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A hodoscope of silicon surface barrier detectors for low energy pion photoproduction experiments
N U C L E A R I N S T R U M E N T S A N D M E T H O D S 79 (197o) I 3 4 - I 4 O ; © N O R T H - H O L L A N D
PUBLISHING
CO.
A H O D O S C O P E OF SILICON SURFACE B A R R I E R D E T E C T O R S F O R LOW ENERGY PION PHOTOPRODUCTION
EXPERIMENTS
J. E. BATEMAN* and W. R. HOGG Department of Natural Philosophy, University of Glasgow, Glasgow W.2, Scotland Received 7 November 1969 The detection of recoil charged particles from reactions produced by 300 MeV bremsstrahlung on light nuclei has been carried out using a hodos:ope of totally depleted multi-element silicon surface barrier detectors. A large effective detection solid angle and a wide angular range have been achieved with good
angular resolution. The charged particles were identified using the silicon range-energy relation technique..The development of the multi-element detectors and the experimental properties of the hodoscope are described.
1. Introduction
range 300-60 ° . F o r the charge signal, the rise time o f pulses was a b o u t 15 ns in C1 a n d a b o u t 25 ns in C2, thereby allowing a p r o m p t fast coincidence to eliminate incoherent b a c k g r o u n d . Clipping o f the energy signal was used to avoid pile up o f electron pulses. F o r d e u t e r o n s the threshold for detection was ~ 3 M e V a n d b a c k g r o u n d electrons d e p o s i t e d a m a x i m u m o f only 90 keV in C1, below coincidence threshold. The 32/~m thickness o f C1 also minimized multiple scattering corrections. Energy resolutions o f a b o u t 100 keV (fwhm) were sufficient to allow mass d i s c r i m i n a t i o n o f detected particles.
F o r low energy p h o t o p r o d u c t i o n r e a c t i o n s to be identified by detecting the recoil c h a r g e d particle (e.g. the deuterons in ? + d ~ ~z° + d ) the requirements imposed on the detection technique include: a. well defined angle o f emission (e.g. 4- 1°); b. large solid angle (e.g. 10 msr with this d e t e c t o r solid angle a n d accepting an energy interval o f 10 M e V in the b r e m s s t r a h l u n g b e a m o f the G l a s g o w electron s y n c h r o t r o n , a yield o f a b o u t one c o u n t per h o u r f r o m the process ? + d ~ n ° + d is o b t a i n e d , assuming a m e a n differential cross section o f 5/~b s r - 1 ) ; c. tolerable response on exposure to low energy p h o t o n , electron a n d n e u t r o n b a c k g r o u n d s ; d. m o d e r a t e l y fast response time for coincidence w o r k (e.g. 10 ns); e. low energy t h r e s h o l d o f d e t e c t i o n (e.g. 5 M e V deuterons); f. g o o d energy r e s o l u t i o n in o r d e r to identify different types o f c h a r g e d particles. In a study o f the c o h e r e n t process
C1
C2
Bremsst rahlung
C3
Beam
? + d--* n O + d , using the G l a s g o w electron s y n c h r o t r o n , the experim e n t required the d e t e c t i o n o f recoil d e u t e r o n s in the energy range 6 to 20 M e V and over l a b o r a t o r y p o l a r angles o f 30 ° to 60 ° . T h e a b o v e r e q u i r e m e n t s were m e t by a telescope o f four silicon surface b a r r i e r d e t e c t o r s so a r r a n g e d t h a t the first two were m u l t i - e l e m e n t a r r a y types, followed by two thicker c o n v e n t i o n a l detectors (fig. 1). The telescope was placed in the v a c u u m o f the cryogenic target 1) (fig. 2). The d e t e c t o r s C1 and C2 b o t h h a d 12 strips each 0.5 m m wide by 10 m m high s u r r o u n d e d by a g u a r d area. A coincidence between a strip in C1 and one in C2 defined an a n g u l a r r e s o l u t i o n o f _ 1½° a n d a solid angle o f a b o u t ½ msr in each o f 144 bins in the a n g u l a r * Now at Physics Department, University of Durham. 134
/
I~ 2mm Thick
i
i
Counter
|
Fig. 1. a. Schematic view of the surface barrier detector hodotcope, b. Plan view of the liquid deuterium target and the desector hodoscope. The details of target structure are omitted for clarity.
