A scintillation chamber-image converter system for space experiments

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(I964) 336-340; © N O R T H - H O L L A N D

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A S C I N T I L L A T I O N C H A M B E R - I M A G E C O N V E R T E R S Y S T E M FOR SPACE E X P E R I M E N T S J. R. W A T E R S

American Science and Engineering Inc., Cambridge, Mass. Received 21 M a r c h 1964

A new type of detector which is especially suited to nuclear experiments in space is described. It consists of a track imaging filament scintillation chamber coupled to an image converter tube of high gain that has direct electrical read-out. The detector is small, light weight, low power and uses no magnets or voltages

higher t h a n 4 kV. Its application to two typical experiments is described. T h e s e are a v-ray telescope to detect high energy p h o t o n s a n d an e x p e r i m e n t to m e a s u r e the a n g u l a r distribution o f p r o t o n s a n d electrons in space.

1. Introduction

satellite measurements so far the only physical results known to the author have been from a balloon 2) which is, of course, much simpler technically.

In making measurements of trapped radiation, cosmic rays and similar particle fluxes in space there are some unique difficulties encountered that do not apply to the normal laboratory experiment. Most of these are fairly obvious and come into the category of physical limitations, whereby we mean that size, weight and power requirements must be reduced to the minimum. The experimental apparatus must be designed for long life and be able to yield the m a x i m u m amount of data from the least amount of hardware. In general, these requirements have been met so far by the use of fairly simple detectors such as Geiger counters, ion chambers, scintillation counters and semiconductor detectors. We are now approaching the stage, however, where some of the physical questions to be answered require more comprehensive and elaborate instrumentation and we shall describe a system that, although reasonably complex when compared to the sensors mentioned above, yet is simple enough to yield vast amounts of new information and enable more ambitious experiments to be planned. The detector to be described is based on the scintillation chamber 1) that was developed for laboratory use but has been largely superseded by the spark chamber. This is primarily because the spark chamber can be made larger in size and is easier to build, being less dependent on the availability of high quality image intensifiers. However, for space experiments the spark chamber is not obviously the best instrument since the size of the detector is limited by the spacecraft. Additional complications arise from the need for pressurisation and the interference problem between the spark and the transistorized electronics in the vehicle. The problem of obtaining the information from the spark chamber can be solved fairly easily by use of either a digitized television system, magnetic core storage from wire chambers or the acoustic type of spark detector. Although several groups are known to be working on the application of spark chambers to

2. Scintillation chambers A scintillation chamber 1) forms an image of the light from the track of a charged particle through the scintillating material rather than merely collecting the light as in a scintillation counter. Thus the output of such a device is a track " p h o t o g r a p h " similar in principle to those obtained from cloud or bubble chambers. The scintillation light is extremely weak so that it must be amplified by image intensifiers before it can be recorded on film. The chamber exists in two basic forms: in one the scintillator is a block of material, such as a Nal(TI) crystal, which is coupled to the cathode of the first image intensifier by a lens3). In the other form, the scintillator is made in thin filaments a millimeter or so diameter which are pressed against the glass window of the first tube4). Many hundreds of filaments are stacked together, not in optical contact, to constitute the sensitive volume of the chamber. The light from an excited filament is piped to its ends and goes through the tube window to reach the cathode. The photoelectrons from the first cathode are either accelerated or multiplied, depending on the type of tube, until they finally strike an output phosphor to give an amplified image of the incident light pattern. Optical gains of l05 are available from multi-stage tubes. However, it is usually necessary to couple an image intensifier to a second one by a lens (or fiber optics) for greater overall light gain in order to record the tracks of minimum ionizing particles through the scintillation chamber. The data is finally recorded on film or by a sensitive television camera. Such scintillation chambers have been operated successfully in the laboratory and a device has been built by Doolittle and Graves 5) for flight in a balloon. They plan to investigate the charge and energy spectra of primary cosmic rays from Z = 2 to Z = 8 at a height 336

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of 100000 feet. Their track forming scintillator is a NaI(T1) crystal coupled to two three-stage magnetically focussed image intensifiers. These tubes require an overall voltage of 45 kV weight about 30 pounds and are 27" long. The output of the second intensifier is to be recorded in a special camera which presses the film against the fiber optic output window. The experiment has to be performed inside a pressurised gondola. The gross weight of their system is about 100 pounds. Such a system would require considerable modification before it would be suitable for a satellite flight, not only to reduce the weight and size but also in more fundamental matters such as the data read-out and the "ruggedization" of the image intensifiers.

