (16)|
|External Links: USPTO, USPTO Assignment, Espacenet|
mvsNToR Robert J. Schneeberger March 15, 1960 R. J. scHNEEBERGER IMAGE DEVICE Filed May 11 2,928,969 IMAGE DEVICE i Application May 11, 1956, Serial No. 584,231 3 claims. (ci. 313-65) 'This invention relates to image producing systems and more particularly to tubes in which a radiation image is received on an input screen and produces output signals which may be used to reconstruct the image.
This invention isfparticularly useful in pickup tubes such as described in my copending application entitled X-Ray Image intensifying Device, Serial No. 416,879, filed March 17, 1954, and assigned to the same assignee. In image intensifying devices of this type, a radiation image such as X-rays is focussed upon a fluorescent layer of a phosphor material in the input screen which excites the phosphor therein producing a Alight image which, in turn, causes a closely adjacent photocathode electrode to provide an electron image due to photoelectronemission at a rate proportional to the brightness of each element. The electrons in the photoelectron image vgenerated by the input screen are accelerated to a velocity of the order of to2() kilovolts and focussed toa reduced size of electronimage upon-a target electrode. The target is comprised of a semi-insulating ,layerv e of a material exhibiting, the property of electron bornbardment induced conductivity with a conductive backing plate on the bombarded side of the semi-insulating layer and a grid support member on the free surface of the backing plate. The electrons in the electron image from the photocathode of the input screen penetrate through the thin electron permeable conductive backing layer onto the target to produce in the semi-insulating layer (resistivity of about 1012 ohms per cubic centimeter) what may be thought of vas a conductivity inverted image duplicating the spaced distribution of Ythe electron image from the photocathode and so of the radiation image.
In the scanning section of the image tube, an electron beam is provided on the opposite side of the target with respect to the input screen and scans the free face of the semi-insulating layer to bring the entire surface to a similar potential as thatappli'ed to the cathode of the electron beam generating gun. `With noradiationnimage projected on the input screen, the exposed face `of the semi-insulating layer of thetarget facing the electron gun is maintained at the cathode potential of the electron scanning beam. A positive potential is applied to the conductive backing layer of the Vtarget of the order of 50 volts positive with respect 4to the cathode potential of the scanning beam and Va field of 50 volts exists between the free surface of the semi-insulating layer and the backing layer side. When a radiation image l is projected onto the input screen, photoelectrons from the photocathode layer,` ofthe input screen bombard the target and induce conductivity in the 'semi-insulating layer. The semi-insulating layer acts like a leaky capacitor resulting` in the scanned or free surface of the semi-insulating layer changing from the cathode potential offthe scanning gun to some positive potential less th'an`50 volts. This Veffect on Athe semi-insulating layer'-` my be referred to as a conductive image. The change United i States Patent O Y, '2,928,969 Patented. Mar. 1'960 ICC 2 in potential of the surface elements of the scanned surface of the semi-insulator is proportional tothe intensity of electron bombardment. When the low velocity scanning electron beam moves over the surface of the target,
each target surface element will be restored to the' tion obtains a very good signal to noise ratio at low video or low light level signals.
levels of radiation energy. The target described using a semi-insulating material such as arsenic trisulfide exhibits amplifications as high as 1000 with an accelerating`voltage of 20 kv. It is advantageous to obtain .as high a signal to noise ratio as possible. This may be improved by obtaining an output current from the target as large as possible. For example, a signal to noise ratio of,V 50 provides a relatively snow-free picture. It is found that a target current ofl07 amperes is needed to produce a signal which is 50 times the noise power inherent of the first stage of the video amplifier., For example, with a target amplification of 1000, a photoemission current of 10-1o amperes is required to produce an output current of 10"t from the target. A typical cathode will emit electrons at the rate of 1010 amperes at the lowest fiuoroscopic levels used in X-rays and would provide ;a relatively snow-free picture. v
It has been found, however, that the image intensifiers, described in Vthe above-mentioned application and in my copending application entitled Bombardment Conducting Target, Serial No. 525,594, Yfiled August l, 1955, now U.S. Patent 2,900,555, issued August 18,
1959, and assigned to the lsame Vassignee'as the present i 4 by the noise of photoelectric emission from the photo-V cathode of the device. The image orthicon does not have as much amplification as the image device described in the above-mentioned applications. The image orthicon utilizes amplification of the return portion of the scanning beam. The disadvantage of this type -of amplification is that the vmaximum` return beam current and, therefore, the maximum noise'occurs with minimum It is, therefore, more desirable to place internal amplification within the tube ahead of the scanning beam and derive, if possible, lthe amplified signal from the target electrode itself. This type of amplification contributes only a negligible amount of noise to the amplified signal obtained from the target r electrode.