HODOSCOPEOF SILICONSURFACEBARRIERDETECTORS
135
MYLAR WINDOW ~
~
TOCORNELL CHAMBER
CRYOSTAT MTOuUBN[TNO DETECTOR TELESCOPEASSEMBLY
E ~
ECTRONICVACUUM SEAL THRO'S
PHOTON
BEAM~ ./ /
[Zig. 2. Plan view of the target chamber. Few previous attempts have been made to utilise semiconductor detectors in this field 2) due chiefly to the difficulty of simultaneously attaining adequate angular definition and solid angle. This limitation (imposed by the size of available detectors) is removed by the present design which provides a potential solid angle of ~ 50 msr distributed over 30 ° of laboratory angle with 4- 1.5 ° resolution. Angular distributions can be thus measured in a single data run.
¢ ' / / /
2. Production of the detectors
The basic technique of manufacture is a standard one for producing transmission silicon surface barrier detectors3). The structure of the mounted detectors is similar to that described by Awcock and Young 4) [-fig. 3]. The pattern of the 12 strips with guard area is produced by evaporation of aluminium (back contact) through two masks, carefully registered. Each rectangular strip (10 m m × 0.5 ram) is an independent detector surrounded by a dead area of silicon 0.!5 m m wide which is itself separated from the dead space of the neighbouring strip by an aluminium evaporation of width 0.15 ram, continuous with the surrounding evaporation. The guard area structure was chosen to give well defined position signals. Connections to the detector strips were made with 12 #m diameter gold wire pasted to the aluminium with silver epoxy paste (Johnson and Mathey Ltd). The other end of these wires was secured to the pyrophillite ring. Some of these connections went open-circuit during the setting up of the telescope, resulting in a smaller solid angle than designed.
7.!m
]
Aluminium Evaporation
]
Silicon Masked Aluminium Evaporation
Guard Ring C . . . . . tion
\
C ....c t l o n ~
\/
Output leads to / ~ " ~ r e-a m p ,i f i e r s
/
/
/
/ Support
/ ~ Gold f r o n t contact
\ P y r o p h y l l i t e Rings \ Silicon slice (~22mm dia)
Fig. 3. a. Plan view of aluminium face of a multi-element detector, b. Cross section of a multi-element detector.
136
J. E. B A T E M A N A N D W. R. H O G G
3. Performance of the detectors It will be noted that the geometry-defining electrode is on the back of the detector so that the device becomes position sensitive only when it is totally depleted. The requirements of total depletion and low noise were not completely met especially in C2 (500 #m) because of an intrinsic characteristic of the silicon used. The silicon slices, cut parallel to the ( 1 - 1 - 1 ) crystal plane, showed a radial doping variation which resulted in a trough of low resistivity in the centre of the sliceS). For example in C1 (32/~m) total depletion was achieved in the edge strips with a bias of 30 V but the centre strips required 50 V. Excessive over voltage can lead to a rise in detector noise but, to compensate for the resistivity variation, a self adjusting bias arrangement was used by inserting high resistances (44 Mr2) in series with each strip6). The guard areas were set at a higher potential than the strips thereby causing a transverse electric field across the dead space between the guard area and strip. This tended to push the effective boundary of the active area towards the evaporation and so improve its definition. The independence of the strips was checked in two ways. A strong un-collimated alpha source (5.5 MeV) was placed close to the array detector. The outputs of two adjacent strips were put in coincidence with a resolving time of 50 ns and an energy threshold of 250
keV. No coincidence was observed during 5 min of observation. Secondly a collimated alpha source (image size on detector about 0.1 m m diameter) was scanned across the detector, giving the response shown in fig. 4. This indicates that the guard area serves to isolate the strips from each other. The definition of active area of a strip in the array detector is not delineated by geometry alone but also by the method of deriving the signal from the strip. Particles entering the dead silicon will share the charge between the active strip and the guard area, but favouring the latter due to the electric field configuration. In this case the reduced amplitude of the analogue signal will result in the rejection of this "event" because there will not be a good fit to the range-energy relations for that detector. The other mechanism for transporting the charge to the active strip is diffusion which gives rise to a slow pulse. The logic signal, however, used to denote a real event makes use of the fastest component of the output pulse. Thus slow pulses will not trigger the coincidence circuits. The overall result of these effects is to reduce the uncertainty in the definition of the active area to about 50 gin.
