US3043974A - Electron discharge devices - Google Patents

Description

July 10, 1962 J; D. MOGEE 3,043,974

ELECTRON DISCHARGE DEVICES Filed Feb. 27, 1959 3 Sheets-Sheet 1 INVENTOI? ATTORNE Y5 July 10, 1962 J. D. MCGEE ELECTRON DISCHARGE DEVICES 3 Sheets-Sheet 2 Filed Feb. 27, 1959 INVENTOR %M gwguvkcblk MW, wiimm ATTORNE Y5 July 10, 1962 J. D. MCGEE 3,043,974

ELECTRON DISCHARGE DEVICES Filed Feb. 27, 1959 3 Sheets-Sheet 5 IN VE N TOR MD X573 BY K MI I w g muk kw ATTORNEYS 3,043,974 ELECTRGN DISCHARGE DEVEQES James Dvvyer McGee, London, England, assignor to National Research Development Corporation, London,

England Filed Feb. 27, 1959, Ser. No. 7%,tl99 Claims priority, application Great Eritain Mar. 6, 1958 '7 Claims. (Cl. 313--65) This invention relates to electron discharge devices and their manufacture.

In the manufacture of some electron discharge devices, it is necessary first to mount the electrodes in an envelope and afterwards to coat or otherwise to process one or more of the mounted electrodes by the admission of a vapour into the envelope. If the vapour is permitted to reach others of the mounted electrodes, however, contamination results, which is detrimental to the operation of the completed device.

One object of the present invention is to provide a manufacturing technique whereby such undesirable contamination is avoided by confining admitted Vapour to certain regions of the envelope containing an electrode to be processed and by excluding the vapour from the other regions of the envelope containing electrodes which would be contaminated.

Another object of the invention is to provide an electron discharge tube of novel construction, whereby such a manufacturing technique is facilitated.

One class of electron discharge devices to which this invention relates is photo-electric tubes having a photocathode formed by the action of an alkali metal vapour upon a base metal, and the invention will be explained with reference to this class of tube.

During the manufacture of these tubes, the photo-sensitive surface of the photo-cathode, is formed after the electrodes have been introduced into a vessel and the vessel evacuated by treatment of a base metal layer with an alkali metal vapour. The most sensitive types of such photo-cathode surfaces include caesium and the formation of photo-cathodes by caesium vapour will be referred to herein by way of example.

Normally, the caesium is introduced into the vessel as vapour or caesium vapour is generated in the tube itself. The vapour condenses on and reacts with the base metal of the cathode, for example antimony, to form the required photo-sensitive surface.

If the other electrodes of the device are also exposed to the caesium vapour, condensation will occur on these electrodes as well. The resultant surface has a low work-function and this condition would facilitate the liberation of electrons from these other electrodes due to the high po tential electric fields present during the operation of the device. Such spurious currents, leading to excessive dark current and causing spurious background eifects, limit the maximum voltages which can be applied between the electrodes.

In U.S. Patent No. 2,899,576, there is described an electron discharge device comprising a vessel having first and second chambers containing at least one electrode and the second chamber being adapted for the admission of vapour thereto, the device also comprising an electrode coated while inside the vessel by vapour condensation thereon and adapted to occupy first and second alternative positions, the first serving to close the said aperture during the coating operation and the second being in operative relationship to the electrode or electrodes in the first chamber.

In that specification there is described, by way of example, a single stage image intensifier tube having a vessel comprising two chambers, the first leading into the second by way of an apertured flange. The second chamber 3,043,914 Patented July 10, 1962 contains electrodes, which are maintained at a high potential in operation, and a fluorescent screen. A glass plate provided with a coating of antimony is adapted to occupy a first position in which the antimony coating faces into the first chamber and ,the plate seals the 'aperturein the flange in a vapour-tight manner. A photo-cathode surface is formed on the plate by admitting caesium vapour into the first chamber. The vapour combines with the antimony layer to form the photo-cathode layer but the vapour is prevented from passing into the second chamber by the vapour-tight seal between the glass plate and the intermediate chamber containing electrodes to be maintained at high potentials when the device is in operation, the said flanges having movable plates associated therei with, the plates being adapted in one position to close the aperture in the associated flange in a vapour-tight manner and in another position to occupy an operative position in relation to the other electrodes of the device.

