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Segmented compton suppression spectrometer using bismuth germanate
Nuclear Instruments and Methods in Physics Research 222 (1984) 463-466 North-Holland, Amsterdam
463
SEGMENTED COMPTON SUPPRESSION SPECTROMETER USING BISMUTH GERMANATE A. O L I N , P.R. P O F F E N B E R G E R and D.I. B R I T T O N TR1UMF and Unwersi(v of Victoria, 4004 Wesbrook Mall, Vancouver, B.C., Canada V6T 2A3
Received 10 November 1983
A Compton suppression spectrometer has been constructed for use in experiments performed in an intense beam environment. The high rate capability is achieved by the use of a segmented configuration of bismuth germanate crystals. Performance characteristics of this instrument have been measured both with radioactive sources and in the environment of a stopping ~r beam.
1. Introduction
2. Description of spectrometer
An important background when photon spectra are measured with Ge detectors comes from photons of higher energy which Compton-scatter out of the detector volume. A number of practical devices have been constructed to reduce this background in which the Ge detector is surrounded by a large volume of NaI(T1) scintillator [1,2]. The Compton-scattered photons are detected in the scintillator with high efficiency, and this signal is used to veto these events. NaI(Tl)-based systems work well with radioactive sources, but the large 127I(n, n') cross section limits their usefulness in an intense beam environment. Here the veto rate can be very high, leading to large deadtimes, and a considerable number of 127I T-rays appear in the Ge spectrum. Bismuth germanate (BGO) has a number of advantages over NaI(TI) for this application. The properties of this material as a y-ray scintillator were first described by Nestor and Huang [3]. The (n, n') cross section is much lower [4] compared to the photon efficiency, and Ge(n, n') y's are already present due to the detector material, so that few additional lines are introduced by the scintillator. The compactness of the BGO allows more flexibility in the positioning and shielding of the detector. However, due to the low light yield of BGO, care must be taken to obtain good timing and efficiency for low energy photons [5]. In this paper we describe a segmented BGO Compton suppression spectrometer (CSS) developed at TRIUMF for use with intense pion beams. This discussion includes the physical construction, signal processing methods, and energy and timing resolution of the counters. Finally, the suppression achieved both with sources and under beam conditions is presented.
The spectrometer, fig. 1, is constructed of 16 5.5 x 5.5 x 8.0 cm3 BGO crystals arranged in separate halves and encased in aluminum. The Ge entrance hole is perpendicular to the direction of the incoming photons in order to detect the predominant forward-scattered photons. Each pair of crystals is optically separate and viewed by a 5 cm phototube. The pulse amplitudes vary by about 10% over the length of each pair of crystals, except for the front crystals, where the constriction due to holes causes an 80% loss in amplitude. To overcome this, these crystals are viewed by additional phototubes. The design would be improved by optically isolating each of the individual front crystals to increase the segmentation. These crystals were grown and assembled by Harshaw Chemical Company. The phototubes (EMI 9907 with a rubidium bialkali cathode) are coupled directly to the crystals with Dow-Corning optical grease. The tubes, magnetic shielding, bases, and preamplifiers are mounted and
_INCIDENT PHOTONS ~~1~
~
P.M.T.'S
LDETEC#0
-
~BGO
CRYSTALS
Fig. 1. Schematicof the BGO Compton suppression spectrometer. 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
464
.4. Olin et al. / Segmented Compton suppression spectrometer
supported in a light-tight enclosure. The front of the crystal is shielded with steel, tungsten (which provides the limiting collimation) and borax. The Ge counter used is a p-type coaxial intrinsic detector with a 1.8 keV resolution at 1332 keV. In order to obtain a reasonable low energy threshold with BGO, which has a low photon yield compared to NaI(TI), it is best to work in the single photoelectron mode. This is achieved by using low-noise amplifiers mounted directly on the phototube bases. A fast amplitier on the anode signal is used for timing, while the dynode signal is sent through a shaping amplifier to achieve energy resolution and pulse-pair discrimination. With 1900 V on the phototube we obtain typically an amplitude of 70 mV for the anode single photoelectron pulse and 50 mV for the dynode pulse, while the rms noise level is < 5 mV. In fig. 2 are shown oscilloscope traces of the resulting pulse shapes for single photoelectrons and for 662 keV -y's, An anode-dynode coincidence is used to generate the Ge veto pulse. To achieve a high count rate capability, each segment of the BGO is independently instru-
mented, with the 80 ns veto pulse being considerably shorter than the retiring time of the individual segments ( - 600 ns depending on pulse heights). The number of photoelectrons required to be within the 80 ns veto gate can be controlled by adjusting the dynode discriminator level. At a total BGO count rate of 200 kHz, count rates in individual segments do not exceed 50 kHz, corresponding, to a worst case segment inefficiency of 3%. At these count rates, the Ge deadtime due to random vetoes is 2%. The deadtime observed in beam will differ from these values because of the multiplicity of coincident photons produced by stopping pions.