It is accordingly an object of my invention toV provide an improved type of pickup tube.
It is another object to provide an improved pick-up tube having Yhigh sensitivity and high signal to noise ratio.
It is another object to provide preamplification of an electron image section of 'a pickup tube prior to bombarding the target electrode. Y
These and other objects are effected by my invention as will be apparent from the following description taken in accordance with the accompanying drawing through-- outwhich like reference characters indicate like parts Y and in which: Y -Figure 1 ,is afschematic view in lon'gitudin'al section of a 'tube embodying the iprinciples of :my invention;
Fig. 2 is a view in section and enlarged scale of the electron multiplier structure utilized in Fig. l;
Fig. 3 is a view in section and enlarged scale of a modified electron multiplier structure that maybe embodied in Fig. 1;
Fig. 4 is a view in section and enlarged scale of a modified form of electron multiplier that may be embodied in Fig. 1;
, Fig. 5 is a view in section and enlarged scale of a modified form of electron multiplier that may be embodied in Fig. l; and
Fig. 6 is a view in section and enlarged scale ofra modified form of target that may be embodied in Fig. l. Y
Referring in detail to Figs. l and 2, a vacuum tight enclosure or envelope 1t) is provided which may be ofany suitable material such as glass. and of any suitable configuration. In the specific embodiment shownythe envelope 18 may be broken down for purpose of explanation into an image section and a scanning section. The scanning section is enclosed within a neck portion 12 of the envelope and the image section enclosed within a connecting larger tubular body portion 14. The body portion 14 is closed off by a face plate member 16.
An input light sensitive screen 18 is positioned at one end of the envelope 10 near the face plate 16 and is comprised of a thin glass curved plate Ztl'k coated on its inner concave surface with a thin layer 22 of a transparent conductive material such as stannic oxide. A photoemissive layer 24 of a suitable material such as cesiated antimony is provided on the conductive coated side of the glass support plate 18. The convex or outer surface'of the glass plate 18 is coated with a layer of fluorescent material 26 such as zinc cadmium sulfide. Theinputscreen described above is of the type where theradiations vto be intensified are projected onto the phosphor layer 26, which is sensitive to the input radiatiomand converted into radiations to which the photoemissive layer 24 is sensitive. It is obvious in those applications where suitable photoemissive material is available and sensitive to the radiations to be amplified that the fluorescent layer. 26 may be dispensed with. It is also possible to support the input screen 18 on the inner surface of the face plate 16. The transparent coating 22 may also be omitted if photoemissive material is sufficiently conductive.
The radiation image which is projected onto the input screen 18 through the face plate portion 16 vof the envelope 10 excites the radiation sensitive fluorescent layer` 26 and the light thus generated withinthe fluorescent layer 26 is transmitted through the glass support member 20 and the transparent conductive layer 22 to the photoemissive layer 24. The light image thus generated within the fluorescent layer 26 generates a co-rresponding electron image at the surface of the photoemissive layer 24 which is a replica of the radiation image projected onto the input rscreen 18. An output lead 28 for the input screen 18 is connected to the transparent conductive coating 22 and is connected on the exterior portionof thev tube to the negative terminal of a suitable potential source represented by a battery 70.
A suitable electron lens system and electron multiplier assembly is provided within the image section andfocusses and accelerates a contracted amplified replica of the electron image from the input screen 18 onto a target 50. The target is comprised essentially of a metallic support grid structure 52 having a large open area. On the opposite side of the support grid 52 with respect to the input screen a continuous backing layer 54 of a suitable conductive material such as aluminum is provided thin enough to be permeable to electrons accelerated from the input screen 18. On the surface of the backing layer 54 opposite with respect to the support grid 52, there is deposited a thin layer 56 of a semi-insulating material of a resistivity greater than 1012 ohms per cubic centimeter in the unexcited state such as arsenic trisulfde, antimony trisulde or amorphous selenium, which' exhibits the property of electron bombardment induced conductivity. The backing layer 54 is provided with a lead 53 to the exterior of the envelope 10. The lead 58 is connected through a resistor 60 to the positive terminal of a battery 62. The negative terminal of battery 62 is connected to ground. The potential of the battery 62 may be of the order of 50 volts. The output signal from the target 18 may be derived by means of the capacitor 64 and resistor 66 connected in series to the lead 58 of the target and to ground. The output of the circuit may be then obtained by connecting an amplifier across the resistor 66.
