US2699511A - Storage tube for invisible radiation - Google Patents

Description

Jan 11, 1955 E. E. sHELDoN STORAGE TUBE FOR INVISIBLE RADIATION 2 Sheets-Sheet l Filed May 4, 1951 Twl TIQ- E DDD Mr-III D El D Jan. ll, 1955 E. E. sHl-:LDON

STORAGE TUBI: FOR INVISIBLE RADIATION 2 Sheets-Sheet 2 Filed May 4, 1951 JNVENTOR. [ow/m0 EMA/vuil. SHELDo/v BY y@ 6 States This invention relates to an improved method and device for intensifying images and refers more particularly to an improved method and device for intensifying images formed by the impingement of invisible radiations (which term is meant to include X-rays and other invisible radiations such'as gamma rays and the like, and also irradiation by beams of atom particles such as e. g. neutrons) on a fluorescent or other reactive screen, and it is a continuation'in part of my patent application Serial No. 741,803 for Method and Device for Intensification of Images, filed on April 16, 1947, now U. S. Pat. No. 2.555,423.

One primary object of the present invention is to provide a method and device to produce intensified images. This intensification will enable to overcome the inefficiency of the present fluoroscopic examinations. 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 normal visual acuity in reference both to detail and to the contrast.

Another object of this invention is to make it possible to prolong the fluoroscopic examination since it will reduce markedly the strength of radiation affecting the patients body. Conversely, the exposure time or energy necessary for the radiography may be reduced.

Another object is to provide a method and device to store X-ray images which was not possible until now.

The present intensifying devices concerned with reproduction of X-ray images are completely unsatisfactory,

kbecause at low levels of fluorescent illumination, such as we are dealing with, there is not enough of X-ray photons to be absorbed by fluorescent or photoelectric Vscreens used in such devices. Therefore the original X-ray image can be reproduced by them only with a considerable loss of information. It is well known that the lack of sullicient number of X-ray quanta cannot be remedied by the increase of intensity of X-ray radiation, as it will result in damage to the patients body. This `basic deficiency of the X-ray examination was overcome in my invention by using an X-ray exposure of a strong intensity but of a short duration, and storing the invisible X-ray image for subsequent inspection for the desired length of time without any need of maintaining the X-ray irradiation. The X-ray beam, therefore, can be shut off while reading the stored X-ray image and in this way the total X-ray exposure received by the patient is not increased, in spite of using bursts of a great X-ray intensity.

In order to obtain the objects of this invention a special X-ray sensitive image tube had to be designed, Fig. 1. This novel X-ray image tube is characterized by elimination of the optical lens system present in other image tubes which resulted in -30 fold gain in the light reaching the photocathode. Then by the combined use of a novel photoemissive pick-up system and storage system of the tube, of aV multiplier section of the tube, of a novel electron image amplifier system, of the electronic acceleration and of the electronic image diminution the intensification of the luminosity of the original image exceeding the ratio of 1000-1 was accomplished.

The elimination of the optical system present in other image tubes to focus the fluorescent image on the photocathode of the tube was accomplished by positioning within the X-ray sensitive image tube ofthe screen, consisting of combination of X-ray transparent, light reflecting layer, of X-ray fluorescent, or reactive layer, and of the photoemissve layer. All layers are placed in close apposition to each other to prevent the loss of definition. The fluorescent and photoemissive layers are separated only by a very thin light transparent, chemically inactive, barrier layer of a thickness not exceeding 0.15 millimeter. The previous combinations of fluorescent and photoemissive layers were not ysuccessful because of detrimental ice chemical interaction of both layers, due to lack of a barrier between them, therefore the introduction of light transparent barrier layer represents a very important part of this invention. The photoemissive layer is of semitransparent type. This layer is characterized by emission of electrons on the side opposite to the side of the incident light. The photoelectrons emitted from the photoemissive layer in a pattern corresponding to the incident light pattern are accelerated and focussed by means of magnetic and/or electric fields on the novel image amplifying system.