3. Measurements with sources
The resolution of the individual crystals, measured with a 137Cs source using a charge ADC with a 60 ns gate, is typically 25-30%. The single photoelectron peak appears at 6-10 keV. These values are higher than the results obtained by Moszynski et al. [5], who obtained 15% resolution and 2 keV/photoelectron using a 2 cm diameter × 3 cm crystal. The relative efficiencies for photon collection in these two crystals were calculated using a Monte Carlo code developed by Anderson and Salomon [6]. For reasonable values of the attenuation coefficient (30-50 cm) and diffuse reflectivities (0.80-0.99) we find a × 3-4 loss of photons in the larger crystal. Thus the difference between ref, [5] and the present measurement can be attributed to photon statistics. After consideration of the 60 ns and 300 ns decay times of BGO, the effective low energy threshold is approximately 50 keV. The energy and timing of photons from a 137Cs source scattered from a scintilla, tor into the BGO has been observed to fall comfortably within these limits. F I00©0
.......................................
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!
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I
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o
Fig. 2. Oscilloscope traces of the preamplifier output signals. (a) and (b) are single photon signals from the anode and dynode preamplifiers (scale 100 mV/div × 100 ns/div) and (c) and (d) are 662 keV ),-ray signals (scale 500 mV/div×100 ns/div).
.
l
I
400
80o
_
ENERGY
~2oo
~6oo
(keV)
Fig. 3. Suppression of 6°Co Compton-scattered photons. The lower curve shows the subset of the total events(upper curve) which are not vetoed by the CSS.
A. Olin et al. / Segmented Compton suppression spectrometer
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ENERGY (keV) Fig. 4. Suppression of backgrounds following ~- stopping in 2°spb. (a) Prompt events - 3 ns < t.r < 3 ns. The structures at 575 keV and 1273 keV are the pionic 5-4 and 4--3 X-rays. (b) Delayed events 10 ns < t.r < 80 ns. The dominant triangular structures result from Ge(n. n')y reactions in the detector.
The detector response may be characterized by the peak-to-total count ratio, Qu, and the corresponding ratio for Compton-suppressed events, Q~. The suppression factor is then given by
465
recorded the energy and time relative to the pion stop of the photon in the Ge detector, the energy and time of all BGO segments that fired, and the presence of a BGO veto signal. All events occurring in a 200 ns window after the pion stop, or in a 450 ns window after a stop in the degrader (with no subsequent stop in the target) were recorded. Since the degrader was carefully shielded from the Ge detector, the latter requirement allowed us to obtain a calibration source spectrum under beam conditions. The effect of the CSS in-beam with the Pb target is shown in fig. 4. The suppression achieved in the prompt spectrum is clearly worse than was achieved out of beam. This is attributed to the large multiplicity of y's and neutrons emitted after ~r capture in heavy nuclei. The observed suppression is consistent with a background predominantly composed of a continuous distribution of y-rays, which could originate directly from v capture in the target or from Compton scattering and n-capture ~,'s in the shielding. A suppression factor closer to the out-of-beam value was observed in the Na target prompt spectrum. Losses of photopeak counts due to the imposition of the CSS veto, as determined by measuring the area of the prominent 575 keV ~r 5 - 4 2°spb X-ray, were approximately 10%. The suppression factor obtained in the delayed spectrum, fig. 4b, is significantly greater than the out-of-beam value. The dominant y ' s in this spectrum come from (n. n') interactions in the Ge detector, whose photopeak intensities are reduced to half by the CSS. Veto signals in the BGO produced by the scattered neutron represent the most likely mechanism to produce this enhanced suppression. The suppression observed in the in-beam source spectrum was the same as the out-of-beam value.