In a preferred form of the invention each apertured flange is provided with a tortuous passage between adjacent chambers. I

By the term tortuous passage it'is meant that there is no straight line-of-sight passage between the two chambers for atoms or molecules. Such a passage may be provided by a tube extending from one chamber into the other, the tube being of zig-zag, twisted or curved form.

It is preferred to coat the internal surface of the tube with a material which has strong chemical aflinity for alkali metal atoms, and hence effectively traps such atoms when they come into contact'with the internal surfaces, as they'must do in attempting to pass through a tortuous passage.

In such a device both plates may be subjected to the action of vapour introduced into or generated in the first and third chambers, the vapour being excluded from the second, the intermediate, chamber by the vapour-tight seals at each end.

In one form of the invention applied to a two-stage image intensifier, the second chamber" is itself divided into two parts by an apertured flange, provided witha tortuous passage, the first part adjacent the first chamber containing electrodes associated with the first stage of the device and the second part adjacent the third cham ber containing at least some of the electrodes associated with the second stage of the device. In this embodiment, the second plate is adapted in one position to seal the second and third chambers in a vapour-tight manner and in another position to be located against'the flange between the two parts of the second chamber, in

which position it is in operative relationship to the assess a into the next in sequence through an apertured flange. This device has three movable plates adapted in one position to seal off each chamber of the vessel from the next in. a vapour-tight manner. three plates may be treated by vapour in the first and third chambers, the vapour being excluded from the second and fourth chambers containing the other electrodes. Subsequently, all three plates are moved into operative positions in relation to the other electrodes.

In order that the invention may be more clearly understood, three embodiments will now be described in detail, by way of example, with reference to the accompanying drawings, of which:

FIG. 1 is a diagrammatic longitudinal sectional drawing showing the arrangement of a two-stage image .intensifier tube during its manufacture;

FIG. 2 shows the tube of FIG. 1 after its manufacture is completed;

FIG. 3-shows the arrangement of a three-stage tube during manufacture;

FIG.'4 shows the tube ofFlG. 3 after its manufacture is completed;

FIG. 5 shows a two-stage tube, which is a shortened form-of the three-stage tube of FIG. 3 during manuf-acture;

. FIG. 6 shows the tube of FIG. 5 after its manufacture is completed and FIG." 7 is a diagrammatic drawing of a catch for holding a plateagainst a flange.

In FIG. 1 a cylindrical envelope 1 has transparent plane end faces and is divided into three chambers 2, 34 and 5. The chamber 3.4 is itself divided into two parts 3 and 4. The chambers or chamber parts 2, 3, 4 and 5 lead each into the next in sequence through metal apertured flanges 6, 7 and '8 respectively.

Tubes 9 and 11 extend into the chambers 2 and 5 respectively and an exhaust tube extends into the chamber 3.

A- bent tube 12, opening intothe chamber 3,-passes through the flange 6 andextends into the chamber 2. Similarly, a .bent tube 13, opening into the chamber 4, passes through the flange 8 and extends into the chamber 5. Both the tubes 12, 13 are coated internally with a material, which has an affinity to alkali metal atoms. The material may be metallic antimony, metallic tin, tin oxide or the graphite dispersion sold under the registered trademark Aquadag.

The chamber part 3 contains electrodes 14 having lead out wires passing through seals in the envelope 1 to external connections 15. The electrodes 14 are associated with the first stage of the tube and each is maintained at a high potential relative to the next when the completed tube is in operation.