The amplification section of the tube consists of one or a few screens each of them composed of a very thin light-reflecting, layer, and of a photoemissive layer in close apposition to each other. It is necessary to include a very thin light transparent, chemically inactive barrier layer between the fluorescent and photoemissive layers in order to prevent their chemical interaction, which should be of `a thickness not exceeding 0.15 millimeter. .The electrons from the pick-up section of the image tube are focussed by magnetic or electrostatic fields on the fluorescent layer of a screen described above. The luminescence ofthe fluorescent layer of the amplification screen will cause the emission of electrons from the photoemissive layer of thc screen. This process can be repeated a few times, using a few screens described above resulting in 10-100 times intensification of the original electron image.

In another modification to be used in this invention there is an additional multiplier section which consists of a multiplier and can give an additional intensification of the electron image by secondary electron emission.

The electrons leaving the amplifying section are acr celerated by means of high Voltage electro-static fields.

The accelerating system can be of a conventional type well known in the art. Much better results with higher voltages will be achieved with an electrostatic multilens system. Next the electron image is stored in the special storage target. The storage of the electron image allows the inspection of the X-ray image for a desired time without the need of maintaining X-ray irradiation during the reading. This saves so much energy in X-ray exposures that the patients body will not be impaired even with prolonged examination.

Next the electron image is demagnied which results in its additional intensification. The electron diminution of the image, in order to gain its intensification is well known in the art, therefore does not have to be described in detail.

The diminished electron image is projected on the fluorescent screen at the end of the tube where it can be viewed by the observer directly or by means of an optical magnifying eye piece, through the light transparent end wall of the tube.

The use of an optical eye piece to magnify optically the electronically diminished image appearing on the fluorescent screen, is also well known in the art, therefore does not need further description.

The combination of the above described features of the X-ray sensitive image tube allows to obtain intensification of the original X-ray image which was the primary objective of this invention. Having such a marked intensification of the original X-ray image it will be possible now to use a much finer grain of fluorescent screens than was practical until now and to improve this way detail and contrast of the final image, which was another purpose of this invention. 'f

The invention will appear more clearly from the, following detailed description when takernin connection with the accompanying drawings by way of example only,- preferred embodiments of the inventive idea.

In the drawings:

Fig. 1 is a cross-sectional view of the X-ray image storage tube.

Fig. 2 is a cross-sectional view of a modification of the X-ray image storage tube.

Fig. 3 is a cross-sectional view of the X-ray storage image tube in combination with optical system.

Fig. 4 is a cross-sectional view of a modification of the X-ray image storage tube.

electron pervious layer, .of a fluorescent Fig. is a cross-sectional View of a modification of the X-ray image storage tube.

Figs. 6 and 6a are a plan view of the storage target.

Fig. 7 is a diagrammatic view of the neutron sensitive photocathode to be used in neutron storage tube.

Figs. 8 and 9 show a modication of a neutron sensitive photocathode.

Fig. is a cross-sectional view of a modification of the X-ray image storage tube.

The face 1 of the image tube 4@ shown in Fig. 1 must be of a material transparent to the type of radiation to be used. Inside of the face of the tube there is a very thin visible light reflecting, X-ray transparent layer 2 such as of aluminum which prevents the loss of light from the adjacent fiuorescent layer 3. An extremely thin barrier layer 4 separates the uorescent screen 3 from the adjacent photoemissive layer 5. The refiecting layer 2, the tiuorescent layer 3, the separating layer 4, and photoemissive layer 5 form together a composite photocathode Sa which converts X-ray images into electron images. it is obvious f that the composite photocathode may be of convex shape instead of flat type. The fluorescent layer 3 and photoemissive layers 5 should be correlated so that under the influence of the particular radiation used. there is obtained a maximum output of photoemission. More particularly the fiuorescent layer should be composed of aY material having its greatest sensitivity to the type of radiation to be used, and the photoemissive material likewise shouid have its maximum sensitivity to the wave length emitted by the fluorescent layer. Fluorescent substances that may be used are willemite, or other zinc silicates, BaPbSOy, zinc selenides, zinc sulphidcs, calcium fluoride or calcium tungstate, with or without activators. The satisfactory photoemissive materials for the composite photocathode will be caesium oxide, caesium oxide activated by silver, caesium with antimony, caesium with bismuth or arsenic or antimony, with lithium or potassium. The barrier layer 4 between the fiuorescent and photoemissive surfaces can be an exceedingly thin transparent film of mica, glass, ZnFz, or organic substance such as nitrocellulose or gelatine, of silicon or of a suitable plastic, or of a conducting material such as known under the trade name Nesa.

