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Low energy pion detection by a silicon surface barrier telescope
NUCLEAR
INSTRUMENTS
AND METHODS
157 ( 1 9 7 8 )
29-34;
(~)
NORTH-HOLLAND
PUBLISHING
CO.
LOW ENERGY PION DETECTION BY A SILICON SURFACE BARRIER TELESCOPE* R. M. SEALOCKt , H. S. CAPLAN and M. K. LEUNG**
Saskatchewan Accelerator Laboratory, University of Saskatchewan, Saskatoon, Canada S7N 014/0 G. J. LOLOS and S. HONTZEAS
Department o/Physics, University al Regina, Regina, Canada $4S 0,42 Received 17 May 1978 Four telescopes of three (2-AE, l-E) silicon surface barrier detectors each, mounted in the focal plane of a magnetic spectrometer, have been used to detect positive pions in the energy range from 4.7-17.9 MeV and negative pions from 14.1-17.9 MeV. Positive pions from 4.7-12.7 MeV were stopped in the third detector while positive and negative pions from 14.1-17.9 MeV were detected in transmission. For energies greater than 7.4 MeV aluminum moderators were placed in front of the first detector to degrade the pion energy. Energy spectra show well resolved pion peaks with extremely low background. Double differential cross sections for the J2C(e, zr+)12B, e' reaction have been measured.
1. Introduction Pion production and scattering experiments have for the most part been limited by the detectors available to pion energies greater than 20-30MeV. Since the pion-nucleus final state Coulomb interaction and the strong Coulomb interference increase with decreasing pion energy, larger effects will be observed for lower pion energies. For this reason we have built a detector for pions with energies as low as 5 MeV. Pions are usually detected by telescopes of plastic scintillators, sometimes with Cherenkov anticoincidence counters to discriminate against positron and electron background"2). Pions have been detected indirectly by counting positrons from the t~+--,v+v+e decay following the r c + ~ / z + + v decay3). By combining scintillators and detection of the pion decay 4MeV pions have been observed4). Lithium drifted silicon detectors have been used to detect positive pions but not below 30 MeV 5). None of these methods has detected negative pions with energies lower than 21.6 MeV 6). Although silicon surface barrier detectors (SSBD's) have never before been used to detect pions, we find that a telescope of SSBD's has good energy resolution, excellent background rejection, and the ability to detect low energy pions of either charge. Stopped positive pions have been detected * Supported by the National Research Council of Canada. * Joint appointment with Department of Physics, University of Regina, Regina, Canada $4S 0A2. ** Present address: Keith Consulting Engineers, Uranium City, Saskatchewan, Canada SOJ 2W0.
from 4.7-12.7MeV. Positive and negative pions from 14.1-17.9MeV have been detected in transmission. 2. Equipment A telescope consists of two transmission mounted 300/~m thick by 300 mm 2 active area SSBD's and one 2000/zm thick by 300 mm 2 active area SSBD. The detectors are mounted as compactly as the individual housings will allow in order to minimize the effects of multiple scattering. An aluminum moderator may be placed in front of the first detector to reduce the pion energy. Fig. 1 shows the arrangement of the detectors and moderator and the disposition of 4 telescopes in the focal
ze--
U ~ -..... ~, ~___ ...... ~........... __t=~_~( utrunWindow ~ Hous,ng AE AE
E
Moderator Fig. ]. "]he upper part of the diagram shows the disposition of the telescopes on the spectrometer focal plane. The lower part shows one telescope with a moderator in place.
