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A Two-stage Electrostatic Image Intensifier with a Large Photocathode Area
A Two-stage Electrostatic Image Intensifier with a Large Photocathode Area A. W.CZ'OODHEAlI, 1). (4. TAYLOR ant1 1'. SCHAUEN 111ullarrl Reserircti Lr~boratoriee,Urdhill, S u r r e y , Eiiglanil
INTRODUCTION This is a progress report on the development) of a n image converter for use by the United Kingdom Atomic Energy Authority. The tube is intended as part of a syst.em for making observations of Cerenkov radiation, or for particle tracking. The properties required of the tube are best defined by considering, as an example, particle tracking with a soint,illation chamber. A stream of particles enters a chamber composed of st'acked plast,ic fibres, The particles .will generate light' in the fibres which are traversed and tthis of an image intensifier.. Only a few of can be fed to the ph~t~ocat~hode t,hese t,racks will be of interest so that an auxiliary detection mechanism will be necessary to identify the event, to be selected. Whilst the identification and selection processes are being carried out the image of the event must be stored in t,he system. This can be done by utilizing the decay time of a fluorescent screen. Subsequent tjo the sbore there must be a gating circuit to enable the appropriate ima,ge to be selected and amplified. The complet,e system will consist of a first' stage image converter with a screen having tjhe appropriate decay characteristics for storage and a second stage image converter which can he shutt'ered, followed by a high-gain intensifier. Thus there will be t3wo tubes: the first will process the information which is received and t,he second will provide sufficient amplificat,ion for the event t.0 be recorded. A bransmission secondary emission intensifier will provide the main amplification and i t is intended that t h e t,ube under development. should fulfil the other functions. It is intended that the tiibes shall be coupled optically and this can be done most eficient#lyif image arid object are of about. equal size. The photocathode of the t.ransmissiori secondary emission intensifier is about one inch in diameter antl so the final screen of the tube i n development will be of similar size. I n order to provide the gating antl storage facilities there must, be at least, t.wo stages of image conversion and the phot,on gain of this part of the system must be at least as large as the losses in the coupling system. T o achieve the maximum gain the two st,ages will be incorporated into the same envelope, the coupling being by means of a phosphor-photocathode sandwich. So as to 105
106
A. W. WOODHEAD, D. Q. TAYLOR A N D P. SCHAGEN
extract the maximum information from the events t o be studied it is necessary that the first photocathode be as large as possible. We have chosen t o make this of 150 mm diameter. The main requirements can now be summarized in terms of a tube. This should be a two-stage image converter coupled by a phosphorphotocathode sandwich ; the second stage should incorporate a shutter mechanism. The input photocathode will be 150 mm in diameter and the final image some seven or eight times smaller. ELECTRON-OPTICS Theoretical Considerations
An image reduced in size by the amount specified is most readily achieved by an electrostatic focusing system, I n two-stage tubes, however, this is not without disadvantages. Figure 1 is a schematic drawing of the electrode system of the tube. The electron-optics are based upon a system of concentric spherical Cathode 1
Anode/l
/Focus electrode 1
Screen 1
Cath,sde 2 Focus elytrode 2
\
Shutter
,
Anbde 2
Fro. 1. Schematic diagram of the electron-optical system.
