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A New Technology for Transferring Photocathodes
A New Technology for Transferring Photocathodes P. DOLIZY and R. LEGOUX Laboratoirea d’&lectronique et de Phyeique Appliqude, L~med-Brduannea,Prance
INTRODUCTION The conventional method of preparing the photoemissive layer in a photoelectric tube is by the thermal evaporation of antimony and alkali metals from sources mounted inside the tube. Some of these sources must be located either directly in view of the photocathode, so as to achieve uniform evaporated layers, or in such a manner that the molecular diffusion rate is relatively high. It is easily understood that such sources prohibit the use of quite a number of electrode configurations, particularly those in which the electrodes are placed close to the photocathode. Moreover, with the conventional method, the alkali vapour permeates throughout the tube during processing, and very often interacts chemically with the surface of the electrodes. This can enhance undesirable effect,s such as leakage currents, field emission, and parasitic photoemission. It can be said that, as a general rule, the performance of the photocathode is dependent to a large extent on the internal structure of the tube. These troubles, often encountered with conventional tube processing methods, can be avoided by the so-called “transfer technique”. The main features of this technique are as follows. (1) The substrate, on which the photoemissive layer is to be formed, is isolated from all other component parts of the photoelectric tube, so that it alone is exposed to the materials evaporated during the processing of the photocathode, (2) The several evaporation sources are grouped together in an auxiliary enclosure, which is located in front of the substrate and is maintained in that position throughout the processing of the photocathode, after which it is removed. (3) The photoemissive layer is only transported to its final position in the tube, if, after stabilization and cooling, it has the required photoelsctrio properties. 367
368
P. DOLIZY AND R . LEGOUX
DESIGNO F THE “TRANSFER” TUBES The design of tubes to be made by the transfer process is strongly influenced by the way they are to be pumped and processed in the special chamber. The envelope of a tube to be made by this process is divided into two parts, each of them containing elements of the tube itself. One of these two parts is often the input window of the tube, and in many msea, this is used as the photocathode substrate. The two parts are eventually joined together by metal rings (Pig. 1). However, the tube is first pumped and the photocathode processed with the two parts separated. The two parts are then joined by an indium seal which is made by pressing the two rings together, one of these having a groove Glass window and Dholocathode substrate
Metal rings
FIG.1. Indium compression seal a t front of phototube. The electrode structure in the lower half of the tube is not shown.
to hold the indium and the other a tongue. Indium is particularly suitable for the purpose because of its great malleability, its low vapour pressure at high temperature, and its great resistance to oxidation. The seal-off tip, which is usual on convendional tubes, is of course no longer required.
THETRANSFER EQUIPMENT A photograph of transfer equipment capable of handling photocathodes up to 120 mm in diameter is shown in Fig. 2. A cross-section of the cylindrical transfer enclosure is shown in Fig. 3. This enclosure is made of stainless steel and the top, which is in the form of a bell-jar of glass or metal, is removable. The two halves of the tube to be processed can be seen in the figure. One half of the tube, pre€erably that including the photocathode substrate, is fastened to the upper part of
NEW TECHNOLOGY BOR TRANSFERRING PHOTOCATHODES
369
the enclosure with a clamping ring held by two columns. The other is set up in the lower part of the enclosure on a movable table guided by the columns and connected to a hydraulic press. At a later stage this lifts up the lower half of the tube for the seal to be made to the upper half. All the evaporation sources necessary for the preparation of the photocathode are grouped together in an auxiliary enclosure which is open at the top and can be moved sideways by a second mechanism.
FIG.2. The transfer apparatus.
The system is pumped first with a cryogenic pump (zeolite and liquid nitrogen) to about 6 x l o T 4torr and then either with an oil diffusion pump having two refrigerated baffles in series or with a Penning ion pump. The ultimate vacuum as measured by the ionization gauge shown in Fig. 3 is generally of the order of torr. The entire equipment is outgassed for 16 h during each pumpiiig cycle by heating coils at a temperature of 260°C, while the transfer bell-jar is brought to between 400°C and 460°C. When the bake-out is terminated, the photoemissive layer is processed in the conventional way but with modifications to allow for the large volume of the processing chamber. The pressure is then about torr.