30
R, M. S E A L O C K
plane of a 127° double focussing magnetic spectrometer. The first and third detectors of each telescope were calibrated with 5.48 MeV ~z particles from a 241Am source placed in the target position. Alphaparticle count rates vs magnetic field of the spectrometer are shown for one telescope in fig. 2. Energy calibrations were determined from the centroids of the regions of overlap of the two curves. The energy acceptance of each telescope was calculated from the area of overlap and the centroid field using relativistic kinematics for pions. The use of moderators allows the detection of stopped positive pions with energies up to about 12 MeV. When a stopped pion decays to a muon and a neutrino the pulse height in the third detector is enhanced by 4.12 MeV. Above about 12 MeV pions have sufficient range straggling that the decay m u o n may escape the third detector's active volume, complicating the pulse height spectrum. Therefore, pions with energies greater than 12.7 MeV were detected in transmission. In this case, the moderator reduced the pion energy so that the stopping power was increased and the pions were more easily distinguished from electron X - - F I R S T DETECTOR
1200
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or positron background. Five moderator thicknesses were chosen to stop pions of five different energies in the midplane of the third detector. The thicknesses, unique to each telescope, were calculated for the pion energies incident on the centre of the telescope when the spectrometer field was such that 8, 9, 10, 11 or 12 MeV pions were focussed on the centre of the focal plane. Moderator thicknesses ranged from 0.3 to 2.8 mm. The moderator diameter was chosen slightly larger than that of the detector's active area to minimize multiple scattering effects due to the moderator. The 4 telescopes were mounted in a vacuum chamber that was bolted to the exit of the spectrometer. A C A M A C controlled stepping motor, external to the vacuum, drove 4 wheels that held the moderators. Any of the 5 thicknesses or an empty slot could be selected. The detection system was developed using the University of Saskatchewan electron linear accelerator which produces 1 lls wide bursts of electrons with energies up to 200 MeV at a repetition rate of 3 6 0 H z and an average beam current of 20/~A. The beam is m o m e n t u m - a n a l y s e d and arrives at the target with a m o m e n t u m spread, Ap/p, of typically 0.35%. Because of the large neutron and 7ray background associated with electron beams, the detectors must be well shielded. Therefore the detector v a c u u m chamber was surrounded by a m i n i m u m of 10 cm of Pb and 38 cm o f borated paraffin.
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3. Electronics Standard nuclear instrumentation electronics were used to identify pions. The major functions of the electronics were amplification and summing
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FIELD (Gauss) Fig. 2. Calibration of the first and third detector of one telescope with 5.48 MeV ~z particles from a 241 A m source placed in the target position. T h e m o m e n t u m acceptance (fwhm zlp/p) o f this telescope is 0.696%.
Fig. 3. A simplified block diagram of part of the electronics for one teslescope.
LOW
ENERGY
PION
of detector outputs and recognizing triple coincidences. Two ADC's per telescope were enabled by the triple coincidence signal and read by an online SDS 920 computer. Fig. 3 is a simplified block diagram of part of the electronics for one telescope. For each detector there was an Ortec* 122S preamplifier, spectroscopy amplifier, and timing single channel analyser (TSCA). One amplifier output went to the TSCA and a second output to a sum amplifier. For the second detector one amplifier output was split so that it went to an ADC and to the sum amplifier. Amplifier time constants were set at 0.5/~s, the lowest available. Thus the pulse height included energy from the 26 ns pion decay but contributions from the 2.2/~s muon decay were minimized. Amplifier gains were adjusted to yield equal V/MeV so that for stopped pions the pulse height variations due to energy straggling in the detectors would cancel when the amplifier outputs were summed. The TSCA's were operated in the window mode for the first two detectors and as a lower level discriminator for the third detector. A precision pulser calibrated against a 24~Am cz-particle source was used to set the TSCA energy levels. The levels were set so as to ensure that energy straggling could not result in lost events. For stopped pions the third detector lower level was set just below the energy deposited if the decay muon left no energy in the detector. Triple coincidences among TSCA outputs were recognized by an AND gate. Its output was used to enable two ADC's which digitized both the sum of the 3 pulse heights and the second detector pulse height. Th~ 122S preamplifiers were not designed for SSBD's but were the only preamplifiers on hand in sufficient quantity. Energy and timing resolution suffered from preamplifier noise but were adequate to detect pions. Energy resolution was typically 75 keV fwhm for 5.48 MeV ~z-particles. Energy losses in the detectors' dead layers were only a few keV. Because the energy loss in the front two detectors was small (0.4-1.1 MeV), the poor signal to noise ratio caused large time jitter in the TSCA timing output. The TSCA outputs were 500 ns wide and the effective coincidence resolving time for 3 detectors was 500 ns. As will be shown, accidental coincidences were almost nonexistent, primarily due to the energy requirements Ortec, Inc., U.S.A.