A plurality of electron multiplier stages are positioned between the input screen 18 and the target electrode 50. In the specifiic device shown, the images are inverted between stages by electrostatic focussing. Magnetic focussing may also be used. In the specic embodiment, two multiplier stages are shown between the input screen 1S and the target 50, the number of multiplier stages is, dependent upon the amount of amplification desired.
The electron multiplier electrode 40 used in each stage is shown in more detail in Fig. 2. The electrode 40 is comprised of a thin optically transparent supporting sheet 42 of a material such as glass or aluminum. oxide. Positioned on the surface of the support sheet 42 facing the input screen 18 is a layer 44 of a suitable fluorescent material such as zinc sulfide which generates light upon electron bombardment. Positioned on the opposite surface of the support sheet 42 is a layer of aI photoemissive material 46 such as cesiated antimony which generates ,electrons in response to light radiation from the fluorescent layer 46. The electron multiplication within each stage is of the order of 10 with an accelerating voltage of 10 kilovolts. By increasing the acceleration potential between stages to 30 kilovolts an amplification of 40 may be obtained. VThe Yrequisite potential for the electron multiplier electrodes 40 within the envelope 10 may be supplied by any suitable voltage source and in the specific embodiment is represented by the battery 70 and a bleeding resistor 72. A tap is provided on the resistor 72 for each of the multiplier electrodes 40 so that a potential difference of the order of 10 kilovolts is provided between each multiplier electrode stage. The potential is applied to the photoemissive layer 46.
Focussing electrodes 32, 34 and 36 are provided between the input screen 18 and the first multiplier electrode 40 for focussing the electron image from the screen 18 to an inverted image of reduced size of the first multiplier electrode 4f). The electrode 32 may be a short cylindrical metallic section at the potential of the photocathode 24, for example 30 kilovolts, and is positionednear the screen 1S. The electrode 34 is similar in structure to the electrode 32 and positioned at a greater dis-` tance from the screen 18 and at a more positive potential. than the photocathode 24 or negative 29 kilovolts. The electrode 36 consists of a tapered tubular member. The smaller opening of the electrode 36 is toward the input screen 18 and the larger opening is closed by the first multiplier electrode 40. The electrode 36 is at the same potential as the first multiplier electrode 40. In the specific embodiment, this potential may be ten kilovolts more positive than photocathode 24 or 20 kilovolts.
Similar focussing electrodes and acceleration voltages may be used in the second stage and also to focus the electron image from the second electrode 40 to the,
which is caused to scan a raster on the exposed surface. of the semi-insulating layer 56 of the target 50.1231'.y
means of detlecting coils 75, focussing coil 76 and align'- covering the mesh 102 with water and applying the ortrates through to the photoemissive layer 24 `of the input screen 18 and causes emission of the electrons. The electrons emitted ,across lthe surface form an electron image which duplicates the space distribution and intensity of the radiationfimage projected onto the input screen 18,. The electron image thus generatedfrom the photoga'nic material ina suitable solution onthe surface of the water. As the organic material expands out on the surface of the water, the solution evaporates leaving only the organic film material. The water is then removed allowing the organic film to settle onto the mesh support 102. The organic film may 'then be dried and a scattering layer 104 o-f a high atomic number greater than 25, such as gold, is` evaporated onto the free surface of the organic film. The thickness of the scattering layer 104 may be of the order'of 100 angstroms or less. Since emissive layei 24 of the input screen 18 is accelerated and focussed by means of potential applied to electrodes 32; 34, 36 and 40 from the battery 70. yThe electron image is thus inverted and reduced in size .and the electrons bombard the fluorescent layer 44 of the first multiplier elect-rode `40 with energy of about vl0 kilovolts. The electron bombardment of the layer 44 causes emission of light which, in turn, penetrates theglass support layer 42 and causes emission of electrons from the photo'-V emissive layer 46 in the form of an electron image which duplicates the space distribution of the radiation image. The electron image generated in the first electron multiplier electrode 40 is, in turn, accelerated, lfocussed a'rid inverted to the second multiplier electrode 40 where a' similar multiplying action occurs and then is accelerated, focussed and inverted onto the target electrode 5d. The lectron image striking the target 50 penetrates the conductive backing layer 54 and* produces in Vthe semiinsulating layer 56 what may be thought of as the conductivity image duplicating in the space distribution the electron image and, therefore, the radiationima'ge pro-- jected onto the input screen 1S.
