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The time resolution of bismuth germanate scintillation detectors
Nuclear Instruments and Methods 197 (1982) 591-592 North-Holland Publishing Company
591
L e t t e r to the Editor THE TIME RESOLUTION
OF BISMUTH
GERMANATE
SCINTILLATION
DETECTORS
S.A. W E N D E R , G . F . A U C H A M P A U G H University of California, Los Alamos National Laboratory, Los Alamos, NM, 87545, U.S.A. and N.W. HILL Oak Ridge National Laboratory, Oak Ridge Tennessee, 37830, U.S.A. Received 28 December 1981
The time resolution of a bismuth germanate scintillation detector was measured as a function of y-ray energy from 1 MeV to 24 MeV. The measured time resolution of the 6°Co cascade is (2.1 -+0.2) ns. The time resolution improves at higher y-ray energies and is (890-+60) ps at around 20 MeV.
The relatively large efficiency for detection of y-rays makes bismuth germanate (BGO) an attractive alternative to NaI detectors in many nuclear physics applications. In a recent publication Drake et al. [1] compare the efficiency, energy resolution, and sensitivity to neutron backgrounds of a 7.62 cm diameter by 7.62 cm long cylindrical B G O crystal to a 15.3 cm diameter by 25.4 cm long NaI crystal in the y-ray energy range from 4.4 to 24 MeV. For y-rays around 22 MeV that study concludes the following: (1) the two detectors have similar efficiencies, (2) the BGO detectors have approximately 40% poorer energy resolution, and (3) the sensitivity of the BGO detector to neutrons is approximately 2% that of NaI. In many experiments it is necessary to accurately determine the time at which an event occurred. For example, in our particular application we detect ),-rays following neutron reactions induced by a white neutron source. Since the energy of the neutron is determined by time-of-flight, the neutron energy resolution is correlated to the time resolution of the detector system. In this report we describe the time characteristics of a B G O detector for y-rays in the energy range from 1 to 24 MeV. The crystal used in these tests was 5.0 cm in diameter and 7.62 cm long. One end was tapered to a diameter of 3.8 cm and the other end was tapered to a diameter of 4.5 cm. The 4.5 cm end was coupled to the face of an R C A 8850 photomultiplier tube (PMT) using optical coupling grease. The P M T was operated at - 2 5 0 0 V . The anode output pulse had an amplitude of 200 mV with a rise time of 5 ns for y-rays near 1 MeV. The timing pulse was obtained from this anode signal using 0167-5087/82/0000-0000/$02.75 © 1982 North-Holland
an Ortec 934 constant fraction timing discriminator (CFTD). An upper and lower energy level was established using two other CFTDs. The overlap coincidence of the timing signal with the lower level discriminator and the inverted upper level discriminator was used to trigger a time-to-amplitude converter (TAC). The time resolution of the BGO detector around 1 MeV was measured using the 6°Co y-ray cascade in coincidence with a fast plastic (NE 110) scintillator. Both detectors were not collimated. Since the two 60 Co y-rays were not resolved in the B G O crystal, we placed an energy window around both transitions. The T A C spectrum shown in fig. 1 was obtained with the P M T voltage and electronics optimized for this configuration. The time calibration in fig. 1 was 0.33 ns/channel. The time resolution for higher energy y-rays was measured by detecting the 3,-rays produced when the 800 MeV proton beam from the L A M P F accelerator bombarded the tungsten target and the W N R white neutron source facility. The time structure of the beam pulse consisted of an approximately 4 t~s wide macropulse at a repetition rate of 80 Hz. Each macro-pulse consisted of micro-pulses 200 ps wide separated by 5 ns. The y-rays from the production target illuminated the entire crystal. T A C spectra of the micro-structure of the beam were acquired corresponding to several y-ray energy windows. Fig. 2 shows typical T A C spectra of the micro-pulse time structure for various energy windows. Since the coincidence peak from the 6°Co cascade is asymmetric, its full width at half maximum (fwhm) is obtained graphically. The average fwhm of several runs is (2.1 -+ 0.2) ns. At higher y-ray energies the time peaks become more symmetric and their fwhm were obtained
592
S.A. Wender et a L / Time resolution of BGO scintillation detectors Ex(MeV)
I00 ioc
02
05 i
60Co CASCADE
I
I
Io
20
i J i ii
I
5.0
I
I0.0
80
~
50
60
I= v
x
Iz
FWHM =21 n
,?,4c
L~Oz o
z 0 t--
(/}
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=,
=,
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2C
o
s~
j
o
I.O Z tlJ
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02
I i.~ 20 5o CHANNEL NUMBER
IO
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Fig. 1. Plot of the coincidence TAC spectrum for the 6°Co y-ray cascade.