4. Experimental arrangement of the photoproduction experiment Since the polar angle of an event was defined by the geometrical properties of the detector telescope and its orientation with respect to the bremsstrahlung direc-
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Fig. 4. Illustration of the position sensitivity of an array detector by means of an alpha source (24tAm) tracked across the surface.
137
H O D O S C O P E OF S I L I C O N S U R F A C E B A R R I E R D E T E C T O R S
Subminiature 50n Cables Wires Carrying Taking Strip Signals Leakage Current / SignaIsMonitor~ /
Telescope Locating Stop
\
ConnectionsToC1
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/ ation Alpha Sources
\ Cl/C2 Spacer
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Coaxial Cables Carrying Guard Ring Lines And C3, C4 Signals/ _ /
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\ \
OuterlTetesc/ope ~ . \ Tube ]/ ^L,onnec~lons .. ""'~\ t-rom C2 Paxotin ~ Dowell Securing Connection Panel Front End Of Coaxial Cabtes Telescope For C3,C4 And Guard Ring Connections
Plate Leakage Current Monitor ,uide ForDri verod Electronics
Fig. 5. Diagranaxlatic s~ction]of the detector telescope assembly.
tion, a function of the telescope assembly was to provide an accurately positioned and oriented mounting for the detectors. The apparatus illustrated in fig. 5 fulfilled these requirements as well as allowing the detectors to be withdrawn through a gate valve from the vacuum system of the cryogenic target. The detectors were aligned in the telescope carriage which was in turn aligned with respect to the beam and target. The telescope carriage also facilitated the making of connections from the detectors through seal-through connectors in the vacuum wall. These connections inelude coaxial signal cables, bias and guard area connections, and leakage current monitor wires (fig. 6). Two weak alpha particle sources (24tAm) near the back of detectors C1 and C2 allowed inspection of the rise-time of pulses during data taking periods. Uniformly fast alpha particle pulse rise times ensured that
each of the strips was fully depleted and functioning normally. The detectors were in operation in the photoproduction experiment for over two months, being used at r o o m temperature in the vacuum of the target. Leakage currents remained stable, and no strips of C1 or C2 failed after the initial setting up. The details of C1 and C2 are given in table 1. The loss of useful strips was due to the following factors. 1. The central four strips of C2 were impossible to deplete totally due to the resistivity modulation in the silicon. 2. One or two strips were lost in C1 and C2 due to the conducting paste contacts going open circuit. 3. The remainder of the losses were due to failures in the subminiature cables between the detectors and the preamplifiers.
TABLE I Parameters o f detectors C1 and C2.
C1 C2
Thickness ~m)
Bias
(v)
Resistivity (.(2.cm)
Leakage current ~A)
Rise time (e particles in back) (ns)
32 + 7 552 ___ 28
~ 30 ~ 300
340 3000
0.9 17.5
15 25
Electronic noise (all strips summed) (keV fwhm) 110 120
138
J. E. B A T E M A N
A N D W . R. H O G G
Fig. 6. D e t e c t o r t e l e s c o p e a s s e m b l y s t r i p p e d d o w n .
The total effective solid angle of the telescope with the above failures was reduced from ~ 50 msr to ~ 15 msr. While this resulted in a loss of statistics, the figure of 15 msr still represented a substantial increase in solid angle when compared with a single orthodox telescope. A measure of the performance of the multi-element assembly can be gauged from the discrimination parameters DP1 and DP2, fig. 7. If the energy deposited in C1 is T1 and in C2 is 7"2 then DPI = (/'1 + T2) 1.73 __ T 1.73
=
AR/K,
where AR is the thickness of C1 and the range-energy relationship is R = K T 1"~3, K being a different constant for protons and deuteronsT). These DP1 and DP2 parameters (generated in software) were used to separate protons and deuterons. The data outside the peaks are due to particles which hit the detectors in the dead silicon surrounding the strips. DP2 shows unambiguous separation of protons and deuterons for particles transmitted by C2. On the other hand particles stopping in C2 cannot always be identified unambiguously. This resulted from C1 being only 32 #m thick instead of the design value of 50 #m. The separation was made by fitting Gaussian curves to the upper side
of the proton distribution and the lower side of the deuteron distribution. Another problem experienced was pick-up of rf radiation from the synchrotron cavity. The trouble was aggravated by intrinsicly low signal levels and by having unavoidably long leads from the detectors to their preamplifiers. A multi-parameter electronic system processed the signals from the telescope (the signature of an event was three pulse heights and two digital strip numbers) and recorded them on paper tape for later computer analysis. A comprehensive calibration and monitoring system permitted regular checks on the whole detection system during the long data taking runs. The electronic system is described in 8). An absolute measurement of the cross section of the reaction 7 + d--~ n ° + d was required, demanding an exact knowledge of the photon beam and the solid angle subtended by each angular bin of the telescope. The unusual geometry of the detector assembly posed several problems since a. different angular bins viewed different parts of the irradiated target;
HODOSCOPE
OF
SILICON
SURFACE
IO0 A c c e p t a n c e Criteria Protons
DP1 ~ 2.8
Deuterons 3 7~DP! < 5 3
80
DETECTORS
139
taneously. The information derived from the experiment, including the normalisation calculation, was processed on an IBM 360-44 computer. The statistical accuracy and systematic errors were poorer than planned, due in part to the problems already stated and in part due to the closure of the accelerator for nuclear physics purposes.