Our detector uses the principle of a channeled image intensifier 6) but has direct electrical read-out. Figure 1 shows a simplified sketch of the instrument. A filament scintillation chamber is butted against the front glass window which is made thin to minimize the spreading /~

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image converter tube. of light as it passes through. On the back of the window is deposited a conventional photocathode from whirl1 electrons are ejected by light from a filament. Note that the light from a single filament causes electrons to be emitted only from a small area adjacent to that filament. The photoelectrons are imaged onto a bundle of small electron multipliers 7) which can give electron gains of 105 or more. The amplified photo-electrons are drawn out of the far end of the multiplier and attracted to one of a large number of independent anodes. These anodes are aligned with the scintillating filaments so that the electrons produced by the light from one filament are all collected by one anode. Hence, an output pulse will appear on the corresponding anode when a charged particle traverses one filament so that a particle track through the scintillation chamber will

be reproduced as an instantaneous pattern of charge pulses on the anodes. These pulses are then stored so that they may be scanned sequentially for transfer to the telemetry. On reception, the particle track may be reconstructed as it passed through the chamber. We now have to show that the device as described is experimentally feasible and that it can be made to work. A more detailed drawing of a tube configuration that can be fabricated is shown in fig. 2. An electron lens has been added to focus the electrons from the cathode onto the front of the first electron multiplier bundle. This is found to be necessary in order to provide enough room in the tube for the fabrication of the cathode. But for this, proximity or magnetic focusing could be used so shortening the tube. The electron multipliers, which are the heart of this device, have been described by Wiley and Hendee7). They consists of hollow glass tubes with a high resistance inner surface which is a good secondary electron emitter. Electrical contacts are made to their ends and a potential difference of 1000 to 2000 volt is set up along them. Thus there is a uniform axial electric field down the tube and an initial electron striking the inner wall will produce secondaries which drift down the tube making many collisions with the walls so being greatly amplified. Tube diameters from 0.004" to 0.04" have been made with gains up to l 0 6 before regeneration sets in. These devices are made by the Bendix Corporation who have demonstrated 7) an image intensifier containing 5000 tubes 0.004" inside diameter whose output was presented on a phosphor screen and had a resolution of 2 line pairs/mm. Our image tube is essentially identical with this device except for the addition of a second array of multipliers

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and the replacement of the output phosphor with a multiple anode whose connections are brought out of the vacuum through a multi-lead header that is available commerciallyS). Hence, little difficulty should be experienced in its fabrication since no new techniques are involved. The image converter would use tubes 0.01" diameter formed into a bundle 2" × 1.5". A minimum ionizing particle traversing a 1 mm filament of plastic scintillator will give, on the average, 6 photoelectrons from a cathode s ) and if we assume an electron gain of l0 s we shall have a pulse of 6 x 105 electrons on the anode. This corresponds to a charge of 10 -13 coulomb or a voltage change of 20 mV on a 5 pF anode capacity. This pulse will be a few nanoseconds wide and will have to be stored for one or two milliseconds to allow for the sequential scanning. Hence, we propose to use a second bank of electron multipliers close to the ends of the first bank and apply a potential of a few hundred volt between them for proximity focusing. If now the second bank has a similar gain we shall get a very large pulse out which will be limited by the ability of the multipliers to deliver current. For one milliampere, we shall get a 10 V output pulse if the width is 50 ns. This can be stored on the anode capacitor or it can be used to trigger a simple one-active-element device which can store the signal indefinitely. We shall need one such element per anode so there will be several thousand of them. They can be designed to draw current only when triggered so that no major problem is present. The scanning routine, which is initiated on command, decides which of the anodes has received a pulse. An alternative approach to the storage would be to replace the anodes by a low leakage storage target which would be scanned from the back by an electron beam. This would not be so good either for long term stability or since a failure of the scanning beam system would mean complete failure of the detector. Initially, we have been considering a detector that would have 2000 filaments in the scintillation chamber and would require the same number of anodes. The filaments must be aligned with the anodes but this should be simple with the use of suitable jigs. These 2000 channels can be arranged in any suitable shape which need not conform to the shape of the filaments in the scintillation chamber if light pipes are used. In effect, we have 2000 individual photomultipliers in the same package. The tube described is active all the time so that noise electrons produced at the photocathode will be amplified and deposited on the anodes. This rate will be less than one noise electron per second from an area

corresponding to a 1 mm filament. However, there will also be tracks in the scintillator due to background radiation that we do not wish to record. These can be eliminated by repeatedly discharging the output anodes once every few microseconds depending on the background rate. This will erase the background information and will establish the effective integration time of the detector. We would like to re-emphasize the simplicity of this detector when compared with conventional scintillation chamber systems or spark chambers. Since the electron multiplier bundles are about an inch long the overall length of the image converter tube would be about 8" most of which is due to the electrostatic lens. If this were changed to a magnetic lens, the tube length would be about 5". The tube is about 4" diameter, weighs perhaps a pound and has about 3 to 4 kV overall. The electronics for the readout would consists of the 2000 storage elements and a sequential scanner which has to decide which of the anodes have received a pulse. The filament scintillation chamber and auxiliary counters would vary from experiment to experiment but the converter tube would not change provided the number of channels was not altered. These characteristics can be contrasted with those of the conventional scintillation chamber described earlier or with the well known properties of spark chambers. We shall now describe two typical experiments that were the reason for the design of this detector and serve to illustrate its usefulness.