DETECTION
31
of the TSCA's. Measurements of the timing resolution between the front two detectors yielded 44 and 155 ns fwhm for test pulses corresponding to 1.7 and 0.45 MeV, respectively. The measured timing resolution for 17MeV pions (0.46MeV deposited in detectors 1 and 2) was 150 ns fwhm and the maximum observed time difference between the TSCA outputs for the front two detectors was 413 ns. 4. Detector response
To determine the pion yield from a pulse height spectrum one must understand the spectrum shape and sources of background. Deducing cross sections from these yields requires a knowledge of the detection efficiency and mechanisms for losing valid events. The pulse height spectra are modified by range and energy straggling, edge effects, the decay of stopped muons, and background events in one or more detectors in coincidence with a pion. Detection efficiency is reduced by multiple scattering and pion absorption in the detectors. Other causes of loss are absorption in the target or moderator and decay in flight. These processes and background mechanisms for both stopped and transmitted pions are discussed below. The 4.12 MeV decay muons have a range in silicon of about 800#m 7) and thus stop within a sphere of 1600gin diameter. For 2000~zm thick third detectors, pions of energy greater than about 12 MeV have sufficient range straggling 8) that decay muons may escape through the front or back surface of the third detector. If a muon escapes through the back surface, the summed pulse height will be reduced; while if it escapes through the front surface, the second detector pulse height may be enhanced, with the possible result that the pulse height exceeds the TSCA upper level and the event is rejected. Loss of muons causes a low energy tail in the summed pulse height spectrum and a high energy tail in the second detector pulse height spectrum. Energy straggling in the moderator or detectors causes broadening of the pulse height spectra except in the case of the summed spectrum for pion energies that do not require moderators. If a positive pion stops within 800/~m of the edge of the third detector, the decay muon may escape. Because of the spectrometer focussing characteristics, the effective area of each detector is that of the intersection of a 0.65 cm wide strip
32
R.M.
SEALOCK
and a 1.97 cm diameter circle. Since 8.5% of this effective area is within 800/~m of the edge, one expects a low energy tail in the summed spectrum due to edge losses. A Monte Carlo calculation indicated that for this reason 2.1% of the pion events will have pulse heights missing part of the muon energy. When a stopped positive muon decays, the positron energy is between 0 and 52 MeV; and its contribution to the pulse heights depends on both the direction and time of the decay. The effect on the original pulse height of a delayed second pulse was studied using test pulses and 0.5 #s amplifier time constants. Enhancement of the first pulse height was 100% of the second pulse height for delays less than 0.1/~s and fell to 0% at 2.0/~s. A sufficiently prompt muon decay can increase the summed pulse height by as much as 8 MeV if the positron has a maximum path length in the third detector. Scattered electrons and positrons produced in the target that pass through the telescope within 2.0 ~s of a pion event will also increase the pulse heights. Pulse height spectra for stopped 6.6 MeV positive pions and transmitted 15.3 MeV negative pions are shown in fig. 4. Parts a and c are summed pulse height spectra and parts b and d are second detector pulse height spectra. The data are the combined results for one telescope of several runs at different laboratory angles with the same spectrometer magnetic field. No moderator was used for the 6.6 MeV data and a 1.9 mm thick moderator was used for the 15.3 MeV data. The peak in fig. 4a has a resolution of 223 keV fwhm and is fairly well described by a simple theoretical model represented by the solid line. The model includes partial loss of the muon energy at the edge and contributions from the decay positron. Electronic and spectrometer resolutions have been folded in. The solid lines in figs. 4b, 4c and 4d were calculated from energy straggling formulaeg). Possible causes of the poor fit to the high energy sides of the experimental spectra are amplifier gain shifts and electrons or positrons that pass through the telescope within 2.0 gs of a pion. These electrons or positrons can add as much as 1 MeV to the sum pulse height. Although the charged particle detection efficiency of SSBD's is nominally 100%, a particle that enters the first detector near the edge may undergo sufficient multiple scattering to miss the second or third detector. Multiple scattering in the modera-
et al.
tors does not affect the yield because they have larger diameters than the detectors and in-scattering equals out-scattering. If 6.0MeV pions pass through 600 ~tm of silicon, they will emerge with a mean angular divergence of 3.4 ° (ref. 10). For the pr¢sent geometry, 6% of all 6.0MeV pions incident on the first detector will miss the third detector active area due to multiple scattering~l). Nuclear absorption of pions in the detectors also reduces the detection efficiency, either by keeping a pion from reaching all three detectors or by causing too high a pulse in the first or second detector (as in a (Tr,x) reaction). Less than 0.5% of the incident pions are absorbed in the detectors ~). Pions can also be absorbed in the target or moderator and these losses are estimated at less than 0.5%. Pion yields are corrected for decay in flight. The distance from the target to the focal plane, along the central ray of the spectrometer, is 2.5 m, resulting in survival probabilities ranging from (b) '
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ENERGY
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Fig: 4. Pulse height spectra for stopped 6.6 MeV positive pions and t r a n s m i t t e d 15.3MeV negative pions. Both s u m m e d (a and c) and second detector (b and d) pulse height spectra are shown. T h e theoretical curves are explained in the text.