370
P. DOLIZY AND R. LEQOUX
FIQ.3. Croae-section of the transfer encloaure.
NEW TECHNOLOGY FOR TRANSFERRING PHOTOCATHODES
37 1
During the processing, the sensitivity of the photocathode is monitored using the light from a tungsten filament lamp placed above the bell-j ar, As soon as the photocathode has cooled to a temperature of between 40°C and BO'C, and has stabilized, then, providing that the required photoelectric properties have been achieved, the tube is ready to be closed. The pressure at this stage is torr. In the closing process the sensitizing enclosure is first moved to a lateral position in the belljar leaving room for the tube to be lifted up (Fig. 4). The two parts of
Processing enclosure
1 I
FIG.4. Showing how the processing enclosure is moved to one side and the tube is closed.
the tube are then joined by compressing the indium seal. The photocathode is now in its operating position. Air is let into the bell-jar and the tube can now be removed from its mechanical supports and is ready for use. OF THE METHOD ADVANTAGES The advantages of the method are numerous. Apart from those that have already been noted, it avoids the poisoning of component parts of the tube by the physical and chemical action of alkali vapours during the photocathode processing. This applies in particular to the following; the fluorescent screens of image tubes, surfaces which must be highly insulating, and electrodes which must be free from field emission or photoemission. Photocathodes made using the method are comparable to those
372
P. DOLIZY AND R. LEQOUX
obtained by conventional methods. Trialkali photocathodes with sensitivities higher than 200 pA/lm have been achieved on glass and metal substrates. The uniformity of the photocathodes is also improved because the evaporation sources can be placed at greater distances from the substrate than are possible in conventional tubes.
FIQ.5 . High current photodiode. Anode t o cathode spacing 2 mm.
After a photocathode has been processed it can be tested before its introduction into a tube, and if inadequate it can be rejected, thus avoiding throwing away the entire tube. A most important aspect of the process is that it makes possible the construction of tubes in which the dimensions and shape are such that photocathode processing would not be practicable by conventional methods. As an example of such a tube, Fig. 5 shows a high current
FIR.6. High-speed shutter tubes.
NEW TECHNOLOGY FOR TRANSFERRING PHOTOCATHODES
373
photodiode in which the distance between the cathode and anode is 2 mm. Figure 6 shows a family of fast shutter tubes for which the photocathode diameters range from 40 mm to 120 mm, and the cathode to phosphor distances from 2 mm to 10mm. These are more fully described in another paper in this volume.?
DISCUSSION s. MAJUMDAR: 1. What current can you draw from the high current photodiodes? 2. In your image tubes, does the cathode performance deteriorate after many hours of operation? J . GRAF: 1. These diodes deliver a linear rosponse up to 10 A for an applied voltage of 3 kV and a pulse of 1 psec. The saturation current is 20 A. 2. We have not seen any change in cathode performance on a tube tested for 5 h continuous running. I t must be noticed that these types of tubes aro designed only for pulse operation. M. ROME: What improvements are found in dark current by the use of the transfer technique? Would you please compare the dark current of tubes with the same photocathode, (e.g. type 5-20),of similar sensitivity, which differ only that some are conventionally prepared and others by the transfer method? J. GRAF: For the same 5.20 sensitivity in two photomultiplier tubes, one conventionally prepared and the other by the transfer method, the transfer tube has a dark current nearly hundred times lower than the conventional tube. R. DECKER: 1. Is the tube isolated or just shielded from the photocathode processing chamber? 2. Is it possible to process more than one cathode a t one time? 3. How far do the bellows have to deflect to make a seal? J . GRAR: 1. The body of the tube is only separated from tho sensitizing enclosure containing the alkali dispensers. There is not a tight separation between the dispensers and the body of the tube. 2. Yes, it is possible to process simultaneously several photoemissive layers in the transfer equipment. 3. The deflexion of the bellows depends upon the height of the sensitizing enclosure which is itself a function of the diameter of the cathode to be processed. The maximum dellexion is 40 cm. R. AIREY: Have you attempted to effect an indium seal by bringing the parts together in the presence of the molten metal, thus eliminating the need for a high pressure hydraulic ram? J . GRAF: The sealing of tubes by means of a molten metal joint can be done provided that the gases desorbed by the joint inside the tube do not spoil the characteristics of the photoemissive layer and of the tube structure.
t See p. 989.