The thickness of the separating layer should not exceed 0.15 millimeter. The light transparent separating layer 4 in the photocathode 5a may be deposited on the fiuorescent layer 3 so that it doesnt require any support by the walls of the tube. In modification of the composite photocathode the light transparent layer 4 may be attached to the walls of the tube by means of metallic rings 1 andV may provide then support for other layers.

The electron image obtained in the pick-up section is now accelerated by electrode 6 and is transferred to the first screen of the amplifying section 7 by means of focussing magnetic or electrostatic fields which are not indicated, since they are well known in the art and would only serve to complicate the illustration.

The amplifying section 7 uses one or a few successively arranged vspecial screens 7a each of them consisting of an electron pervious, light-reflecting layer E, of-a fluorescent layer 9, of 'light transparent barrier layer 10, and of photoemissive layer lli. Fluorescent substances Vthat may j be used are willemite or other zinc silicates, zinc selenides,

zinc sulphide, calcium iiuoride or calcium tungstate with or without activators. The satisfactory photoemissive materials will be caesium oxide, caesium oxide activated by silver, caesium with antimony, with bismuth or arsenic or antimony with lithium or potassium. The barrier layer 10 between the fiuorescent and photoemissive surfaces can be an exceedingly thin transparent filmy or mica, glass, ZnFz, or organic substances such as c. g. nitrocellulose or gelatine, or of conducting material known in trade as Nesa, of silicon or of a suitable plastic. The separating layer 4 should be as thin as possible and should not exceed the thickness of 0.15 millimeter'. The amplification achieved by this system results in marked intensification of the original image.

In some applications it may be preferable to use inA conjunction with amplifying system the electron multiplier section consisting of one or a few'stages of secondary electron multipliers which serves to intensify further the electronic image. In such a case the electron image from the pick-up section of the tube is focussed by means of magnetic field on the first stage of the multiplier section. The

secondary electrons from the first stage are focussed thel y.

same Way on the second stage of the multiplier section and so on. CsO1Cs or AgzMg multpliers provide a good secondary electron emissione The electrons emerging from the amplifying section are now accelerated and imaged by means of electromagnetic or electrostatic fields 47 to the desired velocity, giving thus further intensification of the electron image. Next the electron image is diminished by means of electromagnetic or electrostatic lenses to the desired size, resulting in image intensification proportional to the square power of the linear diminution and is projected on the storage target.

Thc use of a storage target improves markedly signal to noise ratio resulting in pictures of much better detail and contrast. The storage target is shown in Fig. 6 and consists of a thin perforated sheet of woven conducting wire mesh 41a. On the side of the target opposite to the photocathode there is deposited by evaporation storage material such as CaFz, 41b in such a manner that openings 41c in the target should not be occluded. In some cases on the side of the target facing the photocathode there is deposited by evaporation a thin metal coating.