110
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The multi-element semiconductor telescope described has permitted the use of silicon detectors in a scattering experiment with good angular resolution, adequate solid angular acceptance and a wide angular range. It has thus been possible to take advantage of the special qualities of a silicon surface barrier detector telescope (i.e. low detection energy threshold, good charged particle discrimination capability, and insensitivity to low energy neutrons, photons and electrons) to perform a low cross-section pion photoproduction measurement with a low intensity bremsstrahlung beam. Kinematical regions have thus been studied which hitherto had remained unexplored due to technical difficulty. Useful experimental accuracy was achieved, but limited in the present case due to development difficulties with the multi-element detectors and curtailment of the data-taking time. It is considered that the technique could be usefully adapted to study other photoprocesses by the detection of recoil charged particles. The authors wish to express their appreciation to Miss M. Low for assistance in the production of standard detectors, to Microponent Developments Ltd. for production of the evaporation masks, to Mr. Ahmed of Solid State Nutronics for his perseverance in the production of the multi-element detectors and to Mr. J. B. Mundell for his collaboration in the entire project. Thanks are also due to Prof. P. I. Dee for his interest and support. J.E.B. acknowledges financial support from S.R.C. during the early part of this work.
T
DP2
Fig. 7. Line printer histogram of the values of (a) DPl and (b) DP2 for a batch of events. b. the photon beam intensity varied over the target area9); c. the effective solid angle of an angular bin varies rapidly within the strip of target which it views. These difficulties were overcome by normalising to the known cross section 1°' 11) ? + d - ~ p + n , and in the experiment protons and deuterons were detected simul-
References 1) E. H. Bellamy, W. R. Hogg and D. Miller, Nucl. Instr. and Meth. 7 (1960) 293. ~) J. R. O'Fallon, L. J. Koester, Jr., J. H. Smith and A. I. Yavin, Phys. Rev. 141 (1966) 889; also, M. Blecher, Thesis (University of Illinois, 1968) unpublished. J. W. Staples, Thesis (University of Illinois, 1969) unpublished. 3) R. J. Fox and C. J. Borkowski, 1EEE Trans. Nucl. Sci. NS-9 (1962) 213. 4) M. L. Awcockand D. C. Young, A.E.R.E. Report R-4710. s) W. Schtiler, Rev. Sci. Instr. 38 (1967) 1374.
140
J. E. BATEMAN AND W. R. H O G G
6) j. E. Bateman, A new position sensitive silicon surface barrier dE/dX detector, Proc. Int. Syrup. Nucl. electronics (Versailles, 1968). 7) F. S. Goulding, D. A. Landis, J. Cerny and R. H. Pehl, IEEE Trans. Nucl. Sci. NS-13 (1966) 335. 8) j. E. Bateman and J. B. Mundell, A multiparameter elec-
tronic system for analysing and recording data from a hodoscope of silicon surface barrier detectors, to be published in Nuch Instr. and Meth. 9) L. I. Schiff, Phys. Rev. 83 (1951) 252. 10) L. Allen, Phys. Rev. 98 (1954) 705. 11) A. Whetstone and J. Halpern, Phys. Rev. 109 (1957) 2072.
PUBLISHING
CO.