LOW
ENERGY
0.305 to 0.532 for 5.0 and 17.0 MeV pions, respectively. A variety of processes with charged and neutral particles contribute to background in the pulse height spectra. Charged particles other than pions may be detected and accidental coincidences from neutron and 7-ray room background may imitate the pion pulse heights. In most cases, essentially all of the events in pulse height spectra like those in fig. 4 are pions. However, for negative pion measurements at angles less than about 50 °, background from scattered electrons obscured the pion peaks. The counting rate with no coincidence requirement was less than 1 count per telescope per 100beam pulses for a spectrometer field corresponding to 7.0 MeV positive pions and a spectrometer angle of 90 ° . Background count rates with electron beam energies below the pion threshold were less than 1% and 2% of pion count rates with 200 MeV incident electrons for positive and negative spectrometer fields, respectively. With the spec!rometer field set for 7.0 MeV positive pions and the moderator normally used for 12.0 MeV pions in front of the first detector, count rates were 2.6% of the pion count rate with the correct absorber.
PlON
33
DETECTION
5. uC cross section m e a s u r e m e n t s The system has been used to measure cross sections for the 12C(e, n+)]2B, e ' reaction at several laboratory angles. Fig. 5 shows 90 ° double differential cross sections with statistical uncertainties for 5-16 MeV positive pions. A 102 mg/cm 2 thick natural carbon target was used and the electron beam energy was 200 MeV. Count rates per telescope varied from about 50 to 500 pions per hour. The horizontal bars reflect the target thickness and the energy acceptance of the spectrometer. The data points have been plotted at the pion energies that correspond to production in the centre of the target. There is good agreement among the individual telescopes. The telescopes were also used to detect 15 MeV protons from the 12C(e, p)I1B, e' reaction. No moderator was necessary to stop 15 MeV protons in the third detector. Proton yields were measured before and after pion runs to check consistency. 6. D i s c u s s i o n and c o n c l u s i o n s Improvements to the present detection system are being incorporated. Both timing and energy resolution will be improved by using Ortec 142B preamplifiers. By reducing the timing resolution to
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ENERGY (MeV) Fig. 5. Double differential cross sections and statistical uncertainties for the 12C(e, 7r + )12B, e' reaction at an incident electron energy of 200 MeV. The symbols denote different telescopes.
34
R.M.
SEALOCK
the 20 ns range, accidental coincidences and electron and positron background will be reduced. This will allow negative pion cross section measurements at more forward angles. With an improved signal to noise ratio, smaller energy losses could be observed; and the transmission measurements could be extended to higher energies. Future measurements will be made with a 12-channel C A M A C A D C so that the pulse height in each detector can be recorded and events stored individually for off-line histogram building. Variations on the present technique have certain advantages. By removing the third detector and using a fast double coincidence both positive and negative pions with energies as low as 5 MeV could be detected in transmission. The third detector could be replaced by a scintillator and photomultiplier to detect higher energy stopped positive pions. Electron or positron background could be reduced by placing a Cherenkov anticoincidence counter behind the telescope. The utility of SSBD's for the detection of low energy pions is apparent, even with unsophisticated electronics. In particular, SSBD's have better energy resolution than scintillators, can detect pions of lower energy, and can easily detect pions of both charges.
et al.
The authors wish to thank Dr. R. G. Winter for his help in the initial stage of the project. The advice and assistance of the physicists and staff of the accelerator laboratory must also be acknowledged.
References I) D. Axen, G. Duesdieker, L. Felawka, C. H. Q. Jones, M. Salomon and W. Westlund, Nucl. Meth. 118 (1974) 435. 2) M. J. Saltmarsh, B. M. Preedom, R. D. Edge Darden, III, Nucl. Instr. and Meth. 105 (1972) 3) G. Audit et al., Phys. Rev. C15 (1977) 1415.
Ingrain, G. Instr. and and C. W. 311.
4) D. W. G. S. Leith, E. M. Lawson, R. Little and G. M. Lewis, Nocl. Instr. and Meth. 29 (1964) 341. s) K. Shoda, H. Ohashi and K. Nakahara, Phys. Rev. Lett. 39 (1977) 1131. 6) F. Borkowski, Ch. Schmitt, G. G. Simon, V. Walther, D. Drechsek W. Haxton and R. Rosenfelder, Phys. Rev. Lett. 38 (1977) 742. 7) R. M. Sternheimer, Phys. Rev. 115 (1959) 137. 8) R. M. Sternheimer, Phys. Rev. 117 (1960) 485. 9) S. M. Seizer and M. J. Berger, in: Studies in penetration o/ charged particles in matter (Natl. Acad. Sci., Publ. 1133, Washington, D.C., 1964) p. 187. Io) J. B. Marion and B. A. Zimmerman, Nucl. Instr. and Meth. 51 (1967) 93. ll} W. C. Dickinson and D. C. Dodder, Rev. Sci. Instr. 24 (1953) 428.