Between the photocathode and the storage target, in a close spacing to the target, there is mounted a fine mesh conducting screen 42. On the side of the storage target, opposite to the photocathode there is disposed a meshed metal electrode 43, which repels electrons during the writing phase of operation and attracts electrons during the reading phase. Adjacent to said meshed metallic screen there is disposed a fluorescent screen 44 provided with a metallic light reflecting electron transparent layer 44a such as of aluminum. The reflector electrode 43 during writing is kept at the potential negative to the photocathode 5a. Therefore the photoelectrons transmitted through the perforated target are repelled by said refiector electrode and have to fall back on the storage target 41 and deposit thereon varying charges at successive points according to the pattern of X-ray image. The best way of operating my system is-to have the storage target surface at zero potential or at photocathode potential and then to write on it positive, which means to deposit positive charges. This can be accomplished by adjusting the potential of the surface of the storage target so that its secondary emission is greater than unity.

The photoelectrons having the pattern of X-ray image after passage through openings in the target are repulsed back by the refiector electrode 43 because in this phase of operation its potential is lower than that of the storage target. The impingement ofphotoelectron beam causes secondary electron emission from the target 4i greater than unity. The secondary electrons are drawn away by the mesh screen 41a of the storage target which is connected to the source of positive potential. As a result a positive charge image is formed on the perforated target 41 having the pattern of the original X-ray image. This charge image can be stored in the target asrlong as hours, if the storage material is CaFz.

In the reading phase of operation of my system the target 41 is scanned by a slow electron beam 5f? from the electron gun 52. The electron beam is focused by magnetic or electrostatic fields 49 and is decelerated by the electrode 42 which may be in the form of a ring or ofV meshed screen. The defiecting fields and synchronizing circuits are not shown in order not to complicate the drawings. It is obvious that all fields controlling the scanning-beam are inoperative during the writing phase of Vthe operation. In the same way the fields controlling the photoelectron beam are not operating during the reading phase of the operation. A part of the scanning electron beam 50a passes through the perforations 41C in the target 41. The charge imagevon the target controls the passage of the scanning electron beam 59a acting in the similar manner to a grid in the electron tube.

The electron beam 50 in the reading phase of operation passes through the openings in the target 4l., is modulated by the stored charges on it and strikes the uorescent screen 44 producing thereby a light image having the pattern of the original X-ray image. The fluorescent screen 44 is provided with an electron transparent, light-refiecting backing 44a such as of aluminum. At the same time the open mesh metallic screen 43 may be used as a collector for electrons to be converted into video signals and transmitted to distant receivers. ln case no transmission of stored Vimage is desired the image 'a broad electron beam from the electron reproduced on the fluorescent screen 44 can be'marke'dly intensified by using instead of the scanning electron zbeam gun 52 covering all storage target 41 or a fiat ribbon scanning beam covering one line of the image.

Video signals can be obtained not only fromthe transmitted electrons of the scanning beam but as well from the electrons 5912 of said scanning beam returning to the electron gun. This part of the electron beamis also modulated by the charge image on the target 41 but is of reverse polarity than the transmitted electrons. The non-transmitted part of the scanning electron beam returns to the multiplier section 46. They are multiplied there and then are converted into video signals. This arrangement by using multiplication ofelectrons allows a marked intensification of Avideo signals. The returning electron beam 50b contains two groups of electrons. VOne group are electrons which are reiiected specularly from theV target. Another one are electrons which are reflected non-specularly if means were scattered. These two groups can be separated from each other before reaching the multiplier. There are many ways to separate these two groups of electrons, all well known in the art. The best method is to introduce an additional helical motion into a primary scanning beam. Then the scattered electrons in the returning beam will be on one side of the specularly reiiected electrons. Therefore it will be possible to direct scattered electrons into aperture of the multiplier, while stopping the reflected electrons by the edge of the multiplier aperture. The use of the scattered electrons increases markedly sensitivity of the system because it reduces the inherent shot noise of the scanning electron beam.

Video signals have the'pattern of the original X-ray image. They are amplified and transmitted by coaxial cable or by high frequency waves to receivers. Receivers may be of various types such as kinescopes, facsimile receivers, in combination with electrographic cameras and others may be used to reproduce images for inspection or recording. The accelerating, focusing, and detiecting fields as well as synchronizing circuits are not shown as they are well known in the art and would only complicate drawings.

