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A method to suppress Cherenkov background in the measurement of decay time
EJSEViER
Nuciearilnstruments and Methods in Physics Research A 405 f 1998) 176- 178
Letter
NUCLEAR ~NS~UM~S a ME~ODS IN PHYSICS RESEARCH SecttonA
to the Editor
A method to suppress Cherenkov background the measurement of decay time
in
Chong Wu, Zhaomin Wang, Zizong Xu* Department of Modern Phwics, Unicersir?,qf Science and Technology of China,Hefei Anhui 230027, People ‘s Republic qf China Received 3 March 1997: received in revised form I8 July 1997
In a traditional start-stop mode of single-photon time resolution spectrometer with cascade y-rays as excitation (Fig. l), y1 and yZ are emitted simultaneously (Ar < 1 ps for “Co, At = 0 for “Na). A fast scintillator (plastic) coupled with a fast photomultiplier (PMI) accepts y1(y2) to generate the start signal of TAG. ;f2(y1)fires the tested crystal and a ffuorescence pulse of the crystal is produced. A single-photoelectron-sensitive photomultiplier PM2 looks the sample with such a small acceptance that on an average less than one photon per fluorescence pulse can reach the photocathode of PM2. The signal from the PM2 is produced by a single photoelectron as “stop” signal of TAC. The start signal can be properly taken as the zero time (time origin). The single sampling of the fluorescence pulse of the tested crystal as the stop signal gives the time distribution of the fluorescence pulse. The distribution of the time interval between start and stop signals describes the time distribution of a fluorescence pulse when event samples are large enough.
* Corres~nding author. TeI.: + 86 5.51 3601178: fax: -I- 86 551 3631760; e-mail: xu~lx~.mp~y.ust~.edu.~n.
However, the decay time measurement is significantly distur~d by the Cherenkov background produced by the correlation y-ray which hits PM2 glass window. In the case of Fig. la, when PM 1 is fired by one of the two y-rays, the i&W angle correlated y passes through the sample. The sample is usually not so thick that a part of the original y-rays and some Compton scattered y-rays are able to pass it and be converted into photoelectron or Compton recoil electron in the glass window of PM2. In the case of Fig. 1b, one of cascade y-rays fires the PMl, the other y has a possibility to hit the sample and generates scattered Compton y-rays from the sample into the glass window causing secondary electrons in the window. Moreover, there is a possibility that when one of the cascade y-rays is received by PMl, the other does not hit the sample, but is directly received by the glass window of PM2 and a secondary electron in the window is produced. If secondary electrons produced by the correlation y-ray have kinetic energies over 200 keV (n = 1.458, I$ > l), the electron will emit Cherenkov photon in the entrance window of PM2 The Cherenkov photon may be converted into stop signal accepted by TAC. ff there is a correlated start signal, the decay time signal will display an acute peak. The effect makes the measurement of fast component very uncertain.
0168-9002/98/$19.00 $1 1998 Elsevier Science B.V. All rights reserved. PII SOl68-9002(97)01!?1-6
plastic scintillator PM1 sample
PM2
sample
(a)
PM:! (b)
Fig. I. Start-stop mode with cascade y-ray excitation: (a) the axis of PM 1 and PM1 on the line: (b) the axis of PM I perpendicular to PM3.
Fig. 2. The decay time for BGO without gate.
We add a coincidence gatei based on the traditional delayed coincidence method [ 1.23 in order to suppress this Cherenkov background. The scintillater to be measured is coupled with a PMT, the output of which drives a gate. The start signals are
’ The detailed description of the experimental setup and the results of analysis will be submitted to Nuci. Instr. and Meth., shortly.
valid on condition that the gate signais exist simultaneously. The Cherenkov background produced by y-rays hitting directly the glass window of the PMT can be suppressed completely. Fig. 2 is the measured decay time spectra of BGO without the gate. It appears as an acute peak that superimposes in front of the decay time spectra. Fig. 3 is the measured decay time spectra of BGO with the gate. It shows that the acute peak has disappeared. The result is significantly different
178
C. Wu d al. /Nucl. Instr. and Meth. in Phq’s. Res. A 405 (I 998) 176--I 78
300
400
500
600
700
Fig. 3. The decay time for BGO with a gate.
from Fig. 10 in the RESCEU No.37/96 [3]. The BGO crystal has a ten nanosecond component and a 250 ns component by fitting Fig. 3. But there is a 1 ns fast component by fitting Fig. 2. The fast component is not the decay time of the BGO crystal. It is contributed by Cherenkov photon. So the Cherenkov background can be suppressed indeed by a coincidence gate. The random coincidence background is also suppressed greatly.
