US2851624A - Tube sensitive to images of invisible radiation - Google Patents

Description

SPf 9 1958 E. E. sHELDoN 2,851,624

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ing/fn @www JMJ/mv 31 WX United States Patent TUBE SENSITIVE TO MAGES F INVISIBLE RADIATION Edward Emanuel Sheldon, New vork, N. v.

Application October 12, 1951, Serial No. 251,114

11 claims. (C1. 313-65) This invention relates to an improved method and device of intensifying images and refers more particularly to an improved method and device for intensifying images formed by the impingement of X-rays, and other invisible radiations, such as gamma rays and the like, and also irradiation by beams of atom particles such as neutrons, and is for the same subject matter as my U. S. Patent No. 2,555,423.

The main problem in using X-rays or neutrons for medical diagnosis is the danger of causing damage to the patient by radiation. The danger of over-exposure necessitates the used of a very weak X-ray or neutron beam, which means that the X-ray intensity much be very low and we have, therefore, only a small number of X-ray quanta in the invisible image of the human body. If we do not use all of available X-ray quanta, we will not be able to reproduce an image having all the necessary intelligence, no matter how much we will subsequently intensify this image by electronic means. The solution of this problem is to provide an invisible radiation receptor which will utilize all incoming photons of radiation, which means it will have quantum eiciency elo-se to unity. The present X-ray receivers of photoemissive type have a very low quantum eiciency, such as of the order of a fraction of 1% and, therefore, suffer from a basic limitation, as explained above.

Another object of this invention is to provide a method and device to produce intensified images. This intensification will enable the overcoming of the ineticiency of the present fluoroscopic examination. At the present level of illumination of the fluoroscopic image, the human eye has to rely exclusively on scotopic (dark adaptation) vision, which is characterized by a tremendous loss of Anormal visual acuity in reference both to detail and to the contrast.

`In order to obtain improvement in the visual acuity, intensification of the brightness of the order of 1000 is obligatory. Without intensification of luminosity of at least of the order of 1000, the eye is confined to so-called p Another object of thisinvention is to make it possible to prolong the uoroscopic examination since the invention will allow the` reduction of the strength of radiation aiecting the patients body. Conversely, the exposure time or energy necessary for the radiography may be reduced. The objectives of this invention are obtained by a novel invisible radiation sensitive image tube. This tube has X-ray or neutron receiving screen of a dielectric material,

which exhibits property of becoming conductive and producing a current of electrical charges (electrons and positive holes) in response to X-ray or neutron beam. The

` invisible X-ray or neutron image produces, in the invisible radiation sensitive screen, a pattern of electrical charges with a high quantum etiicency, such as approaching unity. The electrical charges have the pattern of the X-ray or neutron image. They cannot, however, be used directly for reproduction of a visi-ble image with the necessary intensiiication. They are used in my invention to modulate the emission of a strong uncontrolled photoelectron beam. The modulated photoelectron beam will have the pattern of the original X-ray or neutron image. This photoelec- `tron beam can be accelerated, electron-opticall diminished and projected onto a fluorescent screen in the image tube to reproduce a visible image with necessary intensiication, as was explained in my U. S. Patent No. 2,555,423. f

The invention will be better understood when taken in connection with the accompanying drawings.

In the drawings:

Figure l-represents a novel invisible radiation sensitive image tube.

Figure 2 represents a modification of invisible radiation sensitive image tube.

Figure 3 represents an invisible radiation image tube 'a novel invisible radiation sensitive tube 1. The face 2 of the image tube 1 must be of a material transparent to the radiation to be used. Inside of the tube there is a composite screen 3 sensitive to X-rays or neutrons. The screen, shown also in Fig. 4, comprises a thin, radiation transparent, conducting layer 4, such as of aluminum, gold, silver or platinum, and an X-ray or neutron sensitive layer 5 o-f dielectric material, which has property of becoming conductive and producing a current of electrical charges in response to said radiation.