INTRODUCTION The conventional method of preparing the photoemissive layer in a photoelectric tube is by the thermal evaporation of antimony and alkali metals from sources mounted inside the tube. Some of these sources must be located either directly in view of the photocathode, so as to achieve uniform evaporated layers, or in such a manner that the molecular diffusion rate is relatively high. It is easily understood that such sources prohibit the use of quite a number of electrode configurations, particularly those in which the electrodes are placed close to the photocathode. Moreover, with the conventional method, the alkali vapour permeates throughout the tube during processing, and very often interacts chemically with the surface of the electrodes. This can enhance undesirable effect,s such as leakage currents, field emission, and parasitic photoemission. It can be said that, as a general rule, the performance of the photocathode is dependent to a large extent on the internal structure of the tube. These troubles, often encountered with conventional tube processing methods, can be avoided by the so-called “transfer technique”. The main features of this technique are as follows. (1) The substrate, on which the photoemissive layer is to be formed, is isolated from all other component parts of the photoelectric tube, so that it alone is exposed to the materials evaporated during the processing of the photocathode, (2) The several evaporation sources are grouped together in an auxiliary enclosure, which is located in front of the substrate and is maintained in that position throughout the processing of the photocathode, after which it is removed. (3) The photoemissive layer is only transported to its final position in the tube, if, after stabilization and cooling, it has the required photoelsctrio properties. 367
368
P. DOLIZY AND R . LEGOUX
DESIGNO F THE “TRANSFER” TUBES The design of tubes to be made by the transfer process is strongly influenced by the way they are to be pumped and processed in the special chamber. The envelope of a tube to be made by this process is divided into two parts, each of them containing elements of the tube itself. One of these two parts is often the input window of the tube, and in many msea, this is used as the photocathode substrate. The two parts are eventually joined together by metal rings (Pig. 1). However, the tube is first pumped and the photocathode processed with the two parts separated. The two parts are then joined by an indium seal which is made by pressing the two rings together, one of these having a groove Glass window and Dholocathode substrate
Metal rings
FIG.1. Indium compression seal a t front of phototube. The electrode structure in the lower half of the tube is not shown.
to hold the indium and the other a tongue. Indium is particularly suitable for the purpose because of its great malleability, its low vapour pressure at high temperature, and its great resistance to oxidation. The seal-off tip, which is usual on convendional tubes, is of course no longer required.
THETRANSFER EQUIPMENT A photograph of transfer equipment capable of handling photocathodes up to 120 mm in diameter is shown in Fig. 2. A cross-section of the cylindrical transfer enclosure is shown in Fig. 3. This enclosure is made of stainless steel and the top, which is in the form of a bell-jar of glass or metal, is removable. The two halves of the tube to be processed can be seen in the figure. One half of the tube, pre€erably that including the photocathode substrate, is fastened to the upper part of
NEW TECHNOLOGY BOR TRANSFERRING PHOTOCATHODES
369
the enclosure with a clamping ring held by two columns. The other is set up in the lower part of the enclosure on a movable table guided by the columns and connected to a hydraulic press. At a later stage this lifts up the lower half of the tube for the seal to be made to the upper half. All the evaporation sources necessary for the preparation of the photocathode are grouped together in an auxiliary enclosure which is open at the top and can be moved sideways by a second mechanism.
FIG.2. The transfer apparatus.
The system is pumped first with a cryogenic pump (zeolite and liquid nitrogen) to about 6 x l o T 4torr and then either with an oil diffusion pump having two refrigerated baffles in series or with a Penning ion pump. The ultimate vacuum as measured by the ionization gauge shown in Fig. 3 is generally of the order of torr. The entire equipment is outgassed for 16 h during each pumpiiig cycle by heating coils at a temperature of 260°C, while the transfer bell-jar is brought to between 400°C and 460°C. When the bake-out is terminated, the photoemissive layer is processed in the conventional way but with modifications to allow for the large volume of the processing chamber. The pressure is then about torr.
370
P. DOLIZY AND R. LEQOUX
FIQ.3. Croae-section of the transfer encloaure.