INSTRUMENTS
AND METHODS
157 ( 1 9 7 8 )
29-34;
(~)
NORTH-HOLLAND
PUBLISHING
CO.
LOW ENERGY PION DETECTION BY A SILICON SURFACE BARRIER TELESCOPE* R. M. SEALOCKt , H. S. CAPLAN and M. K. LEUNG**
Saskatchewan Accelerator Laboratory, University of Saskatchewan, Saskatoon, Canada S7N 014/0 G. J. LOLOS and S. HONTZEAS
Department o/Physics, University al Regina, Regina, Canada $4S 0,42 Received 17 May 1978 Four telescopes of three (2-AE, l-E) silicon surface barrier detectors each, mounted in the focal plane of a magnetic spectrometer, have been used to detect positive pions in the energy range from 4.7-17.9 MeV and negative pions from 14.1-17.9 MeV. Positive pions from 4.7-12.7 MeV were stopped in the third detector while positive and negative pions from 14.1-17.9 MeV were detected in transmission. For energies greater than 7.4 MeV aluminum moderators were placed in front of the first detector to degrade the pion energy. Energy spectra show well resolved pion peaks with extremely low background. Double differential cross sections for the J2C(e, zr+)12B, e' reaction have been measured.
1. Introduction Pion production and scattering experiments have for the most part been limited by the detectors available to pion energies greater than 20-30MeV. Since the pion-nucleus final state Coulomb interaction and the strong Coulomb interference increase with decreasing pion energy, larger effects will be observed for lower pion energies. For this reason we have built a detector for pions with energies as low as 5 MeV. Pions are usually detected by telescopes of plastic scintillators, sometimes with Cherenkov anticoincidence counters to discriminate against positron and electron background"2). Pions have been detected indirectly by counting positrons from the t~+--,v+v+e decay following the r c + ~ / z + + v decay3). By combining scintillators and detection of the pion decay 4MeV pions have been observed4). Lithium drifted silicon detectors have been used to detect positive pions but not below 30 MeV 5). None of these methods has detected negative pions with energies lower than 21.6 MeV 6). Although silicon surface barrier detectors (SSBD's) have never before been used to detect pions, we find that a telescope of SSBD's has good energy resolution, excellent background rejection, and the ability to detect low energy pions of either charge. Stopped positive pions have been detected * Supported by the National Research Council of Canada. * Joint appointment with Department of Physics, University of Regina, Regina, Canada $4S 0A2. ** Present address: Keith Consulting Engineers, Uranium City, Saskatchewan, Canada SOJ 2W0.
from 4.7-12.7MeV. Positive and negative pions from 14.1-17.9MeV have been detected in transmission. 2. Equipment A telescope consists of two transmission mounted 300/~m thick by 300 mm 2 active area SSBD's and one 2000/zm thick by 300 mm 2 active area SSBD. The detectors are mounted as compactly as the individual housings will allow in order to minimize the effects of multiple scattering. An aluminum moderator may be placed in front of the first detector to reduce the pion energy. Fig. 1 shows the arrangement of the detectors and moderator and the disposition of 4 telescopes in the focal
ze--
U ~ -..... ~, ~___ ...... ~........... __t=~_~( utrunWindow ~ Hous,ng AE AE
E
Moderator Fig. ]. "]he upper part of the diagram shows the disposition of the telescopes on the spectrometer focal plane. The lower part shows one telescope with a moderator in place.
30
R, M. S E A L O C K
plane of a 127° double focussing magnetic spectrometer. The first and third detectors of each telescope were calibrated with 5.48 MeV ~z particles from a 241Am source placed in the target position. Alphaparticle count rates vs magnetic field of the spectrometer are shown for one telescope in fig. 2. Energy calibrations were determined from the centroids of the regions of overlap of the two curves. The energy acceptance of each telescope was calculated from the area of overlap and the centroid field using relativistic kinematics for pions. The use of moderators allows the detection of stopped positive pions with energies up to about 12 MeV. When a stopped pion decays to a muon and a neutrino the pulse height in the third detector is enhanced by 4.12 MeV. Above about 12 MeV pions have sufficient range straggling that the decay m u o n may escape the third detector's active volume, complicating the pulse height spectrum. Therefore, pions with energies greater than 12.7 MeV were detected in transmission. In this case, the moderator reduced the pion energy so that the stopping power was increased and the pions were more easily distinguished from electron X - - F I R S T DETECTOR
1200
O - - T H I R D DETECTOR o x
x
o
o
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et al.