The theory and explanation of this phenomenon is given by the article of S. G. Zizzo and I. B. Platt Detection of X-Ray Quantay by a Cadmium-Sulphide Crystal Counter, Physical Review, volume 76, September l, 1949vl page 701'.. lt is believed that energetic X-ray photons striking X-ray sensitive dielectric materials are able to remove an electron from its place in the matter. The deficiency of an electron can be considered as a positive particle, which is also called a positive hole. Electrons and positive holes move across said insulator under the inuence of electrical field applied by means of conducting electrodes, which are deposited on either side of insulator. The electrons and positive holes form, therefore, a current of electrical charges produced by X-rays. Such materials are CdS, diamond, MgO, ZnS or 'antimony compounds. On the side ofthe layer 5 remote from the X-ray source, there is deposited another thin conducting layer 6. This 'layer may be formed by a plurality of small conducting islands of platinum, gold or silver, which are insulated from each other. Such layer may be produced by covering layer 5 with a line mesh and evaporating a conducting material over said mesh. After evaporation, the mesh screen is removed, leaving a mosaic of conducting islands. Layers 4 and 6 are connected to the terminals of a source of electrical power, such as battery 11, in order to provide a polarizing electrical field across the layer 5. In some instances the layer 6 maybe formed as a continuous conductinglayer 6a of any of materials men- `iton of photoemissive X-ray or neutron receivers.

should be excessive, the stored charge pattern may be removed by changing the polarity of battery.

The X-ray or neutron beam strikes the screen 3 and produces therein a current of` electrical charges having the'pattern of the examined body, as explained above. Screen 3 has quantum efficiency for X-rays close to unity. Therefore, the X-ray image is converted into pattern of electrical charges or potentials without any loss of information, in contradistinction to the opera- The efliciency of the' response of layer 5 to the X-ray or neutron beam can be markedly increased by irradiating layer 5 with red or infra-red light at the time of the X-ray exposure. Some CdS crystals respond better to green light irradiation as it reduces their lag.

In close spacing to the composite screen 3, such as a few microns, there is mounted a ne mesh screen 8 of conducting material. On said mesh screen, there is deposited a' photoemissive layer 9 in such a manner as not to obstruct the openings in the mesh. The pattern of the electrical charges in the layer 5 can be considered as a pattern of various potentials or electrical elds. I discovered that these potentials will modulate the emission of photoelectrons from the photoemissive layer 9, although they are behind said layer, provided the distance between them is very small. The layer 9 is irradiated by a source of light 10 and produces a broad beam of photoelectrons. The emission of photoelectrons from layer 9 depends on electrical potentials i. e., ields in its proximity. The more positive the charges in the layer 5, the more suppressed will be the emission of photoelectrons from layer 9- In this Way, the photoelectron beam will be modulated by the charges in the layer 5 of screen 3, which have the pattern of the original X-ray or neutron image.

In some cases the mesh screen 8 can be used both foi-providing potential for the photoemissive layer 9 and for providing electrical eld across the layer 5.

t The photoelectrons emitted by the layer 9` are now accelerated by means of high voltage electrostatic or electromagnetic ields 18, which may have the form of ring electrodes, to the desired velocity, producing in this manner intensication of the electron image. The electron image is focused by electrostatic or electromagnetic elds 19. The accelerating and focusing electrodes are well known in the art. It is believed, therefore, that they do not have to be described in detail in order not to complicate the drawings.

Next, the electron image is diminished by means of electron lenses to the desired size, resulting in image intensification proportional to the square power of the linear diminution and is projected through the electron ,pervious, light reflecting aluminum layer on the uorescent screen 21, such as made of ne grains of ZnO, Zn silicates or ZnS with appropriate activators, where it can be viewed by observer. The electron-optical diminution of an image is also well known in the art.

In some cases, it may be more desirable to have the fluorescent screen mounted outside of the vacuum tube; in such cases, thin electron transparent layer of chromium or aluminum is placed on the end wall of the vacuum tube made of fernico glass. The image appearing on the fluorescent screen can be viewed directly or by means of an optical eye-piece 23 giving the desired optical magnification of the image. In this way, all objectives of my invention are accomplished. The invisible radiation image is converted into electron image without any loss of information because of high quantum efficiency of the X-ray or neutron sensitive screen and the resulting electron image can be intensied to produce the necessary brightness of reproduced visible image.