A H O D O S C O P E OF SILICON SURFACE B A R R I E R D E T E C T O R S F O R LOW ENERGY PION PHOTOPRODUCTION
EXPERIMENTS
J. E. BATEMAN* and W. R. HOGG Department of Natural Philosophy, University of Glasgow, Glasgow W.2, Scotland Received 7 November 1969 The detection of recoil charged particles from reactions produced by 300 MeV bremsstrahlung on light nuclei has been carried out using a hodos:ope of totally depleted multi-element silicon surface barrier detectors. A large effective detection solid angle and a wide angular range have been achieved with good
angular resolution. The charged particles were identified using the silicon range-energy relation technique..The development of the multi-element detectors and the experimental properties of the hodoscope are described.
1. Introduction
range 300-60 ° . F o r the charge signal, the rise time o f pulses was a b o u t 15 ns in C1 a n d a b o u t 25 ns in C2, thereby allowing a p r o m p t fast coincidence to eliminate incoherent b a c k g r o u n d . Clipping o f the energy signal was used to avoid pile up o f electron pulses. F o r d e u t e r o n s the threshold for detection was ~ 3 M e V a n d b a c k g r o u n d electrons d e p o s i t e d a m a x i m u m o f only 90 keV in C1, below coincidence threshold. The 32/~m thickness o f C1 also minimized multiple scattering corrections. Energy resolutions o f a b o u t 100 keV (fwhm) were sufficient to allow mass d i s c r i m i n a t i o n o f detected particles.
F o r low energy p h o t o p r o d u c t i o n r e a c t i o n s to be identified by detecting the recoil c h a r g e d particle (e.g. the deuterons in ? + d ~ ~z° + d ) the requirements imposed on the detection technique include: a. well defined angle o f emission (e.g. 4- 1°); b. large solid angle (e.g. 10 msr with this d e t e c t o r solid angle a n d accepting an energy interval o f 10 M e V in the b r e m s s t r a h l u n g b e a m o f the G l a s g o w electron s y n c h r o t r o n , a yield o f a b o u t one c o u n t per h o u r f r o m the process ? + d ~ n ° + d is o b t a i n e d , assuming a m e a n differential cross section o f 5/~b s r - 1 ) ; c. tolerable response on exposure to low energy p h o t o n , electron a n d n e u t r o n b a c k g r o u n d s ; d. m o d e r a t e l y fast response time for coincidence w o r k (e.g. 10 ns); e. low energy t h r e s h o l d o f d e t e c t i o n (e.g. 5 M e V deuterons); f. g o o d energy r e s o l u t i o n in o r d e r to identify different types o f c h a r g e d particles. In a study o f the c o h e r e n t process
C1
C2
Bremsst rahlung
C3
Beam
? + d--* n O + d , using the G l a s g o w electron s y n c h r o t r o n , the experim e n t required the d e t e c t i o n o f recoil d e u t e r o n s in the energy range 6 to 20 M e V and over l a b o r a t o r y p o l a r angles o f 30 ° to 60 ° . T h e a b o v e r e q u i r e m e n t s were m e t by a telescope o f four silicon surface b a r r i e r d e t e c t o r s so a r r a n g e d t h a t the first two were m u l t i - e l e m e n t a r r a y types, followed by two thicker c o n v e n t i o n a l detectors (fig. 1). The telescope was placed in the v a c u u m o f the cryogenic target 1) (fig. 2). The d e t e c t o r s C1 and C2 b o t h h a d 12 strips each 0.5 m m wide by 10 m m high s u r r o u n d e d by a g u a r d area. A coincidence between a strip in C1 and one in C2 defined an a n g u l a r r e s o l u t i o n o f _ 1½° a n d a solid angle o f a b o u t ½ msr in each o f 144 bins in the a n g u l a r * Now at Physics Department, University of Durham. 134
/
I~ 2mm Thick
i
i
Counter
|
Fig. 1. a. Schematic view of the surface barrier detector hodotcope, b. Plan view of the liquid deuterium target and the desector hodoscope. The details of target structure are omitted for clarity.
HODOSCOPEOF SILICONSURFACEBARRIERDETECTORS
135
MYLAR WINDOW ~
~
TOCORNELL CHAMBER
CRYOSTAT MTOuUBN[TNO DETECTOR TELESCOPEASSEMBLY
E ~
ECTRONICVACUUM SEAL THRO'S
PHOTON
BEAM~ ./ /
[Zig. 2. Plan view of the target chamber. Few previous attempts have been made to utilise semiconductor detectors in this field 2) due chiefly to the difficulty of simultaneously attaining adequate angular definition and solid angle. This limitation (imposed by the size of available detectors) is removed by the present design which provides a potential solid angle of ~ 50 msr distributed over 30 ° of laboratory angle with 4- 1.5 ° resolution. Angular distributions can be thus measured in a single data run.