s=(1/Qu- l)/(1/Qs-1). Fig. 3 shows the effect of the CSS, for which S = 4.0 was obtained, corresponding to an improvement from Qo = 0.17 to Q~ = 0.46. Note that the reduction in the background is considerably better than × 4 over most of the spectrum, and that the Compton edges from photons that scatter back out through the photon entrance hole contribute significantly. Approximately 5% of the photopeak counts were vetoed.
4. M e a s u r e m e n t s in a pion b e a m e n v i r o n m e n t
The in-beam behaviour of the CSS was studied during a recent experiment to measure the very broad pionic 4 - 3 X-rays in 2°8pb and 2°9Bi, and the 2-1 X-ray in 23Na. The beam of 2 × 105 ~ ' - / s was stopped in 50 g targets of 2°8pb, Bi and N a i l . Plastic scintillators determined the time of the pion stop. The Ge detector was positioned 40 cm from the target, and the total count rate in the BGO was 200 kHz. For each event we
5. D i s c u s s i o n
The performance of this CSS falls somewhat below that of the best NaI(T1) devices as determined by sources [2]. It is also below the performance predicted by Monte Carlo calculations for this geometry, where a suppression factor of - 1 1 for 1 MeV photons was obtained [7]. These calculations do not take into account the inactive material between the Ge detector and the BGO, nor dead layers in the detector core. Thus differences in the Ge detector used might account for the poorer performance. A measure of the overall in-beam performance, the peak-to-background amplitude ratio achieved on the 2°9Bi ~ 5 - 4 X-ray, was 50 : 1, compared with the value of 10 : 1 obtained by BeeJ~z et al. [2]. In making thistcomparison we note that the measurements were performed under rather different conditions, and that beam quality, count rates and timing cuts all affect this ratio. The rate capabilities of the present design make it suitable for use in an intense beam environ-
466
A. Olin et al. / Segmented Compton suppression spectrometer
ment. The reduced backgrounds have made possible the observation of very weak, broadened pionic X-ray transitions. The preamplifiers were designed and constructed in the T R I U M F electronics shop by J. Cresswell and R. Hansen. We wish to thank J. Forsman and P. van Esbroek for their considerable assistance with the final performance testing of the CSS. We also thank our colleagues who participated in the pionic X-ray measurement: G. Beer, R. Kunselman, J. Macdonald, G. Marshall, G. Mason and T. Numao. This work was supported under a grant from the Natural Sciences and Engineering Research Council of Canada.
References [1] P.F. Hinrichsen, D.M. van Patter and H.M. Shapiro, Nucl. Phys. A123 (1969) 250. [2] R. Beetz, W.L. Posthumus, F.W.N. de Boer; J.L. Maarleveld, A. van der Schaaf and J. Konijn, Nucl. Instr. and Meth. 145 (1977) 353. [3] O.H. Nestor and C.Y. Huang, IEEE Trans. Nucl. Sci. NS-22 (1975) 68. [4] M.E. Lone, O. H~iusser, T.K. Alexander, J. Gascon and E. Hagberg, Proc. Int. Workshop BGO~ ed,, C.N. Holmes, Princeton University (1982) p. 237. [5] M. Moszynski, C. Gresset, J. Vacher and R. Ordu, Nucl. Instr. and Meth. 188 (1981) 403. [6] S. Anderson and M. Salomon, Proc. Int. Workshop BGO, p. 158 and TRIUMF preprint TRI-PP-82-44. [7] E. Hagberg, private communication.