Another modification of my invention is shown in Fig. 2. The invisible radiation sensitive image tube 15 has an X-ray or neutron sensitive composite screen 25,

which is similar to the screen 3 described above, except that composite screen 25 is of a perforated type. The layer 26 is of a conducting mesh screen, such as of aluminum, gold, silver or platinum. The layer 27 is deposited on the mesh screen 26 in such a manner as not to obstruct openings therein. The layer 27 is of a dielectric material, which has the property of becoming conductive and producing current of electrical charges (electrons and positive holes) in response to X-ray or neutron radiation. The layer 28 is a conducting mesh screen, such as aluminum, gold, silver or platinum and is deposited on the layer 27 in proper alignment with the screen 26, so that the openings in both screens will be aligned with each other. The mesh screen 28 may also be mounted in close spacing to the layer 27 instead of being deposited thereon. A source of electrical power,

such as battery 11, is connected to the conducting layers 26 and 2S to provide a polarizing electrical field across the layer 27. The X-ray or neutron image is converted by said screen 25 into a pattern of electrical charges or potentials, as was explained above.

A source of photoelectrons 14 or an electron gun producing a broad beam of electrons is disposed within the image tube t-o provide a broad uncontrolled beam of electrons. This beam of electrons is accelerated and isV projected on the X-rayand neutron sensitive composite screen 25 by the electrical elds 15. The screen 25 has a pattern of charges or potentials thereon, which Icorresponds to the original X-ray image. The passage `of electrons fro-m the source 14 through said perforated target depends on potentials present around its openings.y The transmitted electron beam is modul-ated by the pattern of said potentials and will have, therefore, the pattern of the original invisible radiation image. The transmitted electron beam is now accelerated, is electron-optically diminished and is focused on the electron reactive screen 24 having a fluorescent layer 2l and backing layer 2t) of an electron transparent conducting material, such as aluminum. The accelerating fields 30 may be electrostatic or electromagnetic, the focusing elds 29 may Ialso be of electrostatic or electromagnetic type. They are well known in the art. The fluorescent image produced by impingement of electron beam on the fluorescent screen 21 is of brightness, which allows the-eye to operate with a full visual acuity for detail and contrast.

In another modification of my invention shown in Fig. 3, an additional feature of the storage of invisible radiation image is added. The operation of the X-ray or neutron intensifying storage tube 36 is similar to the tubes 1 and 15 described above. The invisible radiation image is converted in the screen 3a into a current of electrical charges having the pattern of said image. 'Ihe screen 3a comprises an X-ray or neutron radiation transparent conducting layer 4, such as of aluminum, gold, silver or platinum and X-ray or neutron sensitive layer 5 of dielectric material, which has property of producing a current of electrical charges in response to lsaid radiation. Such materials are CdS, diamond, MgO, ZnS or antimony compounds. In the preferred embodiment of this invention, layer 6 is omitted and instead, a mesh screen 6b is disposed in close spacing to the uncovered side of the layer 5', as shown in Fig. 6. The mesh screen is connected to one terminal of the battery 11; the other terminal is connected to the layer `4. In this way, electrical-field is provided across the layer 5.

In 4close spacing, such as a few microns, to the composite screen 3a, there is mounted a fine mesh screen 8 of conducting material. On said mesh screen, there is deposited a photoemissive layer 9 in such a manner as not to obstruct the openings in the mesh. The pattern of the electrical `charges on the layer 5 .can be considered as a pattern of various potentials or electrical elds. These potentials will modulate the emission of photoelectrons from the photoemissive layer 9, although they are behind said layer. The layer-9'is irradiated by a source of light ...midi

1 0 and produces` a strong beam offphotoelectrons. The emission of photoelectrons from the layer 9 depends on electrical iields inits proximity. The more positive the charges in the layeriS, the more suppressed will be the emission of photoelectrons from the layer 9. In this way,

' the photoelectron-beamwill'be'modulated by the charges in the screen 3a, which have the pattern of the original invisible'image; The'- photoelectron-imageis accelerated andvfocused bythe electromagnetic or electrostatic iields 34 and.34a.on the perforatedzstorage `target 31. The focusingand accelerating fields are notindicated in detail as they are welll knownin the' art and will only serve to complicate" the drawings. Sometimes it is better to demagnify thephotoelectron` image'electron-optically before projecting it on saiditarget; This can be done bythe use of electron lenses. THeperforated storage target 31 is of dielectric. material,1 such as'of" quartz,V precipitated silica, CaFz, BaF2, mica or glass; The storageI target 31 may be mounted inlthetube b'yfjmeansof metallic rings or may befdeposited: onla fine mesh. screenV 3a so that openings in said screen are not obstructed. In' such a case, the storing dielectric .surface 31b vshould face the' -photoelectron beam.