¢ ' / / /
2. Production of the detectors
The basic technique of manufacture is a standard one for producing transmission silicon surface barrier detectors3). The structure of the mounted detectors is similar to that described by Awcock and Young 4) [-fig. 3]. The pattern of the 12 strips with guard area is produced by evaporation of aluminium (back contact) through two masks, carefully registered. Each rectangular strip (10 m m × 0.5 ram) is an independent detector surrounded by a dead area of silicon 0.!5 m m wide which is itself separated from the dead space of the neighbouring strip by an aluminium evaporation of width 0.15 ram, continuous with the surrounding evaporation. The guard area structure was chosen to give well defined position signals. Connections to the detector strips were made with 12 #m diameter gold wire pasted to the aluminium with silver epoxy paste (Johnson and Mathey Ltd). The other end of these wires was secured to the pyrophillite ring. Some of these connections went open-circuit during the setting up of the telescope, resulting in a smaller solid angle than designed.
7.!m
]
Aluminium Evaporation
]
Silicon Masked Aluminium Evaporation
Guard Ring C . . . . . tion
\
C ....c t l o n ~
\/
Output leads to / ~ " ~ r e-a m p ,i f i e r s
/
/
/
/ Support
/ ~ Gold f r o n t contact
\ P y r o p h y l l i t e Rings \ Silicon slice (~22mm dia)
Fig. 3. a. Plan view of aluminium face of a multi-element detector, b. Cross section of a multi-element detector.
136
J. E. B A T E M A N A N D W. R. H O G G
3. Performance of the detectors It will be noted that the geometry-defining electrode is on the back of the detector so that the device becomes position sensitive only when it is totally depleted. The requirements of total depletion and low noise were not completely met especially in C2 (500 #m) because of an intrinsic characteristic of the silicon used. The silicon slices, cut parallel to the ( 1 - 1 - 1 ) crystal plane, showed a radial doping variation which resulted in a trough of low resistivity in the centre of the sliceS). For example in C1 (32/~m) total depletion was achieved in the edge strips with a bias of 30 V but the centre strips required 50 V. Excessive over voltage can lead to a rise in detector noise but, to compensate for the resistivity variation, a self adjusting bias arrangement was used by inserting high resistances (44 Mr2) in series with each strip6). The guard areas were set at a higher potential than the strips thereby causing a transverse electric field across the dead space between the guard area and strip. This tended to push the effective boundary of the active area towards the evaporation and so improve its definition. The independence of the strips was checked in two ways. A strong un-collimated alpha source (5.5 MeV) was placed close to the array detector. The outputs of two adjacent strips were put in coincidence with a resolving time of 50 ns and an energy threshold of 250
keV. No coincidence was observed during 5 min of observation. Secondly a collimated alpha source (image size on detector about 0.1 m m diameter) was scanned across the detector, giving the response shown in fig. 4. This indicates that the guard area serves to isolate the strips from each other. The definition of active area of a strip in the array detector is not delineated by geometry alone but also by the method of deriving the signal from the strip. Particles entering the dead silicon will share the charge between the active strip and the guard area, but favouring the latter due to the electric field configuration. In this case the reduced amplitude of the analogue signal will result in the rejection of this "event" because there will not be a good fit to the range-energy relations for that detector. The other mechanism for transporting the charge to the active strip is diffusion which gives rise to a slow pulse. The logic signal, however, used to denote a real event makes use of the fastest component of the output pulse. Thus slow pulses will not trigger the coincidence circuits. The overall result of these effects is to reduce the uncertainty in the definition of the active area to about 50 gin.
4. Experimental arrangement of the photoproduction experiment Since the polar angle of an event was defined by the geometrical properties of the detector telescope and its orientation with respect to the bremsstrahlung direc-
STP, IP No 5
STRIP No4
80-
DETECTOR
AC21
70® 60-
to I-. Z :z)
o
50-
40®
IJ_ 0
30-
20©
I0-
T O,O8
I O.IO
O. 12
O J4
O.16
DISTANCE
O.18 ACROSS
I 0.20
I O.22
FACE
OF
O.24
I 0.26
DETECTOR
I 0.28
I O.30
cm
Fig. 4. Illustration of the position sensitivity of an array detector by means of an alpha source (24tAm) tracked across the surface.