The chamber part 4 contains electrodes 16 leading through seals to external connections 17. The electrodes In this condition, all

a One operation in the manufacture of the tube is to subject the antimony layer to the action of caesium vapour whereby a photo-cathode layer of antimony-caesium is formed.

A multiple-layer screen 25, provided on one face 26 with a layer of antimony, is movable in the chamber part 4 and is adapted, in the position in which it is shown in FIG. 1, to seal the aperture in the flange 8 in a vapour-tight manner. In this embodiment, the screen 25 comprises a fluorescent layer and a-layer of antimony. Another operation in the manufacture of the tube is to subject the antimony layer to the action of caesium vapour, whereby the layer is convertedinto a photocathode layer.

The two operations referred to above may be carried out at the same time or separately as is more convenient. Referring to FIG. 1, the envelope 1 is evacuated by Way of the exhaust tube 10. Although the plate 23' forms a vapour-tight seal with the flange 6, the gases from the chamber 2 pass into the chamber 3 by way of the tube 12 and are exhausted with the gases in the latter chamber. Similarly, the gases from chamber 5 pass into the chamber 4 by way of the tube 13 and are similarly exhausted through the tube 10.

Caesium vapour is then admitted into the chamber 2 by way of the tube 9. As an alternative, the tube 9 may be omitted and caesium vapour generated within the chamber 2 itself.

The whole tube is meanwhile baked at a temperature of 150 C. to 170 C. At this temperature the caesium combines with the antimony layer on the face 24 to form a'photo-sensitive compound. Also at this temperature, caesium has sufiicient vapour pressure to difluse throughout the tube and contaminate the electrodes 14 and 16 if it were permitted to enter the chamber 34. However, the plate 23 forms a vapour-tight seal with the flange 6. The caesium atoms enter the tube 12 but, owing to the configuration of the tube 12, necessarily strike the inside surface of the tube before reaching the chamber part 3 and are thereupon absorbed by the coating material in the tube.

In exactly similar manner, caesium vapour is admitted to the chamber 5 by way of the tube 11, or alternatively generated "Within the chamber. The surface 26 of the plate 25 is formed into a photo-sensitive layer, in the same Way. Diffusion of vapour into the chamber 3-4 is prevented by the vapour-tight seal between plate 25 and flange 8 and by the coating material inside the tube 13.

16 are associated with the second stage of the tube and are similarly maintained at high potentials under operative conditions.

The chamber 5 contains an electrode 18 leading through .a seal to an external connection 19. The electrode 18 is also associated with the second stage of the tube.

The flanges 6 and 7 are provided with external confreedom 20- and 21 respectively.

A further external connection 22 passes through a seal to make connection with the aluminum backing of a Referring now to FIG. 2, after the formation of the photo-cathode surfaces 24- and 26, the tubes 9 and 11 are sealed off at pips 27 and 29 respectively and, after evacuation of the vessel 1 is complete, the exhaust tube 10 is similarly sealed off at 28.

After release of catches, clips or any other convenient devices for retaining the plate 23 in position against the flange 6 the plate 23 is turned round, so that the photosensitive surface 24 faces into the chamber part 3. The catches or other devices are then secured so that the plate 23 is again retained against the flange 6. Electrical connection is thereby made from the photo-sensitive layer to the external connection 20. These catches may conveniently be similar to those described with reference to FIG. 7.

By releasing similar catches, not shown, for retaining the plate 25 against the flange 8, and with the surface 26 still facing in the same direction, the plate 25 is allowed to drop on to the flange 7. The antimony-caesium layer on plate 25 is sufliciently robust not to be impaired by this operation. The plate 26 is retained against the flange 7 by further catches, not shown, so that electrical connection to the photo-sensitive layer is made to the external connection 21.

A lens system 33 is provided which focuses light rays 35 from an object 34 to form an image of the object on the photo-sensitive surface 24.