After the stored image has been read and no further storage is desired it may be erased by the use of the scanning electron beam 50 by adjusting the potential of the storage target to the value at which the secondary electron emission of .its storingrsurface' is below unit. In such a case the targetvwill chargefnegatively to the potential of the electron gun cathode. The potential of the reflector in the erasing phase of operation must -be more negative than that ofthe Vstorage target, so that the scanning electron beam will-be repelled to the'target.

It is obvious that instead of composite photocathodeV a a single layer photocathode 5b of a material emitting electrons when vexcited by X-rays such as of lead, gold, bismuth or uranium may be used. This modification is shown in Fig. 2. The operation of the X-ray image tube 70 is similar to the tube 40 described above. The only difference being that in this tube the photoelectron image having the pattern of the X-ray image is deposited as a positive charge image on the storing surface ofthe storage target 41h facing the photocathode. The photoelectron image may be deposited on the opposite side of the target, as well, if areflector electrode of mesh 43 is provided for this purpose as was explained above.

In another modification of my invention shown in Fig. 3 the X-ray image is converted into a iiuorescent image 32a in the fiuorescent screen 31 outside of the X-ray image storage tube 33. The fluorescent image is projected by an optical system, preferably of a reflective type 34 on the X-ray storage tube. having a photocathode 3S of a material emitting electrons suchas of CsSba, CsOAg, or `of lithium or potassium on antimony or bismuth. The iiuorescent image projected onthe photocathode produces a photoelectron image.V The rest ofthe operation of this X-ray storage tube is the same as described above.

It is obvious that thecomposite photocathode, the electron gun and the perforated target may be disposed in many different ways and it is to, beunderstood that the various modifications of their mutual arrangement come within the scope and spirit of my invention. One of such modifications is shownrby the way of examplefonly `in Fig. 4. The X-ray image tube S3 operates in the same way as the tube 70, Vthe vonly difference being .the` photoi6 electron image is projected. on the storage target atan angle which requires the use vof arcuate focusing fields.

Another modification of X-ray storage tube in which the photocathode and electron gun are disposed at the oppositel ends of the tube is shown in Fig. 5. This `arrangement is suitable only for converting the stored X-ray images into video signals and cannot be used for immediate reproduction of X-ray images in the same tube. In this lmodification of my invention, shown in Fig. 5, the invisible X-ray image of the examined object 32 is converted by the composite photocathode 5a which has been described above into a photoelectron image. The photoelectron image is accelerated by the electrode 47 and is focused by the magnetic or electrostatic fields 48 on the perforated storage target 64, also described above.

Between the photocathode and the storage target, in a` close spacing tothe target, there is mounted :a ine mesh conducting screen 65. On the side'of the storage target, opposite to the photocathode there is disposed a meshed metal electrode 67. The recctor electrode 67 during writing is kept at the potential negative vto the photocathode 5a. Therefore the photoelectrons transmitted through the perforated target are rcpelled by said reflector electrode and have to fall back on the storage target 64 and deposit thereon varying charges at successive points according to the pattern of the X-ray image. The photoelectron image may be also stored on the side of the storage target facing the photocathode. In such a case the storing surface should face the photocathode and the mesh electrode 67 is not necessary. The best way of operating my system is to have the storage target surface at zero potential or at photocathode potential and then to write on it positive, which means to deposit positive charges. This can be accomplished by adjusting the potential of the surface of the storage target so that its secondary emission is greater than unity.

The photoelectrons having the pattern of X-ray image after passage through openings in the target are repulsed back by the reflector electrode 67 because in this phase of operation its potential is lower than that of the storage target. The. impingement of photoelectron beam causes secondary electron emission from the target 64 greater than unit. The secondary electrons are drawn away by the mesh screen 64a of the storage target which is connected to the source of positive potential. As a resulta positive charge image is formed on the perforated target 64 having the pattern of the original X-ray image. This charge image can be stored in the target as long as hours, if the storage material is CaFz.