or positron background. Five moderator thicknesses were chosen to stop pions of five different energies in the midplane of the third detector. The thicknesses, unique to each telescope, were calculated for the pion energies incident on the centre of the telescope when the spectrometer field was such that 8, 9, 10, 11 or 12 MeV pions were focussed on the centre of the focal plane. Moderator thicknesses ranged from 0.3 to 2.8 mm. The moderator diameter was chosen slightly larger than that of the detector's active area to minimize multiple scattering effects due to the moderator. The 4 telescopes were mounted in a vacuum chamber that was bolted to the exit of the spectrometer. A C A M A C controlled stepping motor, external to the vacuum, drove 4 wheels that held the moderators. Any of the 5 thicknesses or an empty slot could be selected. The detection system was developed using the University of Saskatchewan electron linear accelerator which produces 1 lls wide bursts of electrons with energies up to 200 MeV at a repetition rate of 3 6 0 H z and an average beam current of 20/~A. The beam is m o m e n t u m - a n a l y s e d and arrives at the target with a m o m e n t u m spread, Ap/p, of typically 0.35%. Because of the large neutron and 7ray background associated with electron beams, the detectors must be well shielded. Therefore the detector v a c u u m chamber was surrounded by a m i n i m u m of 10 cm of Pb and 38 cm o f borated paraffin.
x
c/') "ID cO 8OO ~.) (1.) 09
o
3. Electronics Standard nuclear instrumentation electronics were used to identify pions. The major functions of the electronics were amplification and summing
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FIELD (Gauss) Fig. 2. Calibration of the first and third detector of one telescope with 5.48 MeV ~z particles from a 241 A m source placed in the target position. T h e m o m e n t u m acceptance (fwhm zlp/p) o f this telescope is 0.696%.
Fig. 3. A simplified block diagram of part of the electronics for one teslescope.
LOW
ENERGY
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of detector outputs and recognizing triple coincidences. Two ADC's per telescope were enabled by the triple coincidence signal and read by an online SDS 920 computer. Fig. 3 is a simplified block diagram of part of the electronics for one telescope. For each detector there was an Ortec* 122S preamplifier, spectroscopy amplifier, and timing single channel analyser (TSCA). One amplifier output went to the TSCA and a second output to a sum amplifier. For the second detector one amplifier output was split so that it went to an ADC and to the sum amplifier. Amplifier time constants were set at 0.5/~s, the lowest available. Thus the pulse height included energy from the 26 ns pion decay but contributions from the 2.2/~s muon decay were minimized. Amplifier gains were adjusted to yield equal V/MeV so that for stopped pions the pulse height variations due to energy straggling in the detectors would cancel when the amplifier outputs were summed. The TSCA's were operated in the window mode for the first two detectors and as a lower level discriminator for the third detector. A precision pulser calibrated against a 24~Am cz-particle source was used to set the TSCA energy levels. The levels were set so as to ensure that energy straggling could not result in lost events. For stopped pions the third detector lower level was set just below the energy deposited if the decay muon left no energy in the detector. Triple coincidences among TSCA outputs were recognized by an AND gate. Its output was used to enable two ADC's which digitized both the sum of the 3 pulse heights and the second detector pulse height. Th~ 122S preamplifiers were not designed for SSBD's but were the only preamplifiers on hand in sufficient quantity. Energy and timing resolution suffered from preamplifier noise but were adequate to detect pions. Energy resolution was typically 75 keV fwhm for 5.48 MeV ~z-particles. Energy losses in the detectors' dead layers were only a few keV. Because the energy loss in the front two detectors was small (0.4-1.1 MeV), the poor signal to noise ratio caused large time jitter in the TSCA timing output. The TSCA outputs were 500 ns wide and the effective coincidence resolving time for 3 detectors was 500 ns. As will be shown, accidental coincidences were almost nonexistent, primarily due to the energy requirements Ortec, Inc., U.S.A.