137
H O D O S C O P E OF S I L I C O N S U R F A C E B A R R I E R D E T E C T O R S
Subminiature 50n Cables Wires Carrying Taking Strip Signals Leakage Current / SignaIsMonitor~ /
Telescope Locating Stop
\
ConnectionsToC1
~ J J l
/ ation Alpha Sources
\ Cl/C2 Spacer
~
]
Coaxial Cables Carrying Guard Ring Lines And C3, C4 Signals/ _ /
I
I I TORearvacuum
] i i ThroughPane' ~[ ~ ~ ' -~~-zc--z"-/~ ~t ~ Mff~iefrinR°Tde ; iF°rcope
\ \
OuterlTetesc/ope ~ . \ Tube ]/ ^L,onnec~lons .. ""'~\ t-rom C2 Paxotin ~ Dowell Securing Connection Panel Front End Of Coaxial Cabtes Telescope For C3,C4 And Guard Ring Connections
Plate Leakage Current Monitor ,uide ForDri verod Electronics
Fig. 5. Diagranaxlatic s~ction]of the detector telescope assembly.
tion, a function of the telescope assembly was to provide an accurately positioned and oriented mounting for the detectors. The apparatus illustrated in fig. 5 fulfilled these requirements as well as allowing the detectors to be withdrawn through a gate valve from the vacuum system of the cryogenic target. The detectors were aligned in the telescope carriage which was in turn aligned with respect to the beam and target. The telescope carriage also facilitated the making of connections from the detectors through seal-through connectors in the vacuum wall. These connections inelude coaxial signal cables, bias and guard area connections, and leakage current monitor wires (fig. 6). Two weak alpha particle sources (24tAm) near the back of detectors C1 and C2 allowed inspection of the rise-time of pulses during data taking periods. Uniformly fast alpha particle pulse rise times ensured that
each of the strips was fully depleted and functioning normally. The detectors were in operation in the photoproduction experiment for over two months, being used at r o o m temperature in the vacuum of the target. Leakage currents remained stable, and no strips of C1 or C2 failed after the initial setting up. The details of C1 and C2 are given in table 1. The loss of useful strips was due to the following factors. 1. The central four strips of C2 were impossible to deplete totally due to the resistivity modulation in the silicon. 2. One or two strips were lost in C1 and C2 due to the conducting paste contacts going open circuit. 3. The remainder of the losses were due to failures in the subminiature cables between the detectors and the preamplifiers.
TABLE I Parameters o f detectors C1 and C2.
C1 C2
Thickness ~m)
Bias
(v)
Resistivity (.(2.cm)
Leakage current ~A)
Rise time (e particles in back) (ns)
32 + 7 552 ___ 28
~ 30 ~ 300
340 3000
0.9 17.5
15 25
Electronic noise (all strips summed) (keV fwhm) 110 120
138
J. E. B A T E M A N
A N D W . R. H O G G
Fig. 6. D e t e c t o r t e l e s c o p e a s s e m b l y s t r i p p e d d o w n .
The total effective solid angle of the telescope with the above failures was reduced from ~ 50 msr to ~ 15 msr. While this resulted in a loss of statistics, the figure of 15 msr still represented a substantial increase in solid angle when compared with a single orthodox telescope. A measure of the performance of the multi-element assembly can be gauged from the discrimination parameters DP1 and DP2, fig. 7. If the energy deposited in C1 is T1 and in C2 is 7"2 then DPI = (/'1 + T2) 1.73 __ T 1.73
=
AR/K,
where AR is the thickness of C1 and the range-energy relationship is R = K T 1"~3, K being a different constant for protons and deuteronsT). These DP1 and DP2 parameters (generated in software) were used to separate protons and deuterons. The data outside the peaks are due to particles which hit the detectors in the dead silicon surrounding the strips. DP2 shows unambiguous separation of protons and deuterons for particles transmitted by C2. On the other hand particles stopping in C2 cannot always be identified unambiguously. This resulted from C1 being only 32 #m thick instead of the design value of 50 #m. The separation was made by fitting Gaussian curves to the upper side
of the proton distribution and the lower side of the deuteron distribution. Another problem experienced was pick-up of rf radiation from the synchrotron cavity. The trouble was aggravated by intrinsicly low signal levels and by having unavoidably long leads from the detectors to their preamplifiers. A multi-parameter electronic system processed the signals from the telescope (the signature of an event was three pulse heights and two digital strip numbers) and recorded them on paper tape for later computer analysis. A comprehensive calibration and monitoring system permitted regular checks on the whole detection system during the long data taking runs. The electronic system is described in 8). An absolute measurement of the cross section of the reaction 7 + d--~ n ° + d was required, demanding an exact knowledge of the photon beam and the solid angle subtended by each angular bin of the telescope. The unusual geometry of the detector assembly posed several problems since a. different angular bins viewed different parts of the irradiated target;
HODOSCOPE
OF
SILICON
SURFACE
IO0 A c c e p t a n c e Criteria Protons
DP1 ~ 2.8
Deuterons 3 7~DP! < 5 3
80
DETECTORS
139
taneously. The information derived from the experiment, including the normalisation calculation, was processed on an IBM 360-44 computer. The statistical accuracy and systematic errors were poorer than planned, due in part to the problems already stated and in part due to the closure of the accelerator for nuclear physics purposes.