In the reading phase of operation of my system the target 64 is scanned by a slow electron beam 62 from the electron gun 72. The electron beam is focused by magnetic or electrostatic fields 71 and is decelerated hy the electrode 73 which may be in the form of a ring or of meshed screen. The deflecting fields and synchronizing circuit are not shown in order not to complicate the drawing. It is obvious that all fields controlling the scanning beam are inoperative during the writing phase of operation. In the same way the fields controlling the photoelectron beam are not operating during the reading phase of the operation. A part of the scanning electron beam passes through the perforations in the target 64. The charge image on the target controls the 'passage of the scanning electron beam 62 acting in the similar manner to a grid in the electron tube.

It is obvious that all fields controlling the scanning beam arev inoperative during the writing phase of the operation. In the same way the fields controlling the photoelectron beam are not operating during the reading phase of the operation. The scanning electron beam 62 is modulated by the stored charge image. A part of it, 62a, is transmitted through theopenings in the storage target 64, is collected by the mesh electrode and is converted into video signals in the usual manner. Another part, 62b, of the scanning electron beam isreturning to the electron gun 72, is diverted tothe multiplier 63 and after multiplication therein is converted into video signals. Video signals have the pattern of the original X-ray image and are amplified and transmitted by coaxial cable or by high frequency waves to receivers. Receivers may be of various types such as kinescopcs, facsimile receivers, in combination with electrographic camera, and others may be used to reproduce images for inspection or recording. `In some cases it may be moredesirable to have the uorescent screen 44 mounted outside of the vacuum tube,

. in such cases thin electron transparent layer of chromium or aluminum is placed on the end wall 22 of the vacuum tube made of fernico. glass. The image appearing on the uorescent screen can be viewed directly or by means of an optical eye-piece giving the desired optical magnification of the image. In other cases tbe fluorescent screen 44 is substituted by electrophotographic layer or by photographic layer in combination with uorescent screen, or by an electrographic plate, permitting thus to obtain a permanent record of electron image.

In another alternative of this invention the X-ray image tube is curved and the electron beam is deiiected by proper magnetic or electrostatic fields. This arrangement will prevent the positive ions from reaching the photoemissive section.

It is not intended to restrict the scope of this invention to the employement of X-ray or gamma radiations but other corpuscular radiations and suitable reactive layers are intended to be comprehended. Another form of the invention is illustrated in Fig. 7, wherein a neutron reactive layer 26 preferable from the group boron, lithium, gadolinium and uranium or of paraffine is placed within the face of the image tube to act as a photocathode. The protons or electrons liberated from this layer 26 under the impact of neutron radiation will strike through a thin electron pervious chemically inactive barrier layer 4a, a suitable fluorescent layer 27, causing it to uores'ce and activate a suitable photoemissive layer 29 through the light transparent barrier layer 28. In other cases a neutron reactive layer of cadmium or copper will be more advantageous, because of its gamma emission.

In some cases it may be more desirable (Fig. 8) to eliminate the uorescent layer 27 and to cause protons or electrons from the layer 26a to act on electron emissive layer 29a either by apposition, in which case electron pervious chemically inactive barrier layer 30 has to be used to prevent chemical interaction of adjacent layers, or by focussing said protons or electrons on electron emissive layer 29a with magnetic or electrostatic fields (Fig. 9).

The uorescent layer to be used in the neutron sensitive tube may be of similar composition as described above in the X-'ray sensitive image tube, but it has also to be adapted to respond most efficiently to the radiation emitted from neutron sensitive layer by enriching it with proper neutron reactive elements such as boron or gadolinium. The photoemissive layer has again to be correlated with spectral emission of iiuorescentV layer. The amplifying system, the multiplier system, the storage system, the electronic diminution are the same for neutron sensitive image tube and for X-ray sensitive imagetube.