463
SEGMENTED COMPTON SUPPRESSION SPECTROMETER USING BISMUTH GERMANATE A. O L I N , P.R. P O F F E N B E R G E R and D.I. B R I T T O N TR1UMF and Unwersi(v of Victoria, 4004 Wesbrook Mall, Vancouver, B.C., Canada V6T 2A3
Received 10 November 1983
A Compton suppression spectrometer has been constructed for use in experiments performed in an intense beam environment. The high rate capability is achieved by the use of a segmented configuration of bismuth germanate crystals. Performance characteristics of this instrument have been measured both with radioactive sources and in the environment of a stopping ~r beam.
1. Introduction
2. Description of spectrometer
An important background when photon spectra are measured with Ge detectors comes from photons of higher energy which Compton-scatter out of the detector volume. A number of practical devices have been constructed to reduce this background in which the Ge detector is surrounded by a large volume of NaI(T1) scintillator [1,2]. The Compton-scattered photons are detected in the scintillator with high efficiency, and this signal is used to veto these events. NaI(Tl)-based systems work well with radioactive sources, but the large 127I(n, n') cross section limits their usefulness in an intense beam environment. Here the veto rate can be very high, leading to large deadtimes, and a considerable number of 127I T-rays appear in the Ge spectrum. Bismuth germanate (BGO) has a number of advantages over NaI(TI) for this application. The properties of this material as a y-ray scintillator were first described by Nestor and Huang [3]. The (n, n') cross section is much lower [4] compared to the photon efficiency, and Ge(n, n') y's are already present due to the detector material, so that few additional lines are introduced by the scintillator. The compactness of the BGO allows more flexibility in the positioning and shielding of the detector. However, due to the low light yield of BGO, care must be taken to obtain good timing and efficiency for low energy photons [5]. In this paper we describe a segmented BGO Compton suppression spectrometer (CSS) developed at TRIUMF for use with intense pion beams. This discussion includes the physical construction, signal processing methods, and energy and timing resolution of the counters. Finally, the suppression achieved both with sources and under beam conditions is presented.
The spectrometer, fig. 1, is constructed of 16 5.5 x 5.5 x 8.0 cm3 BGO crystals arranged in separate halves and encased in aluminum. The Ge entrance hole is perpendicular to the direction of the incoming photons in order to detect the predominant forward-scattered photons. Each pair of crystals is optically separate and viewed by a 5 cm phototube. The pulse amplitudes vary by about 10% over the length of each pair of crystals, except for the front crystals, where the constriction due to holes causes an 80% loss in amplitude. To overcome this, these crystals are viewed by additional phototubes. The design would be improved by optically isolating each of the individual front crystals to increase the segmentation. These crystals were grown and assembled by Harshaw Chemical Company. The phototubes (EMI 9907 with a rubidium bialkali cathode) are coupled directly to the crystals with Dow-Corning optical grease. The tubes, magnetic shielding, bases, and preamplifiers are mounted and
_INCIDENT PHOTONS ~~1~
~
P.M.T.'S
LDETEC#0
-
~BGO
CRYSTALS
Fig. 1. Schematicof the BGO Compton suppression spectrometer. 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
464
.4. Olin et al. / Segmented Compton suppression spectrometer
supported in a light-tight enclosure. The front of the crystal is shielded with steel, tungsten (which provides the limiting collimation) and borax. The Ge counter used is a p-type coaxial intrinsic detector with a 1.8 keV resolution at 1332 keV. In order to obtain a reasonable low energy threshold with BGO, which has a low photon yield compared to NaI(TI), it is best to work in the single photoelectron mode. This is achieved by using low-noise amplifiers mounted directly on the phototube bases. A fast amplitier on the anode signal is used for timing, while the dynode signal is sent through a shaping amplifier to achieve energy resolution and pulse-pair discrimination. With 1900 V on the phototube we obtain typically an amplitude of 70 mV for the anode single photoelectron pulse and 50 mV for the dynode pulse, while the rms noise level is < 5 mV. In fig. 2 are shown oscilloscope traces of the resulting pulse shapes for single photoelectrons and for 662 keV -y's, An anode-dynode coincidence is used to generate the Ge veto pulse. To achieve a high count rate capability, each segment of the BGO is independently instru-
mented, with the 80 ns veto pulse being considerably shorter than the retiring time of the individual segments ( - 600 ns depending on pulse heights). The number of photoelectrons required to be within the 80 ns veto gate can be controlled by adjusting the dynode discriminator level. At a total BGO count rate of 200 kHz, count rates in individual segments do not exceed 50 kHz, corresponding, to a worst case segment inefficiency of 3%. At these count rates, the Ge deadtime due to random vetoes is 2%. The deadtime observed in beam will differ from these values because of the multiplicity of coincident photons produced by stopping pions.