4. High energy ),-ray telescope The observation of y-rays of energy greater than 50 MeV in space can yield important information on certain astrophysical processes 1°) and measurements by Kraushaar and Clark 11) from a satellite have already yielded significant information. The problem with this experiment is that the expected fluxes are extremely low (about 10 -4 photons/cm2"sec'ster) and that any experimental scheme must be critically examined for spurious events that might occur that could be confused with those due to y-rays. We do not propose to go into detail here in describing how the proposed detector eliminates such spurious events since this is not the aim of this report. We suggest that y-rays be detected by converting a fraction of them to electron-positron pairs in a thin lead plate and by observing the direction of these leptons. For high energy photons, the average angle between the direction of motion of a created lepton and of the incident photon is 29/E degrees where E is the energy of the lepton in MeV. Hence, if we observe the

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m a y seem a small rate but must be c o m p a r e d with the total o f 22 counts obtained by Kraushaar and Clark ~~) f r o m their experiment. The projection of the 7-ray direction can be measured to better than 5 ° so we know that the direction o f origination is inside an 80 ° x 5 ° section o f the celestial sphere. This is 0.12 ster or about 1% of the whole sky showing that the resolution of the simple detector is very good. Using crossed filaments and two converters would give 5 ° x 5 ° resolution with no loss of counting rate.

5. Angular distribution of charged particles Fig. 3. Scintillation chamber design for a high energy ~J-ray telescope. View as seen by the image converter tube. direction of the leptons and their opening angle we shall be able to estimate the direction of the incident ?-ray and obtain a value for its energy~2). The proposed detector is shown in cross-section in fig. 3. The filament scintillation chamber consists of 1950 filaments in a 60 x 35 array split into parts A, B, C as shown. A 1,, lead plate is enclosed between A and B. Section C is separated f r o m B and is close to a plastic Cerenkov counter D which has its front face blackened. The detector operates as follows: A y-ray passes t h r o u g h A, leaving no track, and strikes the lead plate. A b o u t 30% will convert in it and produce a pair o f leptons which will pass through B and C leaving tracks. They will then enter D and cause Cherenkov light to be emitted in the forward direction. A photomultiplier coupled to the back face o f D will detect this light. Additional photomultipliers are coupled to one end o f sections A, B, and C the other ends being coupled to the image converter tube. The signals from the photomultipliers are amplified and the signature A B C D is taken to signify an interaction to be investigated. Accordingly, the image converter readout is started and a " s n a p s h o t " of the chamber taken. This can be examined to ensure that the event was genuine, originated in the lead plate and had the other correct characteristics. Note that we shall actually have a projection of the tracks onto a plane but an additional set o f filaments could be arranged at right angles, if desired, with another converter system to give a stereo view. F r o m examination o f the track picture in B and C the direction o f the y-ray can be deduced. With a detector o f active area 30 c m 2, efficiency 3 0 ~ , and taking the cone o f acceptance o f incoming y-rays as 40 ° half-angle we estimate a counting rate o f 4.5 counts/hour or 100 a day for a flux o f 10 - 4 photons/cmE.sec.ster. This

The angular distribution of protons and electrons relative to the ambient magnetic field in space is o f considerable importance. These distributions are usually measured by using a detector of small field of view, equal to the angular resolution desired, and letting it spin about an axis in order to scan various directions in space. This technique is unsatisfactory if the expected fluxes are low and also because the complete sphere can not be investigated without a multiplicity o f detectors. It is difficult to design a particle detector that has a wide field of view, g o o d angular resolution, high efficiency the ability to distinguish between protons and electrons and to measure their energy. Our scintillation chamber appears to be a detector that fulfils most o f these criteria. In fig. 4 we show a detector that will measure the directions o f protons in the energy range 75 MeV to 120 MeV and electrons from 8 MeV to 20 MeV incident inside a cone o f half-angle 45 ° and with angular resolution of 1°. It consists o f a scintillation chamber split into four equal parts, labelled A, B, C, D, a plastic Cherenkov counter E, a thick plastic scintillator F and a thin scintillator G. A n electron o f suitable energy