DETECTION
31
of the TSCA's. Measurements of the timing resolution between the front two detectors yielded 44 and 155 ns fwhm for test pulses corresponding to 1.7 and 0.45 MeV, respectively. The measured timing resolution for 17MeV pions (0.46MeV deposited in detectors 1 and 2) was 150 ns fwhm and the maximum observed time difference between the TSCA outputs for the front two detectors was 413 ns. 4. Detector response
To determine the pion yield from a pulse height spectrum one must understand the spectrum shape and sources of background. Deducing cross sections from these yields requires a knowledge of the detection efficiency and mechanisms for losing valid events. The pulse height spectra are modified by range and energy straggling, edge effects, the decay of stopped muons, and background events in one or more detectors in coincidence with a pion. Detection efficiency is reduced by multiple scattering and pion absorption in the detectors. Other causes of loss are absorption in the target or moderator and decay in flight. These processes and background mechanisms for both stopped and transmitted pions are discussed below. The 4.12 MeV decay muons have a range in silicon of about 800#m 7) and thus stop within a sphere of 1600gin diameter. For 2000~zm thick third detectors, pions of energy greater than about 12 MeV have sufficient range straggling 8) that decay muons may escape through the front or back surface of the third detector. If a muon escapes through the back surface, the summed pulse height will be reduced; while if it escapes through the front surface, the second detector pulse height may be enhanced, with the possible result that the pulse height exceeds the TSCA upper level and the event is rejected. Loss of muons causes a low energy tail in the summed pulse height spectrum and a high energy tail in the second detector pulse height spectrum. Energy straggling in the moderator or detectors causes broadening of the pulse height spectra except in the case of the summed spectrum for pion energies that do not require moderators. If a positive pion stops within 800/~m of the edge of the third detector, the decay muon may escape. Because of the spectrometer focussing characteristics, the effective area of each detector is that of the intersection of a 0.65 cm wide strip
32
R.M.
SEALOCK
and a 1.97 cm diameter circle. Since 8.5% of this effective area is within 800/~m of the edge, one expects a low energy tail in the summed spectrum due to edge losses. A Monte Carlo calculation indicated that for this reason 2.1% of the pion events will have pulse heights missing part of the muon energy. When a stopped positive muon decays, the positron energy is between 0 and 52 MeV; and its contribution to the pulse heights depends on both the direction and time of the decay. The effect on the original pulse height of a delayed second pulse was studied using test pulses and 0.5 #s amplifier time constants. Enhancement of the first pulse height was 100% of the second pulse height for delays less than 0.1/~s and fell to 0% at 2.0/~s. A sufficiently prompt muon decay can increase the summed pulse height by as much as 8 MeV if the positron has a maximum path length in the third detector. Scattered electrons and positrons produced in the target that pass through the telescope within 2.0 ~s of a pion event will also increase the pulse heights. Pulse height spectra for stopped 6.6 MeV positive pions and transmitted 15.3 MeV negative pions are shown in fig. 4. Parts a and c are summed pulse height spectra and parts b and d are second detector pulse height spectra. The data are the combined results for one telescope of several runs at different laboratory angles with the same spectrometer magnetic field. No moderator was used for the 6.6 MeV data and a 1.9 mm thick moderator was used for the 15.3 MeV data. The peak in fig. 4a has a resolution of 223 keV fwhm and is fairly well described by a simple theoretical model represented by the solid line. The model includes partial loss of the muon energy at the edge and contributions from the decay positron. Electronic and spectrometer resolutions have been folded in. The solid lines in figs. 4b, 4c and 4d were calculated from energy straggling formulaeg). Possible causes of the poor fit to the high energy sides of the experimental spectra are amplifier gain shifts and electrons or positrons that pass through the telescope within 2.0 gs of a pion. These electrons or positrons can add as much as 1 MeV to the sum pulse height. Although the charged particle detection efficiency of SSBD's is nominally 100%, a particle that enters the first detector near the edge may undergo sufficient multiple scattering to miss the second or third detector. Multiple scattering in the modera-
et al.
tors does not affect the yield because they have larger diameters than the detectors and in-scattering equals out-scattering. If 6.0MeV pions pass through 600 ~tm of silicon, they will emerge with a mean angular divergence of 3.4 ° (ref. 10). For the pr¢sent geometry, 6% of all 6.0MeV pions incident on the first detector will miss the third detector active area due to multiple scattering~l). Nuclear absorption of pions in the detectors also reduces the detection efficiency, either by keeping a pion from reaching all three detectors or by causing too high a pulse in the first or second detector (as in a (Tr,x) reaction). Less than 0.5% of the incident pions are absorbed in the detectors ~). Pions can also be absorbed in the target or moderator and these losses are estimated at less than 0.5%. Pion yields are corrected for decay in flight. The distance from the target to the focal plane, along the central ray of the spectrometer, is 2.5 m, resulting in survival probabilities ranging from (b) '
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ENERGY
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Fig: 4. Pulse height spectra for stopped 6.6 MeV positive pions and t r a n s m i t t e d 15.3MeV negative pions. Both s u m m e d (a and c) and second detector (b and d) pulse height spectra are shown. T h e theoretical curves are explained in the text.