3. Measurements with sources
The resolution of the individual crystals, measured with a 137Cs source using a charge ADC with a 60 ns gate, is typically 25-30%. The single photoelectron peak appears at 6-10 keV. These values are higher than the results obtained by Moszynski et al. [5], who obtained 15% resolution and 2 keV/photoelectron using a 2 cm diameter × 3 cm crystal. The relative efficiencies for photon collection in these two crystals were calculated using a Monte Carlo code developed by Anderson and Salomon [6]. For reasonable values of the attenuation coefficient (30-50 cm) and diffuse reflectivities (0.80-0.99) we find a × 3-4 loss of photons in the larger crystal. Thus the difference between ref, [5] and the present measurement can be attributed to photon statistics. After consideration of the 60 ns and 300 ns decay times of BGO, the effective low energy threshold is approximately 50 keV. The energy and timing of photons from a 137Cs source scattered from a scintilla, tor into the BGO has been observed to fall comfortably within these limits. F I00©0
.......................................
]
!
7" 0
I00
I
I
t
o
Fig. 2. Oscilloscope traces of the preamplifier output signals. (a) and (b) are single photon signals from the anode and dynode preamplifiers (scale 100 mV/div × 100 ns/div) and (c) and (d) are 662 keV ),-ray signals (scale 500 mV/div×100 ns/div).
.
l
I
400
80o
_
ENERGY
~2oo
~6oo
(keV)
Fig. 3. Suppression of 6°Co Compton-scattered photons. The lower curve shows the subset of the total events(upper curve) which are not vetoed by the CSS.
A. Olin et al. / Segmented Compton suppression spectrometer
0) I(/00 U3
z
I0(3
0 o 10
I
C
400
800
1200
1600
2000
ENERGY (keV) .
b)
,,Olq
O9 Z
I0 ©
© [D 10 ×10 I
. . . . . . . .
~,
400
~
8C)0
~
I
I
1200
IGO0
2000
ENERGY (keV) Fig. 4. Suppression of backgrounds following ~- stopping in 2°spb. (a) Prompt events - 3 ns < t.r < 3 ns. The structures at 575 keV and 1273 keV are the pionic 5-4 and 4--3 X-rays. (b) Delayed events 10 ns < t.r < 80 ns. The dominant triangular structures result from Ge(n. n')y reactions in the detector.
The detector response may be characterized by the peak-to-total count ratio, Qu, and the corresponding ratio for Compton-suppressed events, Q~. The suppression factor is then given by
465
recorded the energy and time relative to the pion stop of the photon in the Ge detector, the energy and time of all BGO segments that fired, and the presence of a BGO veto signal. All events occurring in a 200 ns window after the pion stop, or in a 450 ns window after a stop in the degrader (with no subsequent stop in the target) were recorded. Since the degrader was carefully shielded from the Ge detector, the latter requirement allowed us to obtain a calibration source spectrum under beam conditions. The effect of the CSS in-beam with the Pb target is shown in fig. 4. The suppression achieved in the prompt spectrum is clearly worse than was achieved out of beam. This is attributed to the large multiplicity of y's and neutrons emitted after ~r capture in heavy nuclei. The observed suppression is consistent with a background predominantly composed of a continuous distribution of y-rays, which could originate directly from v capture in the target or from Compton scattering and n-capture ~,'s in the shielding. A suppression factor closer to the out-of-beam value was observed in the Na target prompt spectrum. Losses of photopeak counts due to the imposition of the CSS veto, as determined by measuring the area of the prominent 575 keV ~r 5 - 4 2°spb X-ray, were approximately 10%. The suppression factor obtained in the delayed spectrum, fig. 4b, is significantly greater than the out-of-beam value. The dominant y ' s in this spectrum come from (n. n') interactions in the Ge detector, whose photopeak intensities are reduced to half by the CSS. Veto signals in the BGO produced by the scattered neutron represent the most likely mechanism to produce this enhanced suppression. The suppression observed in the in-beam source spectrum was the same as the out-of-beam value.