`As previously "explained, without an image projected ontothe input screen 18, there is no conductivity image,
in the insulating layer 56 and the exposed or scanned vsurface of the semi-insulating' la'yer 56 is at the potential of the cathode i'[1 of the scanning electron beam. In the specific structure shown, the cathode 71 of the scanning beam is at ground potential 'while va potential of the order of 50 vo1ts may be applied to the conductive backing layer 54 of the target 50. When a radiation image is projected onto the input screen 1S, the electron image focussed onto the target 50 in a manner previously described results in the conductivity image in thek insula'ting`-l'ayer756 and the respective areas of the exposed su'rface of the semi-insulatinglayer 56 rises to a fraction of the potential applied to the conductive backinglayer 54 of the target 50. This potential pattern on the surface corresponds to the variation of the conductivity image over its surface., n When the electron beam generated by the electrongun 69 strikes a particular element or; area; on the semi-insulatingzlayer surface 56, Vit deposits sufficient electrons to recharge the potential on the surface of the insulator 556 to' that of the cathode 716i the scanning beam. A low velocity electron beam is utilized in this structure. As a result of this recharging action ofthe scanning beam, a corresponding chargeV current flows through the output resistor 66 connected to the target 50 and may be used in awell known manner.
the scattering layer 104 may be very thin, it may be desirable Gto evaporate a` thin support layer 103 of a material such as silicon monoxide or aluminum onto the organic film. The organic layer may then be removed by baking, leaving the layer 103. A secondary electron emissive layer 106 of a thickness of about 600 angstrorns is then evaporated onto the electron scattering film 104- of a suitable insulating material such as potassium chlo- Another modified structure of a multiplier dynode is shown in Fig. 4. The multiplier electrode 110 shown in Fig. 4 consists of alayer 112 of aluminum oxide (A1203) which may be used as the secondary emissive layer with an electron scattering layer 114 deposited on one side thereof and a supporting ring 116 provided around the periphery of the multiplier structure 110. The support ring 116 may be of a suitable material' such as nickel Vor glass. One possible method of preparing the aluminum oxide film is to anodize a thing film of aluminum about 600 angstroms in thickness in a solution of ammonium citrate. This produces a coating of aluminum oxide on each side of the aluminum film. The remaining aluminum layer and the aluminum oxide layer on n only a thin aluminum Voxide film of a thickness of about 600 angstroms.
The output signal may be utilized to modulate a television transmitter or may be connected directly to an image reproducing device and thereby brighten, enlarge or vary contrast by standard television techniques.
VReferring to Fig. 3, there is shown a modified multi- This process isv more fully described in a` copending application entitled Transmission Secondary Emission Dynode Structure, by E. J. Sternglass and W; 'Feibelmam filed May 23, 1956, Serial No. 586,826, and assigned to theA same assignee; this application issued as U.S. Patent No.. 2,898,499 on August 4, 1959.
Another type of multiplier dynode 120 shown in Fig. 5 utilizes an aluminum oxide layer 122 to provide a strong mechanical supporting layer for the dynode. An electron scattering layer 124`of gold is provided on one surface of the support film 122 and a Vsecondary emissive `layer 126 is deposited on the exposed surface of the electron-scattering lmaterial layer 124. The dynode structure shown in Figs. 4 and 5 requires no mesh supportV structure and, therefore, the resolution of the deviceis greatly improved. The necessity of mesh support structurelimits resolution in the image to a degree less than the mesh count of the support member. The use of the aluminum oxide as the support layer for the multiplier structure removes spurious patterns due to interference with other inesh structures in the'device.