LOW
ENERGY
0.305 to 0.532 for 5.0 and 17.0 MeV pions, respectively. A variety of processes with charged and neutral particles contribute to background in the pulse height spectra. Charged particles other than pions may be detected and accidental coincidences from neutron and 7-ray room background may imitate the pion pulse heights. In most cases, essentially all of the events in pulse height spectra like those in fig. 4 are pions. However, for negative pion measurements at angles less than about 50 °, background from scattered electrons obscured the pion peaks. The counting rate with no coincidence requirement was less than 1 count per telescope per 100beam pulses for a spectrometer field corresponding to 7.0 MeV positive pions and a spectrometer angle of 90 ° . Background count rates with electron beam energies below the pion threshold were less than 1% and 2% of pion count rates with 200 MeV incident electrons for positive and negative spectrometer fields, respectively. With the spec!rometer field set for 7.0 MeV positive pions and the moderator normally used for 12.0 MeV pions in front of the first detector, count rates were 2.6% of the pion count rate with the correct absorber.
PlON
33
DETECTION
5. uC cross section m e a s u r e m e n t s The system has been used to measure cross sections for the 12C(e, n+)]2B, e ' reaction at several laboratory angles. Fig. 5 shows 90 ° double differential cross sections with statistical uncertainties for 5-16 MeV positive pions. A 102 mg/cm 2 thick natural carbon target was used and the electron beam energy was 200 MeV. Count rates per telescope varied from about 50 to 500 pions per hour. The horizontal bars reflect the target thickness and the energy acceptance of the spectrometer. The data points have been plotted at the pion energies that correspond to production in the centre of the target. There is good agreement among the individual telescopes. The telescopes were also used to detect 15 MeV protons from the 12C(e, p)I1B, e' reaction. No moderator was necessary to stop 15 MeV protons in the third detector. Proton yields were measured before and after pion runs to check consistency. 6. D i s c u s s i o n and c o n c l u s i o n s Improvements to the present detection system are being incorporated. Both timing and energy resolution will be improved by using Ortec 142B preamplifiers. By reducing the timing resolution to
0.5
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ill
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c"
0.2
0.1
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t
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ENERGY (MeV) Fig. 5. Double differential cross sections and statistical uncertainties for the 12C(e, 7r + )12B, e' reaction at an incident electron energy of 200 MeV. The symbols denote different telescopes.
34
R.M.
SEALOCK
the 20 ns range, accidental coincidences and electron and positron background will be reduced. This will allow negative pion cross section measurements at more forward angles. With an improved signal to noise ratio, smaller energy losses could be observed; and the transmission measurements could be extended to higher energies. Future measurements will be made with a 12-channel C A M A C A D C so that the pulse height in each detector can be recorded and events stored individually for off-line histogram building. Variations on the present technique have certain advantages. By removing the third detector and using a fast double coincidence both positive and negative pions with energies as low as 5 MeV could be detected in transmission. The third detector could be replaced by a scintillator and photomultiplier to detect higher energy stopped positive pions. Electron or positron background could be reduced by placing a Cherenkov anticoincidence counter behind the telescope. The utility of SSBD's for the detection of low energy pions is apparent, even with unsophisticated electronics. In particular, SSBD's have better energy resolution than scintillators, can detect pions of lower energy, and can easily detect pions of both charges.
et al.
The authors wish to thank Dr. R. G. Winter for his help in the initial stage of the project. The advice and assistance of the physicists and staff of the accelerator laboratory must also be acknowledged.
References I) D. Axen, G. Duesdieker, L. Felawka, C. H. Q. Jones, M. Salomon and W. Westlund, Nucl. Meth. 118 (1974) 435. 2) M. J. Saltmarsh, B. M. Preedom, R. D. Edge Darden, III, Nucl. Instr. and Meth. 105 (1972) 3) G. Audit et al., Phys. Rev. C15 (1977) 1415.
Ingrain, G. Instr. and and C. W. 311.
4) D. W. G. S. Leith, E. M. Lawson, R. Little and G. M. Lewis, Nocl. Instr. and Meth. 29 (1964) 341. s) K. Shoda, H. Ohashi and K. Nakahara, Phys. Rev. Lett. 39 (1977) 1131. 6) F. Borkowski, Ch. Schmitt, G. G. Simon, V. Walther, D. Drechsek W. Haxton and R. Rosenfelder, Phys. Rev. Lett. 38 (1977) 742. 7) R. M. Sternheimer, Phys. Rev. 115 (1959) 137. 8) R. M. Sternheimer, Phys. Rev. 117 (1960) 485. 9) S. M. Seizer and M. J. Berger, in: Studies in penetration o/ charged particles in matter (Natl. Acad. Sci., Publ. 1133, Washington, D.C., 1964) p. 187. Io) J. B. Marion and B. A. Zimmerman, Nucl. Instr. and Meth. 51 (1967) 93. ll} W. C. Dickinson and D. C. Dodder, Rev. Sci. Instr. 24 (1953) 428.