O I
t 2
I
I
I 5
i I lit I0
I 20
I
I
I r II 50
I IO0
ETr(MeV)
Fig. 3. Plot of the time resolution as a function of y-ray energy. The left and lower axes refer to time resolution. The straight line is the energy resolution as a function of y-ray energy and refers to the upper and right axes.
IOO
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We would like to t h a n k Dr. R. Klebesadel for the loan of the B G O crystal used in these tests.
% % •
o
by fitting six sequential micro-pulses to G a u s s i a n line shapes. The results of the fits along with the 6°Co result are plotted in fig. 3. Also plotted in fig. 3 is the energy resolution of a typical B G O detector [2]. As seen from the figure, the energy dependence of the time resolution a n d energy resolution are similar. Since the time peak for the 3 - 6 MeV point is not symmetric, a n d its fwhm is o b t a i n e d by fitting to a G a u s s i a n line shape, the plotted value is p r o b a b l y an over-estimate of the time resolution. The fact that the time and energy resolution improves at the same rate with increased y-ray energy suggests that b o t h are limited by the light o u t p u t of the B G O crystal. One of the p r o b l e m s associated with B G O is that the light o u t p u t is typically only 10% of N a I [2]. We are currently investigating methods to improve the light coupling to the P M T with the hope that this will improve the energy and time resolution.
J
20
emeqNi eoe
I
~ e o
~•~
J
40 60 CHANNEL NUMBER
ee
eo~eoe
i
80
Fig. 2. Plot of the micro-pulse structure of the beam for the following energy windows: a) 1.8-2.4 MeV, b) 3.0-6.0 MeV, and c) 15.0-24 MeV.
References
[1] D.M. Drake, L.R. Nilsson and J. Faucett, Nucl. Instr. and Meth. 188 (1981) 313. [2] Properties of BGO detectors, Harshaw Chemical Co., Solon Ohio.
591
L e t t e r to the Editor THE TIME RESOLUTION
OF BISMUTH
GERMANATE
SCINTILLATION
DETECTORS
S.A. W E N D E R , G . F . A U C H A M P A U G H University of California, Los Alamos National Laboratory, Los Alamos, NM, 87545, U.S.A. and N.W. HILL Oak Ridge National Laboratory, Oak Ridge Tennessee, 37830, U.S.A. Received 28 December 1981
The time resolution of a bismuth germanate scintillation detector was measured as a function of y-ray energy from 1 MeV to 24 MeV. The measured time resolution of the 6°Co cascade is (2.1 -+0.2) ns. The time resolution improves at higher y-ray energies and is (890-+60) ps at around 20 MeV.
The relatively large efficiency for detection of y-rays makes bismuth germanate (BGO) an attractive alternative to NaI detectors in many nuclear physics applications. In a recent publication Drake et al. [1] compare the efficiency, energy resolution, and sensitivity to neutron backgrounds of a 7.62 cm diameter by 7.62 cm long cylindrical B G O crystal to a 15.3 cm diameter by 25.4 cm long NaI crystal in the y-ray energy range from 4.4 to 24 MeV. For y-rays around 22 MeV that study concludes the following: (1) the two detectors have similar efficiencies, (2) the BGO detectors have approximately 40% poorer energy resolution, and (3) the sensitivity of the BGO detector to neutrons is approximately 2% that of NaI. In many experiments it is necessary to accurately determine the time at which an event occurred. For example, in our particular application we detect ),-rays following neutron reactions induced by a white neutron source. Since the energy of the neutron is determined by time-of-flight, the neutron energy resolution is correlated to the time resolution of the detector system. In this report we describe the time characteristics of a B G O detector for y-rays in the energy range from 1 to 24 MeV. The crystal used in these tests was 5.0 cm in diameter and 7.62 cm long. One end was tapered to a diameter of 3.8 cm and the other end was tapered to a diameter of 4.5 cm. The 4.5 cm end was coupled to the face of an R C A 8850 photomultiplier tube (PMT) using optical coupling grease. The P M T was operated at - 2 5 0 0 V . The anode output pulse had an amplitude of 200 mV with a rise time of 5 ns for y-rays near 1 MeV. The timing pulse was obtained from this anode signal using 0167-5087/82/0000-0000/$02.75 © 1982 North-Holland