-j In vthe operation of the dynodes 100, il@ and 120 shown in Figs. 3, 4-and 5 into Figl, the electron image generated by the input screen 18 will be accelerated to the first dynode 100, 110 or 120 by a potential of the order of 2000 volts and the incident electrons will strike n the electron scattering layer 104, 114 or 124`and be scati at an angle with respect to normal. The longer the path secondary emissive dynode or electrode 100 shown in Fig. 3 consists of a support mesh 102 of a conductivev material such as copper or nickel having a large percentage of open area. An organic film of a material such as'nitrocellulose may be settled onto the mesh 102Aby of the electron within a given secondary emissive layer the greater will be the amount of secondary emission from the layer. It is desirable that the secondary emissive layer should have a thickness of the order of 600 sible in which the secondary electrons can travel` relatively large distances and thereby escape from much greater depths than in the case of metals, icomplex, cesiated or activated layers or insulators of amorphous structure such as glass. The secondary electrons generated at the surface of the dynode facing the adjacent or second multiplier stage are of low energy of the order of to 5 electron volts. By the selection of accelerating voltages between stages and thicknesses of the electron scattering layer and secondary emissive layer, the incident electrons may be substantially absorbed within each dynode structure. The low energy secondary electrons generated by the iirst dynode will be accelerated to the next stage and so on to the final stage in the manner described above. It has been found that an amplication of the order of from to 10 may be obtained within each dynode structure utilizing the secondary emissive structure. The support layer 122 of aluminum oxide has little effect on the electrons.
Referring in detail to Fig. 6, a modified target structure is shown which may be incorporated into Fig. 1 and is particularly adaptable to the structures shown therein. The target structure 130 provides an aluminum oxide self-supporting film 132 made by the process previosuly described with respect to the secondary'emissive dynodes in Figs. 3, 4 and 5 and provided with a support ring 136 about the periphery of the target. A conductive backing member 134 is deposited on one side of the aluminum oxide layer 132 and a semi-insulating layer 138 is deposited on the free surface of the conductive back plate 134 similar to that described with respect to the target 50 shown in Fig. 1. This type of structure removes the objectionable mesh structure which limits the resolution obtainable in the target and also removes the possibility of spurious patterns set up inv the target due to interference with other mesh structures in the device.
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 changesY and modifications without departing from the spirit and scope thereof.
I claim as my invention:
1. A target electrode comprising a peripheral support ring, a support film of a thickness of the order of 200V to 500 angstroms of a coherent oxide of an `element of the group consisting of aluminum and magnesium disposed and supported on said support ring, a conductive back plate layer deposited on one surface of said 'support lm and a layer of semi-insulating material ex-v hibiting the property of electron bombardment induced conductivity deposited on the free surface of said con-v screen, an electron sensitive target electrode capable ofstoring an electron image and on which the electron image from said input screen is directed,v said target electrode comprised of a relatively thin layer of semi-insulating material exhibiting the property of electron bombardment induced conductivity, a conductive layer on the bombarded side of semi-insulating layer facing said input screen, means for scanning the free surface of said semiinsulating layer with an electron beam of low velocity, means for deriving an electrical signal from said conductive layer of said target in response to said scanning beam passing over said target in a point by point manner, and an electron multiplier assembly positioned between said input screen and said target electrode for amplifying the electron image focussed from said input screen onto said target electrode, said electron multiplier assembly comprising a plurality of transmissive type dynodes, said dynodes comprising a support member and a continuous layer of insulating material which exhibits the property of secondary electron emission from the surface facing said target in response to bombardment of electrons on the other surface facing said input screen.
3. An image pickup device comprising a vacuum tight enclosure having a radiation sensitive input screen capable of producing an electron image corresponding to aradiation image projected onto said radiation sensitive screen, an electron sensitive target electrode capable of storing an electron image and an electron multiplier assembly positioned between said input screen and said target electrode for amplifying the electron image focussed from said input screen onto said target electrode, said target electrode comprised of a relatively thin layer of semi-insulating material exhibiting the property of electron bombardment induced conductivity, said electron multiplier assembly comprising a plurality of transmissive type dynodes, said dynodes comprising a relatively thin layer of insulating material capable of emitting sec-Y ondary electrons from one surface in response to electron bombardment of the opposite surface and a supporting layer provided on the input side of said insulating layer,v
said semi-insulating layer comprising a continuous support film of about 500 angstroms in thickness of a co-I herent oxide of an element of the group consisting of aluminum and magnesium.
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|U.S. Classification||313/377, 313/527, 250/214.0VT, 315/DIG.100, 315/11, 313/376|
|International Classification||H01J31/36, H01J31/49, H01J31/50|
|Cooperative Classification||H01J31/508, H01J31/36, Y10S315/01, H01J31/49|
|European Classification||H01J31/50H, H01J31/36, H01J31/49|