surfaces. One of the main difficulties associated with these systems in two-stage tubes is that the image formed in the first stage is curved in the direction opposite t o that required by the succeeding photocathode. This incompatibility can be overcome to some extent by using, in the second stage, only the centre portion of a large cathode electrode and by introducing a further electrode a t about cathode potential to ensure that the equipotential surfaces take the correct form along the major portion of the electron trajectories. This subsidiary electrode can be used quite conveniently as a shutter electrode. It then remains to flatten the image field in the first stage sufficiently t o reduce the loss of definition a t the edges of the image. This can be done by decreasing the radius of curvature of the cathode plate below the theoretical value. I n order that the second photocathode shall be small compared with the effective tube diameter some demagnification must take place in the first stage. However, if the resolution is limited by the properties
A ‘l’\VO-STAGE ELECTROSTATIC’ IMAU E INTENSIFIER
107
of the fluorescent screens t,lien, for the best performance. this liniitation should be imposed by the final Hcreeii alone. ‘I’hus there must also be some deningnification in the second stage. For these rmsoiis it has been decided to niakc the reduction in irnnge Rizr in approsini;Ltely r q i d steps in each stage. Prcutietrl Wudiw The studies of the electron-optics of the tube have been inatlc in the demonnt,able systeni shown in Fig. 2 . Onc of the problems encountered
with systems of this kind is simulating the phot,ocathode surface in such a way that it is not destroyed by exposure to the atmosphere when 1nodificitti)nsto the system are inude. One niethoci is to usc’ a thin layer of gold deposited upon an ultjm-violet transmitting metlinm and tjo illuminate t,he surface with ultra-violet light. This will provide H suitable ~ ~ l i ( ~ t ( ~ e r i isurface i s s i ~ ~ tbut, ~ changes in shapc of the ciLt Iiotle cannot be made readily and this can sometimes be a difficulty. In tho system show11 in Fig. i? the cathode is a cui~vedcopper sheet8 drilled with sniall holes about 1 niin diameter which are covered wit.h a fine copper mesh. There are 660 meshes per inch and the transniksion is 50:/,. Behind each hole is a heated tungsten filament which acts A H an elect,ron source. Spherical cathode surfaces can be readily obtained
108
A. W. WOODHEAD, U. G . TAYLOR AND P. SCHAGEN
by deforming flat copper sheets hydraulically. All the electrodes are mounted on movable rods which can be controlled externally and the model can be withdrawn from the vacuum chamber for measurements and modifications. Each stage of the tube has been studied separately in the demountable system and a solution has been found in which the mesh can be resolved all over the picture. As the linear reduction in image size is about 3 in each stage this means that 30 rneshes/mm can be resolved on the screen of each section tested separately.
L 60
80
40
20
0
20
40
L 60 80
Distance from centre of cathode (mm) FIG.3. De~nagnifiratiotiax L function of photocathode radius in each stage of the tube.
I n order to obtain some measure of the image distortion that can be expected, measurements of demagnification have been made and these are shown in Fig. 3. The demagnification is expressed as the ratio of the distance of a point from the centre of the cathode to the distance of the image of the point from the centre of the screen. Curve (a) shows results for the first stage and curve (b) for the second stage. Figure 4 shows the variation of the magnification with cathode diameter for the two stages combined. From this curve the form taken by a square on the photocathode when reproduced a t the final screen can be deduced and this i R shown in Fig. 5. The maximum deviation of the curve from the straight line is about 8 % of the total length. According to these measurements, a photocathode of 150 mm diameter will be reduced t o a n image of 53 mm diameter a t the membrane and t o 19 mm at the output screen.
A TWO-STAGE ELECTROSTATIC' IMAGE INTENSIFIER
10'3
The shutter electrode has an aperture without a grid. There is therefore no structure to reduce resolution nor are there any complications which could arise from evaporating the cathode matserialsthrough a grid mesh. The change in potential necessary to switch the tube with
L
80 1'1ct. 4.
,
60
#
!
40
l
l
20
u 0
20
40
60
80
Distance from centre of cathode (mm)
Ovrrnll deinctgnifiwtioti as ct
t'iinctioii
of photocathode rtwbu~.
siich an elect'rode is somewhat higher than woiild be necessary if a mesh were used. However. so long as this is kept within the limits of' normal hard valve circuits this shoiild be no disadvantage. For optimum electron-optical performance the shutter electrode operates ats 100 V positive with respect, to the catchode when 20 kV is applied to this stage. Image cut-off is achieved with about -200 V
110
A . W . WOODHEAD, D . G . TAYLOR AND P . YCHAGEN
relative to the cathode applied to the electrode. The tube can therefore he switched with a pulse of some 300 V amplitude. TUBETECHNOLOGY Figure 6 shows a sirnplifiecl working drawing of the t>ube. A glassmetal construction has been used and each stage is constructed
14'10.
6. A fiirrrplified working drawing of t8het,uhe.
FIG.7. Tho secoiid stage assembly.
separately. The main dimensions of the tube are : diameter approximately 200 inin and overall length 700 inni. The components are of aluminium and are assembled by means of hollow rivets. The phouphor-photocathode sandwich support is either of glass 20-40 p thick or of mica 15-20 p thick and is sealed tjo a metal cup which forms a sett'ling dish for the fluorescent screen. The final screen is argon-arc
A TWO-STAGE ELECTROSTATIC IMAGE INTENSIFIER
111
welded into position and the two halves of the tube are joined by resistance welding. Figure T shows a picture of the second stage assembly. The shutter electrode can be seen mounted on insulated supports. Figure 8 shows the first stage assembly with t,he screen-cathode assembly in place.