The photoelectron image1 is`focused on' the target 31 with velocity` causingLsecondary emission from the target atthe ratio greater thanfunity (S greaterwthan' l). The secondary electrons?. emitted from"the dielectric target are' drawn away by theV adjacent'conducting mesh screen 32. In= this-way,. the.- photoelectronf. imagev is deposited as a positive charge image on the target. It is obvious that photoelectron imagecan also be focused on the target 31 with..velocity,aLwhich secondary electron emission is smaller than unity ('S smaller .than .1). The resulting charge image will then be a negative one. In such a case, the mesh. screen .32 may be omitted.- A. strong broad beam of electrons 35 is emitted fromthe electron gun 37 or from a source of photoelectron's, such'as photoemissive surfaceirradiated Vby light.v Thisifbea'mnSS has=to pass through the"perforated..dielectrictarget-Sl. The passage is modulated by the .chargeimage deposited on said dielectric'rtarget by the action .of the. invisible image, as was explained above.

Therefore-.the .beam .of..electrons which is. .passing through the dielectrictargetwill have' imprinted on it the pattern/:of theoriginal invisible X-ray or neutron image. Theztransmitted electron :beam;:35 is of d a much greater intensity than the original X-ray or neutron image. Therefore, by converting said transmitted electron beam into a visible image in the fluorescent screen 21, a marked intensification of the original X-ray image is obtained. The uorescent screen 21 has an electron transparent, light reecting backing layer 20, such as of aluminum, to prevent back-scattering of light. Instead of fluorescent screen, other electron reactive surfaces may be used, such as photographic films, electrolytic papers of electrographic plates. The transmitted electron beam 35 before its reproduction into visible image may also be intensied by acceleration and electron-optical demagnification, as was explained above.

The storage target 31 may store electrical charges for a long period of time, ranging from a few seconds to a few minutes, depending on the dielectric material used. During the storage time, the X-ray beam may be shut oi, as it is not necessary any longer to maintain the presence of the X-ray image, in contradistinction to all present X-ray devices. This results in a marked reduction of X-ray exposure, which was one of the primary objectives of my invention.

This system of storage of electron beam corresponding to the X-ray image, can also be used in the invisible radiation image tube shown in Fig. 2.

It is obvious that my system of intensification of X- ray or neutron images may be used not only for medical examinations, but for industrial testing or X-ray diffraction studies as well.

The duration of X-ray or neutron induced charges in 6 dielectric layerSfmay vary considerably according to the type' ofmaterial used and the manner of its preparation. In the systemdescribed above duration of said charges was'%,0-V;0 second, so'- that the X-ray exposure had to be maintained throughout the examination. The lag in this system would be verydetrimental because without a fast succession of pictures such as 15-30 in a second, fiickerl or trailing. of images would result. In medical X-ray examinations I discovered that a long duration of induced charges may be on the contrary of a great benefit. If the charges whichhave the pattern of invisible image will persist for a long time such asa few Vseconds. or longer, Iwe will be able to reproduce X-ray images during all-this time without necessity of'maintaining X-ray radiation. Once the charge pattern in layer 5 is formed, the'X-ray beam may be shut oif 'and the uncontrolled. electron beam will be modulated by said charges as long as this charged pattern lasts. Therefore electron beam `will lreproduce during all this time visible images in the fluorescent screen 24. In this system of operationlthe tube 36 acts as a storage tube.

Itis evident thatthe tube. 1 will operate also as a storage tube, as was explained above, if we substitute the composite photocathode-S bythephotocathode 3a shown in Fig. 3 or by the arrangement shown in Fig. 6. Also the tube 15 may be used for storage of images. In such a case; however `the layer-1281of` the composite photocathode 25 preferably should be omitted.

It will thus` be seenf'th'atzthere' is provided a device in which the severalfobjects of'this inventionl are achieved and which islwell. adapted'to meet the conditions of practicalfuse.

As various possible embodimentsfmightlbe made of the above invention and as various changes might be made in the embodiment above set-y forth, it is to be understood that all-mattenhereinzset, forth or shown inthe accompanying drawings, is to. be interpreted as illustrative and not in a limiting sense.