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traverses A, B, C, D, E a n d stops in F so the c h a r a c teristic signature is A B C D E F G which is used to initiate the r e a d - o u t o f the image converter tube. The C h e r e n k o v c o u n t e r E will c o u n t electrons b u t not p r o t o n s so their signature will be A B C D E F G which will also start the r e a d - o u t . The pulse f r o m the thick scintillator F is pulse height a n a l y z e d a n d will give the particle energy. The filaments in A, B, C and D are a r r a n g e d in a b l o c k o f 100 × 5 each with the axes o f A a n d C parallel to each other b u t 30 ° to the axes o f the filaments o f B a n d D. The filaments are left long e n o u g h so t h a t they m a y be bent to b u t t against the faceplate o f one image converter tube. Either scintillating plastic o r glass filaments are suitable. In this way a stereo picture o f the t r a c k t h r o u g h the c h a m b e r can be obtained with one converter tube o f 2000 channels. N o t e t h a t the energy range o f charged particles to which the d e t e c t o r is sensitive can be a d j u s t e d by v a r i a t i o n o f the thickness o f the counters or by the a d d i t i o n o f absorbers. This d e t e c t o r has a sensitive a r e a o f 10 × 10 cm 2 a n d a field o f view o f 45 ° half-angle (1.9 ster) with high detection efficiency a n d a n g u l a r r e s o l u t i o n o f 1° so s h o u l d be very suitable for detecting low fluxes o f particles o f interest a m i d s t a b a c k g r o u n d o f u n w a n t e d events. The resolving time w o u l d be d e t e r m i n e d by the c l e a r i n g time o f the image converter tube and is p r o b a b l y a b o u t 5 microseconds.

6. Summary A new type of d e t e c t o r which is especially suited to sophisticated experiments in space has been described. It is a scintillation c h a m b e r coupled to a channeled image converter tube t h a t has direct digital read-out. The d e t e c t o r is small, light weight, low p o w e r a n d uses no voltages higher than 4 kV. A p p l i c a t i o n to two typical experiments t h a t could use this d e t e c t o r to

a d v a n t a g e are given. These are a high energy v-ray telescope and m e a s u r e m e n t o f the a n g u l a r d i s t r i b u t i o n s o f p r o t o n s and electrons relative to the magnetic field in space. M o s t o f this w o r k was done while I was e m p l o y e d at the Jet P r o p u l s i o n L a b o r a t o r y o f the C a l i f o r n i a Institute o f T e c h n o l o g y to w h o m I s h o u l d like to express my a p p r e c i a t i o n for their support. I should also like to t h a n k Dr. F. C. Hendee o f the Bendix Research L a b o r a t o r i e s , Southfield, M i c h i g a n for m a n y helpful suggestions a n d Professor G. T. R e y n o l d s o f P r i n c e t o n University for his constructive criticism.

References a) Survey articles: G. T. Reynolds, I.R.E. Trans. Nuc. Sci. NS-7 (1960) 115; M. L. Perl, I.R.E. Trans. Nuc. Sci. NS-9 (1962) 236. 2) j. A. DeShong Jr., R. H. Hildebrand and P. Meyer, Phys. Rev. Letters, 12 (1964) 3. 3) For example L. W. Jones and M. L. Perl, Nuclear electronics I (Int. Atom. En. Agen., Vienna, 1962) 165. 4) G. T. Reynolds and P. E. Condon, Rev. Sci. Instr., 28 (1957) 1098; I.R.E. Trans. Nuc. Sci. NS-7 (1960) 115. 5) R. F. Doolittle II and C. D. Graves, Advances in electronics and electron physics vol. XVI (Academic Press, New York, 1962) 535; TRW Space Technology Laboratories Inc., Redondo Beach, Calif. Final Report 9803-6005-RU-000(1963) unpublished. 6) j. Burns and M. J. Neumann, I.R.E. Trans. Nuc. Sci. NS-7 (1960) 142. 7) W. C. Wiley and F. C. Hendee, I.R.E. Trans. Nuc. Sci. NS-9 (1962) 103. 8) Manufactured by Corning Glass Works Inc., Coming, New York. 9) j. R. Waters, G. T. Reynolds, D. B. Scarl and R. A. Zdanis, I.R.E. Trans. Nuc. Sci. NS-9 (1962) 239. 10) p. Morrison, Nuovo Cimento, 7 (1958) 859. xl) W. L. Kraushaar and G. W. Clark, Phys. Rev. Letters, 8 (1962) 106; "Scientific American", 206, no. 5 (May 1962) 52. 12) H. Olsen, Phys. Rev., 131 (1963) 406.