FIG.8 . The fir& stage assembly.
Figure 9 is a picture of a completely assembled tube. Small ion gauges have been built into each stage of these first samples SO that the pressure can be measured and the tube pumped after seal-off. A further requirement when the tube is used with a stacked-fibre scintillation chamber is that, in order to preserve resolution, the photocathode plate should be as thin as possible. I n these early tubes the plates are 2 inm thick but this is not the limit. Experiments with plates just over 1 mni thick show that they will withstand a pressure of two atmospheres. CONCLUSION The tube which has been described has been designed t o form part of a system to study events which are of low light intensity, short duration and which must be selected from ninny other similar events. These requirements and the additional need for a large useful photocathode area pose numerous technological problems, not the least of which is the sheer size of the tube. It has been shown that a tube
112
A. W . WOODHEAD, D . Q. TAYLOR AND P. BCHAOEN
oapable of a high electron-optical performance can be designed and some tubes have been constructed ready for pumping and processing.
PIG.9. The fully msernbled tube.
w.
DISCUSSION
The use of aspherical photocathodes decreases the image planu curvature in image tubes. Has fibre optics been considered as an inter-stage member P A. w. WOODHEAD : Fibre optic windows could be extremely useful particularly when deflection of‘the image in the first 8tage is considered. The difficulty is that such windows have not been easily available in this country. a. w. HUTCHINSON: What possibilities are there for making the tubes with iiltraviolet transmitting windows so that they will be more sensitive to Cerenkov light? A. w. WOODHEAD: There are ultra-violet transmitting glasses which can be sealed to metals of the Nilo-K type and it may be possible to use one of these. Whether or not seals of the diamet#erwhich are used in this tube can be made satisfactorily has yet to be determined. The gain in sensitivity that would reault from using such a glass would certainly seem to be worth while if the technological difficulties can be overcome. R. A. CHIPPENDALE: Which part of the tube does Mr. Woodhead intend to operate at around earth potential? A . w. WOODHEAD: For convenience it is easiest to operate the tube with the membrane at earth potential. R . A. CHIPPENDALE : Would this be expected to cause other difficulties? A. w. WOODHEAD: It is known that with other intensifier tubes a lower background emission is achieved with the first cathode a t “earth”. I t may be necessary to adopt this method of operation with this tube. F. NIKLAS :
INTRODUCTION This is a progress report on the development) of a n image converter for use by the United Kingdom Atomic Energy Authority. The tube is intended as part of a syst.em for making observations of Cerenkov radiation, or for particle tracking. The properties required of the tube are best defined by considering, as an example, particle tracking with a soint,illation chamber. A stream of particles enters a chamber composed of st'acked plast,ic fibres, The particles .will generate light' in the fibres which are traversed and tthis of an image intensifier.. Only a few of can be fed to the ph~t~ocat~hode t,hese t,racks will be of interest so that an auxiliary detection mechanism will be necessary to identify the event, to be selected. Whilst the identification and selection processes are being carried out the image of the event must be stored in t,he system. This can be done by utilizing the decay time of a fluorescent screen. Subsequent tjo the sbore there must be a gating circuit to enable the appropriate ima,ge to be selected and amplified. The complet,e system will consist of a first' stage image converter with a screen having tjhe appropriate decay characteristics for storage and a second stage image converter which can he shutt'ered, followed by a high-gain intensifier. Thus there will be t3wo tubes: the first will process the information which is received and t,he second will provide sufficient amplificat,ion for the event t.0 be recorded. A bransmission secondary emission intensifier will provide the main amplification and i t is intended that t h e t,ube under development. should fulfil the other functions. It is intended that the tiibes shall be coupled optically and this can be done most eficient#lyif image arid object are of about. equal size. The photocathode of the t.ransmissiori secondary emission intensifier is about one inch in diameter antl so the final screen of the tube i n development will be of similar size. I n order to provide the gating antl storage facilities there must, be at least, t.wo stages of image conversion and the phot,on gain of this part of the system must be at least as large as the losses in the coupling system. T o achieve the maximum gain the two st,ages will be incorporated into the same envelope, the coupling being by means of a phosphor-photocathode sandwich. So as to 105
106
A. W. WOODHEAD, D. Q. TAYLOR A N D P. SCHAGEN
extract the maximum information from the events t o be studied it is necessary that the first photocathode be as large as possible. We have chosen t o make this of 150 mm diameter. The main requirements can now be summarized in terms of a tube. This should be a two-stage image converter coupled by a phosphorphotocathode sandwich ; the second stage should incorporate a shutter mechanism. The input photocathode will be 150 mm in diameter and the final image some seven or eight times smaller. ELECTRON-OPTICS Theoretical Considerations
An image reduced in size by the amount specified is most readily achieved by an electrostatic focusing system, I n two-stage tubes, however, this is not without disadvantages. Figure 1 is a schematic drawing of the electrode system of the tube. The electron-optics are based upon a system of concentric spherical Cathode 1
Anode/l
/Focus electrode 1
Screen 1
Cath,sde 2 Focus elytrode 2
\
Shutter
,
Anbde 2
Fro. 1. Schematic diagram of the electron-optical system.