NEW TECHNOLOGY FOR TRANSFERRING PHOTOCATHODES
37 1
During the processing, the sensitivity of the photocathode is monitored using the light from a tungsten filament lamp placed above the bell-j ar, As soon as the photocathode has cooled to a temperature of between 40°C and BO'C, and has stabilized, then, providing that the required photoelectric properties have been achieved, the tube is ready to be closed. The pressure at this stage is torr. In the closing process the sensitizing enclosure is first moved to a lateral position in the belljar leaving room for the tube to be lifted up (Fig. 4). The two parts of
Processing enclosure
1 I
FIG.4. Showing how the processing enclosure is moved to one side and the tube is closed.
the tube are then joined by compressing the indium seal. The photocathode is now in its operating position. Air is let into the bell-jar and the tube can now be removed from its mechanical supports and is ready for use. OF THE METHOD ADVANTAGES The advantages of the method are numerous. Apart from those that have already been noted, it avoids the poisoning of component parts of the tube by the physical and chemical action of alkali vapours during the photocathode processing. This applies in particular to the following; the fluorescent screens of image tubes, surfaces which must be highly insulating, and electrodes which must be free from field emission or photoemission. Photocathodes made using the method are comparable to those
372
P. DOLIZY AND R. LEQOUX
obtained by conventional methods. Trialkali photocathodes with sensitivities higher than 200 pA/lm have been achieved on glass and metal substrates. The uniformity of the photocathodes is also improved because the evaporation sources can be placed at greater distances from the substrate than are possible in conventional tubes.
FIQ.5 . High current photodiode. Anode t o cathode spacing 2 mm.
After a photocathode has been processed it can be tested before its introduction into a tube, and if inadequate it can be rejected, thus avoiding throwing away the entire tube. A most important aspect of the process is that it makes possible the construction of tubes in which the dimensions and shape are such that photocathode processing would not be practicable by conventional methods. As an example of such a tube, Fig. 5 shows a high current
FIR.6. High-speed shutter tubes.
NEW TECHNOLOGY FOR TRANSFERRING PHOTOCATHODES
373
photodiode in which the distance between the cathode and anode is 2 mm. Figure 6 shows a family of fast shutter tubes for which the photocathode diameters range from 40 mm to 120 mm, and the cathode to phosphor distances from 2 mm to 10mm. These are more fully described in another paper in this volume.?
DISCUSSION s. MAJUMDAR: 1. What current can you draw from the high current photodiodes? 2. In your image tubes, does the cathode performance deteriorate after many hours of operation? J . GRAF: 1. These diodes deliver a linear rosponse up to 10 A for an applied voltage of 3 kV and a pulse of 1 psec. The saturation current is 20 A. 2. We have not seen any change in cathode performance on a tube tested for 5 h continuous running. I t must be noticed that these types of tubes aro designed only for pulse operation. M. ROME: What improvements are found in dark current by the use of the transfer technique? Would you please compare the dark current of tubes with the same photocathode, (e.g. type 5-20),of similar sensitivity, which differ only that some are conventionally prepared and others by the transfer method? J. GRAF: For the same 5.20 sensitivity in two photomultiplier tubes, one conventionally prepared and the other by the transfer method, the transfer tube has a dark current nearly hundred times lower than the conventional tube. R. DECKER: 1. Is the tube isolated or just shielded from the photocathode processing chamber? 2. Is it possible to process more than one cathode a t one time? 3. How far do the bellows have to deflect to make a seal? J . GRAR: 1. The body of the tube is only separated from tho sensitizing enclosure containing the alkali dispensers. There is not a tight separation between the dispensers and the body of the tube. 2. Yes, it is possible to process simultaneously several photoemissive layers in the transfer equipment. 3. The deflexion of the bellows depends upon the height of the sensitizing enclosure which is itself a function of the diameter of the cathode to be processed. The maximum dellexion is 40 cm. R. AIREY: Have you attempted to effect an indium seal by bringing the parts together in the presence of the molten metal, thus eliminating the need for a high pressure hydraulic ram? J . GRAF: The sealing of tubes by means of a molten metal joint can be done provided that the gases desorbed by the joint inside the tube do not spoil the characteristics of the photoemissive layer and of the tube structure.
t See p. 989.