In another modification of my invention shown in Fig. l the X-ray sensitive pick-'up tube 76 has a composite storage target 79 shown in Fig. 6a'. The X-ray image is converted in the photocathode a described above into photoelectron beam having the pattern of the X-ray image. The photoelectron image is accelerated and focused on the perforated storage target 79. The target consists of a thin perforated light transparent dielectric such as glass 83. Also a metallic mesh screen can be used instead of a perforated glass. In such a case however, a light transparent dielectric must be deposited on the mesh screen and in such a manner that openings in the screen remain unobstructed. On the side of the glass layer 83 facing the photocathode is deposited a uorescent layer 81 also in such a manner that the openings in the glass are unobstructed. On the side of the uoreseent layer is deposited a light reecting layer 80 such as of aluminum. On the side of the dielectric layer 83 which is away from the photocathode is deposited a photoemissive layer 84 in such a manner that openings in the dielectric layer are not obstructed.

The photoelectron beam causes fluorescence of the layer 81. The uorescent light passes through glass layer 83 and causes emission of electrons from the photoemissive layerv 84. The emitted photoelectrons are led away by Yadjacent collectingrmesh screen 85. As a result a positive charge image is stored on the layer 84. This stored charge controls the passage of the scanning electron beam 87 in the samermanner as was described above. The transmitted electrons of the scanningV beam will strike the fluorescent screen 89 having a light reecting electron transparent backing 83 and will'reproduce therein the X-ray image. The transmitted electrons are focused on "8 the fluorescent screen 89 by means of magnetic or electrostatic iields which are well known in the art.

Storage of X-ray images may also be accomplished by using a light image feed-back system. The iiuorescent X-ray image in this modification of my invention is reproduced on the face of the image tube as was eX- plained above, is projected on a television pick-up tube and produces a photoelectron image therein. The photoelectron image is converted by the pick-up tube into video signals in the manner well known in television. For the purposes of this invention any type of television pick-up tube such as photoemissive type, photoconductive type, or of photovoltaic type may be used. Video signals are sent from the pick-up tube to the kinescope and reproduce there the uoresecent image. The iuorescent image is again projected on the pick-up tube to produce again photoelectron image. In this way an endless stream of iiuorescent light image is produced so that the fluorescent image may be inspected for a desired time without maintaining the X-ray exposure.

The storage tube 76 described above may be also used for the transmitting of X-ray images. In such a case electrons of the scanning beam 87 which are transmitted through the target 79 are made to pass through an apertured electrode disposed between the storage target 79 and the end wall of the tube. By applying suitable eiiection iields to the transmitted electrons, they are m lde to pass through the aperture in the electrode in succession Vcorresponding to various image points. The electrons which pass through the aperture are-fed into a multistage multiplier for intensification. The electrons emerging from the multiplier are converted over a suitable resistor into video signals in a manner well known in television. Video signals are sent to receivers by coaxial cable or by high frequency waves to reproduce a visible image.

It will thus be seen that there is provided a device in which the several objects of this invention are achieved and which is well adapted to meet the conditions of practical use.

As various possible embodiments might be made of the above invention, and as various changes might be made in the embodiment above set forth, it is to be understood that all matter herein set forth or shown l in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

What I claim is: 1. An invisible image sensitive tube having in combination a photocathode comprising uorescent means, light transparent separating means and photoelectric means for receiving said image and converting said image into an electron image, a dielectric storage target having openings therein, means for projecting said electron image on said storage target to store said image in said target, means for irradiating said storage target with an electron beam to modulate said electron beam with said stored image, and means for receiving electrons of said modulated beam.

2. A device as defined in claim 1, in which said light transparent separating means are of a thickness not exceeding 0.15 mm.

3. A device as defined in claim 2, in which said photocathode is deposited on the wall of the said tube.

4. A device as defined in claim 3, wherein the dielectric storage surface of said storage target is on the side opposite to the source of said electron beam.

References Cited in the file of this patent UNITED STATES PATENTS Sheldon Juiy 15.1952

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