surfaces. One of the main difficulties associated with these systems in two-stage tubes is that the image formed in the first stage is curved in the direction opposite t o that required by the succeeding photocathode. This incompatibility can be overcome to some extent by using, in the second stage, only the centre portion of a large cathode electrode and by introducing a further electrode a t about cathode potential to ensure that the equipotential surfaces take the correct form along the major portion of the electron trajectories. This subsidiary electrode can be used quite conveniently as a shutter electrode. It then remains to flatten the image field in the first stage sufficiently t o reduce the loss of definition a t the edges of the image. This can be done by decreasing the radius of curvature of the cathode plate below the theoretical value. I n order that the second photocathode shall be small compared with the effective tube diameter some demagnification must take place in the first stage. However, if the resolution is limited by the properties
A ‘l’\VO-STAGE ELECTROSTATIC’ IMAU E INTENSIFIER
107
of the fluorescent screens t,lien, for the best performance. this liniitation should be imposed by the final Hcreeii alone. ‘I’hus there must also be some deningnification in the second stage. For these rmsoiis it has been decided to niakc the reduction in irnnge Rizr in approsini;Ltely r q i d steps in each stage. Prcutietrl Wudiw The studies of the electron-optics of the tube have been inatlc in the demonnt,able systeni shown in Fig. 2 . Onc of the problems encountered
with systems of this kind is simulating the phot,ocathode surface in such a way that it is not destroyed by exposure to the atmosphere when 1nodificitti)nsto the system are inude. One niethoci is to usc’ a thin layer of gold deposited upon an ultjm-violet transmitting metlinm and tjo illuminate t,he surface with ultra-violet light. This will provide H suitable ~ ~ l i ( ~ t ( ~ e r i isurface i s s i ~ ~ tbut, ~ changes in shapc of the ciLt Iiotle cannot be made readily and this can sometimes be a difficulty. In tho system show11 in Fig. i? the cathode is a cui~vedcopper sheet8 drilled with sniall holes about 1 niin diameter which are covered wit.h a fine copper mesh. There are 660 meshes per inch and the transniksion is 50:/,. Behind each hole is a heated tungsten filament which acts A H an elect,ron source. Spherical cathode surfaces can be readily obtained
108
A. W. WOODHEAD, U. G . TAYLOR AND P. SCHAGEN
by deforming flat copper sheets hydraulically. All the electrodes are mounted on movable rods which can be controlled externally and the model can be withdrawn from the vacuum chamber for measurements and modifications. Each stage of the tube has been studied separately in the demountable system and a solution has been found in which the mesh can be resolved all over the picture. As the linear reduction in image size is about 3 in each stage this means that 30 rneshes/mm can be resolved on the screen of each section tested separately.
L 60
80
40
20
0
20
40
L 60 80
Distance from centre of cathode (mm) FIG.3. De~nagnifiratiotiax L function of photocathode radius in each stage of the tube.
I n order to obtain some measure of the image distortion that can be expected, measurements of demagnification have been made and these are shown in Fig. 3. The demagnification is expressed as the ratio of the distance of a point from the centre of the cathode to the distance of the image of the point from the centre of the screen. Curve (a) shows results for the first stage and curve (b) for the second stage. Figure 4 shows the variation of the magnification with cathode diameter for the two stages combined. From this curve the form taken by a square on the photocathode when reproduced a t the final screen can be deduced and this i R shown in Fig. 5. The maximum deviation of the curve from the straight line is about 8 % of the total length. According to these measurements, a photocathode of 150 mm diameter will be reduced t o a n image of 53 mm diameter a t the membrane and t o 19 mm at the output screen.