110
90
BARRIER
70
5. Conclusion 60 In
"6 lD > LIJ
50
40
"6 30 0 Z 20
I0
/ tO
~:oT t~'o t4o P
~
J
, n
,~ TT So
7.0
t
i~111
~o DP1
n
9:0
l&o
90807050¢-- 5O-
Acceptance Prolons
Criteria
33~DP2 ~ 45
40D e u t r o n s 62~DP2 - 72
"~30O Z 20-
IO-
20
1
/5o
~6o D
p
The multi-element semiconductor telescope described has permitted the use of silicon detectors in a scattering experiment with good angular resolution, adequate solid angular acceptance and a wide angular range. It has thus been possible to take advantage of the special qualities of a silicon surface barrier detector telescope (i.e. low detection energy threshold, good charged particle discrimination capability, and insensitivity to low energy neutrons, photons and electrons) to perform a low cross-section pion photoproduction measurement with a low intensity bremsstrahlung beam. Kinematical regions have thus been studied which hitherto had remained unexplored due to technical difficulty. Useful experimental accuracy was achieved, but limited in the present case due to development difficulties with the multi-element detectors and curtailment of the data-taking time. It is considered that the technique could be usefully adapted to study other photoprocesses by the detection of recoil charged particles. The authors wish to express their appreciation to Miss M. Low for assistance in the production of standard detectors, to Microponent Developments Ltd. for production of the evaporation masks, to Mr. Ahmed of Solid State Nutronics for his perseverance in the production of the multi-element detectors and to Mr. J. B. Mundell for his collaboration in the entire project. Thanks are also due to Prof. P. I. Dee for his interest and support. J.E.B. acknowledges financial support from S.R.C. during the early part of this work.
T
DP2
Fig. 7. Line printer histogram of the values of (a) DPl and (b) DP2 for a batch of events. b. the photon beam intensity varied over the target area9); c. the effective solid angle of an angular bin varies rapidly within the strip of target which it views. These difficulties were overcome by normalising to the known cross section 1°' 11) ? + d - ~ p + n , and in the experiment protons and deuterons were detected simul-
References 1) E. H. Bellamy, W. R. Hogg and D. Miller, Nucl. Instr. and Meth. 7 (1960) 293. ~) J. R. O'Fallon, L. J. Koester, Jr., J. H. Smith and A. I. Yavin, Phys. Rev. 141 (1966) 889; also, M. Blecher, Thesis (University of Illinois, 1968) unpublished. J. W. Staples, Thesis (University of Illinois, 1969) unpublished. 3) R. J. Fox and C. J. Borkowski, 1EEE Trans. Nucl. Sci. NS-9 (1962) 213. 4) M. L. Awcockand D. C. Young, A.E.R.E. Report R-4710. s) W. Schtiler, Rev. Sci. Instr. 38 (1967) 1374.
140
J. E. BATEMAN AND W. R. H O G G
6) j. E. Bateman, A new position sensitive silicon surface barrier dE/dX detector, Proc. Int. Syrup. Nucl. electronics (Versailles, 1968). 7) F. S. Goulding, D. A. Landis, J. Cerny and R. H. Pehl, IEEE Trans. Nucl. Sci. NS-13 (1966) 335. 8) j. E. Bateman and J. B. Mundell, A multiparameter elec-
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