The external connections 20, 15, 21, 17, 19 and 22'are connected in that sequence to increasingly positive potential sources in known manner for tubes of this type. The potential sources are not shown in FIG. 2.. Solenoids 30 and 31 are supplied with a current and provide axial magnetic fields within the chamber 3 and the chamber 45 respectively.

The surface 24 is thus in operative relationship to the electrodes 14 forming the first stage of the tube and the surface 26 is similarly in operative relationship to the electrodes 16 and 18 forming the second stage of the tube.

In the operation of the tube of FIG. 2, the optical image formed by the lens system 33 on the surface 24 liberates electrons to form a corresponding electron image. The surface 24 serves as a photo-cathode and the electrostatic field produced by the potential difference between the electrodes 14 accelerates the electrons liberated therefrom. This electrostatic field, together with the axial magnetic field due to the solenoid fail, produce an electron image at the plate 25 corresponding to the electron image at the surface 24 but having greater energy.

The plate 25, being a multiple-layer intensifying screen, has a fluorescent screen which converts the incident electron image into the corresponding fluorescent light image which is reconverted into an electron image by the photocathode surface 26. The electronsemitted from the photo-cathode are accelerated by the electrodes 16 and 18 and these electrodes, together with the axial magnetic field due to the solenoid 31 produce a high-energy electron image at the fluorescentlayer 32. The fluorescent light image at the fluorescent layer 32 is much brighter than the image formed by the lens system 33.

By reason of the construction of the tube, only the last electrode of the second stage, that is electrode 13, and the walls of the chamber 5 are contaminated by caesium vapour. Spurious currents liberated from the electrode 18 or from the walls of the chamber 5 are subject only to small accelerating force and therefore high potential differences may be applied between successive electrodes of the two stages resulting in considerably more effective operation.

FIGS. 3 and 4 show the construction of a three-stage image intensifier tube. The first and second stages thereof are closely similar to the two-stage tube of FIGS. 1 and 2 and corresponding parts are indicated by the same ref- V erence numerals in FIGS. 1 and 3 and in FIGS. 2 and 4.

Diflerences of detail are shown in that the plate 23 is adapted to be retained against the flange 6 inside the chamber part 3 and, when the plate 23 is turned round from the position shown in FIG. 3 to occupy the position shown in FIG. 4, it is turned round in the chamber part 3. This is by way of being an alternative construction to that shown in FIGS. 1 and 2, but enables the chamber 2 to be made axially shorter, if so desired. The exhaust tube enters the chamber part 4. The final electrode 18 of the tube of FIGS. 1 and 2 is replaced by a third electrode 16 arranged in the chamber 4 and having an external connection 17. The tube 13, providing communication for gases between the chambers 4 and 5, is shown extending into chamber 4.

The envelope 1 of FIGS. 3 and 4 is provided with a further chamber 36 which communicates with the chamber 5 by way of an apertured flange 41. The fluorescent screen 32 is arranged at the end of the chamber 36 remote from chamber 5. The external connection 22 provides an electrical connection with the aluminium backing of the screen 32 by way of a seal in the chamber 36.

Three electrodes 37 associated with the third stage of the tube are provided in the chamber 36, external connections 38 being provided to the electrodes 37 through seals in the envelope 1.

A second plate 39, comprising a multiple-layer screen identical with the plate 25, is retained by catches, not shown, against the flange 41 and, in the position shown in FIG. 3, makes a vapour-tight seal with the flange 41.

In this position, the antimony coated surface 40 faces into the chamber 5. A bent tube 42 opens into the chamber 36 and extends into the chamber 5. The tube 42 is coated internally with the same material as the tubes 12 and 13 and serves an exactly similar purpose admitting to the chamber 5 gases from the chamber 36 but preventing the diffusion of caesium vapour from the chamber 5 to the chamber 36.

In this embodiment, caesium vapour is admitted to chamber 2' and the surface 24 formed as previously described. When caesium vapour is admitted to chamber 5, however, both the surfaces 26 and 4d are formed together. Diffusion of the vapour into the chamber 4 or the chamber 36 is prevented so that none of the electrodes 14, 16 and 37 becomes contaminated.