What is claimedis:

l. An invisible radiation sensitive tubecomprising in combination a perforated screen consisting of a dielectric material, which has the property of, becoming electrically conductive .when irradiated by invisible radiation image and producing electrical charges having the pattern of said invisibley image, vmeansfor producing -a broad beam of electrons, means for projecting said beam of elec* trons on said perforated screen for modulating the passage of said beam of electrons through said perforated screen with said electrical charges on said screen and electron reactive means for receiving said modulated beam of electrons and reproducing said invisible image.

2. An invisible radiation sensitive image tube comprising in combination a screen consisting of dielectric material, which becomes electrically conductive when irradiated by invisible image and produces electrical charges having the pattern of said invisible image, means for producing a first broad beam of electrons disposed in close spacing to said dielectric screen on the side opn posite to the impingement of said invisible radiation image, emission of said beam of electrons being modulated by said pattern of electrical charges in said dielectric screen, a perforated storage target for receiving and storing said modulated first electron beam, means for producing a second broad beam of electrons, the transmission of said second beam of electrons through said perforated target being modulated by said stored rst electron beam on said target and electron reactive screen for receiving said transmitted beam and reproducing said invisible image.

3. An invisible radiation sensitive image tube comprising in combination a screen consisting of dielectrc materal, which becomes electrically conductive when irradiated by said invisible image and produces electrical charges having the pattern of said invisible image, means for producing a first broad beam of electrons disposed in close spacing to said dielectric screen .on the side opposite to the impingement of said invisible radiation image, emission of said beam of electrons. be'- ing modulated by said pattern of electrical charges in said dielectric screen, a storage target for receiving said modulated electron beam and storing said beam, means for producing a second beam of photoelectrons closely spaced to said storage target, emission of said second beam of protoelectrons being modulated by said stored electron beam and a screen for receiving said modulated second photoelectron beam and reproducing said invisible image.

4. A vacuum tub-e comprising in combination a first screen supported by the end wall of said tube and comprising an imperforated layer of material which converts an invisible radiation imagev into an electrical pattern corresponding to the pattern of said invisible image and means for producing a broad beam of electrons modulated with said electrical pattern Aon said screen, said means for producing a broad beam of electrons being separated from said layer producing said electrical pattern, said device furthermore comprising a perforated screen mounted between said layer producing electrical pattern and said means producing a broad beam of electrons, and a second screen for receiving said modulated broad electron beam.

5. A device as defined in claim 4, in which the emission of said broad electron beam is modulated by said electrical pattern on said imperforated layer.

6. A device as defined in claim 4, in which said imperforated layer converts the total image of said invisible radiation simultaneously into an electrical pattern and in which said second screen is supported by the opposite end wall of said tube.

7. A vacuum tube comprising in combination a rst screen havingv an imperforate layer of material which converts an invisible radiation image into an electrical pattern corresponding to the pattern of said image and means for producing a broad beam of electrons modulated with said electrical pattern on said screen, said means for producing Asaid broad beam of electrons being separated from said layer producing said electrical pattern, means for demagniiication of said electron beam, and a second screen for receiving said modulated demagnied broad electron beam and comprising a light reflecting layer and a uorescent layer.

8. A device as defined in claim 7, in which said imperforated layer converts the total image of said invisible radiation simultaneously into said electrical pattern, and said means for producing a broad beam of electrons comprise a photo-emissive layer.

9. A vacuum tube comprising in combination an imperforated screen having a layer of material which converts an invisible radiation image into an electrical pattern corresponding to the pattern of said invisible image and means for producing a broad beam of electrons modulated with said electrical pattern on said screen, said means for producing a broad beam of electrons being spaced a few microns apart from said layer producing said electrical pattern, and a second screen for receiving said modulated broad electron beam and comprising a light reflecting layer and a uorescent layer.

l0. A vacuum tube comprising in combination a screen comprising an imperforated layer of material which converts an invisible radiation image into an electrical pattern corresponding to said invisible image and means for producng a broad beam of electrons modulated with said electrical pattern, said device furthermore comprising a perforated screen mounted between said layer producing said electrical pattern and said means producing a broad beam of electrons, means for demagnication of said broad electron beam, and a second screen for receiving said-modulated broad electron beam.

11. A device as defined in claim 10, in which said means'for producing a broad beam of electrons comprise photoemissive layer.

References Cited in the le of this patent

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