A TWO-STAGE ELECTROSTATIC' IMAGE INTENSIFIER
10'3
The shutter electrode has an aperture without a grid. There is therefore no structure to reduce resolution nor are there any complications which could arise from evaporating the cathode matserialsthrough a grid mesh. The change in potential necessary to switch the tube with
L
80 1'1ct. 4.
,
60
#
!
40
l
l
20
u 0
20
40
60
80
Distance from centre of cathode (mm)
Ovrrnll deinctgnifiwtioti as ct
t'iinctioii
of photocathode rtwbu~.
siich an elect'rode is somewhat higher than woiild be necessary if a mesh were used. However. so long as this is kept within the limits of' normal hard valve circuits this shoiild be no disadvantage. For optimum electron-optical performance the shutter electrode operates ats 100 V positive with respect, to the catchode when 20 kV is applied to this stage. Image cut-off is achieved with about -200 V
110
A . W . WOODHEAD, D . G . TAYLOR AND P . YCHAGEN
relative to the cathode applied to the electrode. The tube can therefore he switched with a pulse of some 300 V amplitude. TUBETECHNOLOGY Figure 6 shows a sirnplifiecl working drawing of the t>ube. A glassmetal construction has been used and each stage is constructed
14'10.
6. A fiirrrplified working drawing of t8het,uhe.
FIG.7. Tho secoiid stage assembly.
separately. The main dimensions of the tube are : diameter approximately 200 inin and overall length 700 inni. The components are of aluminium and are assembled by means of hollow rivets. The phouphor-photocathode sandwich support is either of glass 20-40 p thick or of mica 15-20 p thick and is sealed tjo a metal cup which forms a sett'ling dish for the fluorescent screen. The final screen is argon-arc
A TWO-STAGE ELECTROSTATIC IMAGE INTENSIFIER
111
welded into position and the two halves of the tube are joined by resistance welding. Figure T shows a picture of the second stage assembly. The shutter electrode can be seen mounted on insulated supports. Figure 8 shows the first stage assembly with t,he screen-cathode assembly in place.
FIG.8 . The fir& stage assembly.
Figure 9 is a picture of a completely assembled tube. Small ion gauges have been built into each stage of these first samples SO that the pressure can be measured and the tube pumped after seal-off. A further requirement when the tube is used with a stacked-fibre scintillation chamber is that, in order to preserve resolution, the photocathode plate should be as thin as possible. I n these early tubes the plates are 2 inm thick but this is not the limit. Experiments with plates just over 1 mni thick show that they will withstand a pressure of two atmospheres. CONCLUSION The tube which has been described has been designed t o form part of a system to study events which are of low light intensity, short duration and which must be selected from ninny other similar events. These requirements and the additional need for a large useful photocathode area pose numerous technological problems, not the least of which is the sheer size of the tube. It has been shown that a tube
112
A. W . WOODHEAD, D . Q. TAYLOR AND P. BCHAOEN
oapable of a high electron-optical performance can be designed and some tubes have been constructed ready for pumping and processing.
PIG.9. The fully msernbled tube.
w.
DISCUSSION
The use of aspherical photocathodes decreases the image planu curvature in image tubes. Has fibre optics been considered as an inter-stage member P A. w. WOODHEAD : Fibre optic windows could be extremely useful particularly when deflection of‘the image in the first 8tage is considered. The difficulty is that such windows have not been easily available in this country. a. w. HUTCHINSON: What possibilities are there for making the tubes with iiltraviolet transmitting windows so that they will be more sensitive to Cerenkov light? A. w. WOODHEAD: There are ultra-violet transmitting glasses which can be sealed to metals of the Nilo-K type and it may be possible to use one of these. Whether or not seals of the diamet#erwhich are used in this tube can be made satisfactorily has yet to be determined. The gain in sensitivity that would reault from using such a glass would certainly seem to be worth while if the technological difficulties can be overcome. R. A. CHIPPENDALE: Which part of the tube does Mr. Woodhead intend to operate at around earth potential? A . w. WOODHEAD: For convenience it is easiest to operate the tube with the membrane at earth potential. R . A. CHIPPENDALE : Would this be expected to cause other difficulties? A. w. WOODHEAD: It is known that with other intensifier tubes a lower background emission is achieved with the first cathode a t “earth”. I t may be necessary to adopt this method of operation with this tube. F. NIKLAS :