s=(1/Qu- l)/(1/Qs-1). Fig. 3 shows the effect of the CSS, for which S = 4.0 was obtained, corresponding to an improvement from Qo = 0.17 to Q~ = 0.46. Note that the reduction in the background is considerably better than × 4 over most of the spectrum, and that the Compton edges from photons that scatter back out through the photon entrance hole contribute significantly. Approximately 5% of the photopeak counts were vetoed.
4. M e a s u r e m e n t s in a pion b e a m e n v i r o n m e n t
The in-beam behaviour of the CSS was studied during a recent experiment to measure the very broad pionic 4 - 3 X-rays in 2°8pb and 2°9Bi, and the 2-1 X-ray in 23Na. The beam of 2 × 105 ~ ' - / s was stopped in 50 g targets of 2°8pb, Bi and N a i l . Plastic scintillators determined the time of the pion stop. The Ge detector was positioned 40 cm from the target, and the total count rate in the BGO was 200 kHz. For each event we
5. D i s c u s s i o n
The performance of this CSS falls somewhat below that of the best NaI(T1) devices as determined by sources [2]. It is also below the performance predicted by Monte Carlo calculations for this geometry, where a suppression factor of - 1 1 for 1 MeV photons was obtained [7]. These calculations do not take into account the inactive material between the Ge detector and the BGO, nor dead layers in the detector core. Thus differences in the Ge detector used might account for the poorer performance. A measure of the overall in-beam performance, the peak-to-background amplitude ratio achieved on the 2°9Bi ~ 5 - 4 X-ray, was 50 : 1, compared with the value of 10 : 1 obtained by BeeJ~z et al. [2]. In making thistcomparison we note that the measurements were performed under rather different conditions, and that beam quality, count rates and timing cuts all affect this ratio. The rate capabilities of the present design make it suitable for use in an intense beam environ-
466
A. Olin et al. / Segmented Compton suppression spectrometer
ment. The reduced backgrounds have made possible the observation of very weak, broadened pionic X-ray transitions. The preamplifiers were designed and constructed in the T R I U M F electronics shop by J. Cresswell and R. Hansen. We wish to thank J. Forsman and P. van Esbroek for their considerable assistance with the final performance testing of the CSS. We also thank our colleagues who participated in the pionic X-ray measurement: G. Beer, R. Kunselman, J. Macdonald, G. Marshall, G. Mason and T. Numao. This work was supported under a grant from the Natural Sciences and Engineering Research Council of Canada.
References [1] P.F. Hinrichsen, D.M. van Patter and H.M. Shapiro, Nucl. Phys. A123 (1969) 250. [2] R. Beetz, W.L. Posthumus, F.W.N. de Boer; J.L. Maarleveld, A. van der Schaaf and J. Konijn, Nucl. Instr. and Meth. 145 (1977) 353. [3] O.H. Nestor and C.Y. Huang, IEEE Trans. Nucl. Sci. NS-22 (1975) 68. [4] M.E. Lone, O. H~iusser, T.K. Alexander, J. Gascon and E. Hagberg, Proc. Int. Workshop BGO~ ed,, C.N. Holmes, Princeton University (1982) p. 237. [5] M. Moszynski, C. Gresset, J. Vacher and R. Ordu, Nucl. Instr. and Meth. 188 (1981) 403. [6] S. Anderson and M. Salomon, Proc. Int. Workshop BGO, p. 158 and TRIUMF preprint TRI-PP-82-44. [7] E. Hagberg, private communication.