an Ortec 934 constant fraction timing discriminator (CFTD). An upper and lower energy level was established using two other CFTDs. The overlap coincidence of the timing signal with the lower level discriminator and the inverted upper level discriminator was used to trigger a time-to-amplitude converter (TAC). The time resolution of the BGO detector around 1 MeV was measured using the 6°Co y-ray cascade in coincidence with a fast plastic (NE 110) scintillator. Both detectors were not collimated. Since the two 60 Co y-rays were not resolved in the B G O crystal, we placed an energy window around both transitions. The T A C spectrum shown in fig. 1 was obtained with the P M T voltage and electronics optimized for this configuration. The time calibration in fig. 1 was 0.33 ns/channel. The time resolution for higher energy y-rays was measured by detecting the 3,-rays produced when the 800 MeV proton beam from the L A M P F accelerator bombarded the tungsten target and the W N R white neutron source facility. The time structure of the beam pulse consisted of an approximately 4 t~s wide macropulse at a repetition rate of 80 Hz. Each macro-pulse consisted of micro-pulses 200 ps wide separated by 5 ns. The y-rays from the production target illuminated the entire crystal. T A C spectra of the micro-structure of the beam were acquired corresponding to several y-ray energy windows. Fig. 2 shows typical T A C spectra of the micro-pulse time structure for various energy windows. Since the coincidence peak from the 6°Co cascade is asymmetric, its full width at half maximum (fwhm) is obtained graphically. The average fwhm of several runs is (2.1 -+ 0.2) ns. At higher y-ray energies the time peaks become more symmetric and their fwhm were obtained
592
S.A. Wender et a L / Time resolution of BGO scintillation detectors Ex(MeV)
I00 ioc
02
05 i
60Co CASCADE
I
I
Io
20
i J i ii
I
5.0
I
I0.0
80
~
50
60
I= v
x
Iz
FWHM =21 n
,?,4c
L~Oz o
z 0 t--
(/}
/...
=,
=,
,o o
(g,
taJ tY >-
2C
o
s~
j
o
I.O Z tlJ
I,-
02
I i.~ 20 5o CHANNEL NUMBER
IO
2
4o
Fig. 1. Plot of the coincidence TAC spectrum for the 6°Co y-ray cascade.
O I
t 2
I
I
I 5
i I lit I0
I 20
I
I
I r II 50
I IO0
ETr(MeV)
Fig. 3. Plot of the time resolution as a function of y-ray energy. The left and lower axes refer to time resolution. The straight line is the energy resolution as a function of y-ray energy and refers to the upper and right axes.
IOO
I
o) •
I
•e
•
ee
.o•
eee
-.
o 15o~
I •
ee
50
I
•
:
".. ee
-
..,,.-_.. %
b)
"
V
% •
-,
C)
"~..~"
J
" ...
]
..
%.
We would like to t h a n k Dr. R. Klebesadel for the loan of the B G O crystal used in these tests.
% % •
o
by fitting six sequential micro-pulses to G a u s s i a n line shapes. The results of the fits along with the 6°Co result are plotted in fig. 3. Also plotted in fig. 3 is the energy resolution of a typical B G O detector [2]. As seen from the figure, the energy dependence of the time resolution a n d energy resolution are similar. Since the time peak for the 3 - 6 MeV point is not symmetric, a n d its fwhm is o b t a i n e d by fitting to a G a u s s i a n line shape, the plotted value is p r o b a b l y an over-estimate of the time resolution. The fact that the time and energy resolution improves at the same rate with increased y-ray energy suggests that b o t h are limited by the light o u t p u t of the B G O crystal. One of the p r o b l e m s associated with B G O is that the light o u t p u t is typically only 10% of N a I [2]. We are currently investigating methods to improve the light coupling to the P M T with the hope that this will improve the energy and time resolution.
J
20
emeqNi eoe
I
~ e o
~•~
J
40 60 CHANNEL NUMBER
ee
eo~eoe
i
80
Fig. 2. Plot of the micro-pulse structure of the beam for the following energy windows: a) 1.8-2.4 MeV, b) 3.0-6.0 MeV, and c) 15.0-24 MeV.
References
[1] D.M. Drake, L.R. Nilsson and J. Faucett, Nucl. Instr. and Meth. 188 (1981) 313. [2] Properties of BGO detectors, Harshaw Chemical Co., Solon Ohio.