After sealing off the tubes 9, ltland 11 as before, the plate 23 is reversed and the plate 25 is moved to flange 7, without reversal, as described in the tube of FIG. 2. The plate 39 is released, reversed in the chamber 36 and replaced on the flange 41 in similar manner. An external connection 44 is thereby provided to the plate 39.

The plates 23, 25 and 39 are then in operative relationship to the electrodes 14, 16 and 37 of the first, second and third stages of the tube respectively, as shown in FIG. 4. The connections 44, 38 and 22 are taken in sequence to increasingly positive potential sources not shown. A further. solenoid 43 provides an axial field in the third stage of the tube.

The operation of the tube of FIG. 4 is analogous to that of FIG. 2 but the conversion from electrons to fluorescent light and to electrons again performed by the plate 25 is repeated by the plate 39'. The resultant electrons are accelerated in the third stage to form an electron image of high energy at the screen 32 and the screen 32 reproduces the image formed by the lens system 33 at very much increased brightness.

FIG. 5 shows the construction of a two-stage image intensifier tube. The first and second stages are similar to the second and third stages of the three-stage tube of FIG; 3 and corresponding parts are indicated by the same reference numerals.

In FIG. 5, an envelope 1 is divided into three chambers 4, 5 and 6. The chamber 4 has a metal, apertured flange 7 at one end near the plane front face of the envelope 1. The chamber 4 leads into the chamber 5 by way of a metal, apertured flange 8 and the chamber 5 leads into the chamber 6 by way of a metal, apertured flange 4 1.

The chamber 4 contains electrodes 14 which are associated with the first stage of the tube and are provided with external connections 15. The chamber 6 contains electrodes 16 which are associated with the second stage of the tube and are provided with external connections 17;

The chamber 5 has a side tube 11 leading thereto and the chamber 10 has an exhaust tube 10 leading thereto.

Initially, as shown in FIG. 5, a plate 23, provided on one face 24 with a layer of antimony, is arranged on the flange 8. In this position, the plate 23 seals the aperture in the flange 8 in a vapour-tight manner.

In this embodiment, caesium vapour is admitted by the tube 11 into the chamber 5. The photo-sensitive surface 24 of the plate 23 and the photo-sensitive surface 40 of the plate 39 are formed at the same time. The caesium vapour is confined to the chamber 5, being excluded from the chamber 4 by plate 23 and tube 13 and from chamber 6 by plate 39 and tube 42.

All the three chambers 4, 5 and 6 are exhausted by way of the exhaust tube 10, which is subsequently sealed off at 28. The tube 11 is sealed off at 29.

The plate 23 is then freed, by releasing catches, not shown, and moved to the flange 7 where it is retained by other catches, not shown, without reversal of the plate 23.

FIG. 6 corresponds to FIGS. 2 and 4 of the other embodiments and shows the operative arrangement of the tube of FIG. 5.

FIG. 7 shows in section one catch of two or more similar catches used to retain the plate 23 against the flange 6 in a tube as shown in FIG. 1.

The envelope 1 has an outwardlyextending side tube 46 arranged with its axis inclined to the axis ofv the en- 23 against the flange 6.

The catch is-released by a magnet which attracts the body part upwards in the side tube, compressing the spring 49 and withdrawing the pin 48 to release the plate 23. Upon removal of the magnet, the catch is restored to the position shown in FIG. 7 by the action of the spring 49.

The invention has been described in relation to twostage and three-stage tubes by way of example. The same technique may be applied to tubes of four, five or more stages. For example, if the envelope 1 of the tube of FIG. 3 is extended to provide three further chambers, similar to the chambers 4, 5 and 36, connected in sequence by way of further apertured flanges, a five-stage tube may be constructed. i

What I claim is:

1. An electron discharge device comprising a vessel having at least first, second and third chambers, apertured flanges provided in said vessel for separating said chambers, at least the intermediate chamber'containing electrodes maintained at high potential when the device is in operation, the said flanges having movable plates associated therewith, the plates being adapted in one position to close the aperture in the associated flange in a vapourtight manner and in another position to occupy an operative position in relation to said electrodes, at least one of said plates being associated with two said apertured flanges, for closing the aperture of one of said two flanges and for occupying an operative position in association with the other.

' 2. An electron discharge device comprising a vessel 5 having at least first, second and third chambers, apertured flanges provided in said vessel for separating said chambers, at least the intermediate chamber containing electrodes maintained at high potential when the device is in operation, the said flanges having movable plates associated therewith, the plates being adapted in one position to close the aperture in the associated flange in a vapourtight manner and in another position to occupy an operative position in relation tosaid electrodes, the flanges with apertures thus closed by associated plates having a tortuous passage between adjacent chambers.

3. An electron discharge device as claimed in claim 2, in which the inner surface of the tortuous passage is coated with a material which has an affinity to alkali metal atoms. 5

4. A two-stage image'intensifier comprising a vessel having at'least first, second and third chambers, aperturcd flanges provided in said vessel for separating said chambers, the said flanges having movable plates associated therewith, in which vessel the second chamber is itself divided into two parts by an apertured flange pr 'vided with a tortuous passage, the first part adjacent the first chamber containing electrodes associated with the first stage of the device and the second part adjacent the eeaaava third chamber containing at least some of the electrodes associated with the second stage of the device, and the second plate being adapted in one position to seal the second and third chambers in a vapour-tight manner and in another position to be located against the flange between the two parts of the second chamber, in which position it is in operative relationship to the electrodes of the second stage of the device.

5. A two-stage image intensifier as claimed in claim 4, in which the second plate comprises a multi-layer screen havin a fluorescent layer and a photo-cathode layer in close proximity to each other whereby electrons incident on one face excite the fluorescent layer to emit light photons which in turn liberate photo-electrons from the photo-cathode layer, the latter electrons being emitted from the other face of the plate.

6. An electric discharge device comprising a vessel hav ing at least three. chambers consecutively arranged, apertured members provided in said vessel for separating consecutive chambers, at least one apertured member having a tortuous passage extending between the chambers separated by said member, said electric discharge device further comprising two plate-like electrodes adapted in one position thereof to close the apertures of two of said apertured members, thereby to close one of said chambers against vapour entry thereinto other than by way of said tortuous passage, both said electrodes bearing on one face thereof an electrode surface prepared by vapour condensation thereupon and at least one of said electrodes having an operative position alternative to the said position of closure of said apertured members.

7. For an electron discharge device comprising a vessel having at least first, second and third chambers, a first apertured flange provided in said vessel for separating said first and second chambers and a second apertured flange provided in said vessel for separating said first and second chambers and a second apertured flange provided in said essel for separating said second and third chambers and a plate-like electrode adapted to seal in a vapour-tight manner the aperture of said second apertured flange and having an operative position in said device when posi tioned against said first apertured flange, a method of coating and positioning said plate-like electrode, comprising arranging said electrode in a first position to seal the aperture of said second apertured flange, admitting vapour into the third chamber whereby vapour condensation occurs on the face of said electrode directed into the third chamber and subsequently transposing said electrode to said first apertured flange, thereby to occupy its operative position, the coated face of the electrode still facing towards said third chamber.

References Cited in the file of this patent UNITED STATES PATENTS 2,135,615 Farnsworth Nov. 8, 1938 2,283,413 Cashrnan May 19, 1942 2,374,287 Henry Apr. 24, 1945 2,508,856 Cassman May 23, 1950 2,640,162 Espenschied May 26, 1953 2,683,832 Edwards July 13, 1954 2,697,181 Sheldon Dec. 14, 1954 2,739,244 Sheldon Mar. 20, 1956 2.899576 McGee Aug. 11, 1959

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