References
[I]
L.M. Bol1inger.G.E. Thomas, Rev. Sci. Ins&. 32(1961) 1044. [2] Xu zizong et al., Nucl. Instr. and Meth. A 322 (1992) 239. 133 N. Tsuchida et al.. RESCEU No.37/96. Nucl. Instr. and Meth. A, to be published.
Nuciearilnstruments and Methods in Physics Research A 405 f 1998) 176- 178
Letter
NUCLEAR ~NS~UM~S a ME~ODS IN PHYSICS RESEARCH SecttonA
to the Editor
A method to suppress Cherenkov background the measurement of decay time
in
Chong Wu, Zhaomin Wang, Zizong Xu* Department of Modern Phwics, Unicersir?,qf Science and Technology of China,Hefei Anhui 230027, People ‘s Republic qf China Received 3 March 1997: received in revised form I8 July 1997
In a traditional start-stop mode of single-photon time resolution spectrometer with cascade y-rays as excitation (Fig. l), y1 and yZ are emitted simultaneously (Ar < 1 ps for “Co, At = 0 for “Na). A fast scintillator (plastic) coupled with a fast photomultiplier (PMI) accepts y1(y2) to generate the start signal of TAG. ;f2(y1)fires the tested crystal and a ffuorescence pulse of the crystal is produced. A single-photoelectron-sensitive photomultiplier PM2 looks the sample with such a small acceptance that on an average less than one photon per fluorescence pulse can reach the photocathode of PM2. The signal from the PM2 is produced by a single photoelectron as “stop” signal of TAC. The start signal can be properly taken as the zero time (time origin). The single sampling of the fluorescence pulse of the tested crystal as the stop signal gives the time distribution of the fluorescence pulse. The distribution of the time interval between start and stop signals describes the time distribution of a fluorescence pulse when event samples are large enough.
* Corres~nding author. TeI.: + 86 5.51 3601178: fax: -I- 86 551 3631760; e-mail: xu~lx~.mp~y.ust~.edu.~n.
However, the decay time measurement is significantly distur~d by the Cherenkov background produced by the correlation y-ray which hits PM2 glass window. In the case of Fig. la, when PM 1 is fired by one of the two y-rays, the i&W angle correlated y passes through the sample. The sample is usually not so thick that a part of the original y-rays and some Compton scattered y-rays are able to pass it and be converted into photoelectron or Compton recoil electron in the glass window of PM2. In the case of Fig. 1b, one of cascade y-rays fires the PMl, the other y has a possibility to hit the sample and generates scattered Compton y-rays from the sample into the glass window causing secondary electrons in the window. Moreover, there is a possibility that when one of the cascade y-rays is received by PMl, the other does not hit the sample, but is directly received by the glass window of PM2 and a secondary electron in the window is produced. If secondary electrons produced by the correlation y-ray have kinetic energies over 200 keV (n = 1.458, I$ > l), the electron will emit Cherenkov photon in the entrance window of PM2 The Cherenkov photon may be converted into stop signal accepted by TAC. ff there is a correlated start signal, the decay time signal will display an acute peak. The effect makes the measurement of fast component very uncertain.
0168-9002/98/$19.00 $1 1998 Elsevier Science B.V. All rights reserved. PII SOl68-9002(97)01!?1-6
plastic scintillator PM1 sample
PM2
sample
(a)
PM:! (b)
Fig. I. Start-stop mode with cascade y-ray excitation: (a) the axis of PM 1 and PM1 on the line: (b) the axis of PM I perpendicular to PM3.
Fig. 2. The decay time for BGO without gate.
We add a coincidence gatei based on the traditional delayed coincidence method [ 1.23 in order to suppress this Cherenkov background. The scintillater to be measured is coupled with a PMT, the output of which drives a gate. The start signals are
’ The detailed description of the experimental setup and the results of analysis will be submitted to Nuci. Instr. and Meth., shortly.
valid on condition that the gate signais exist simultaneously. The Cherenkov background produced by y-rays hitting directly the glass window of the PMT can be suppressed completely. Fig. 2 is the measured decay time spectra of BGO without the gate. It appears as an acute peak that superimposes in front of the decay time spectra. Fig. 3 is the measured decay time spectra of BGO with the gate. It shows that the acute peak has disappeared. The result is significantly different
178
C. Wu d al. /Nucl. Instr. and Meth. in Phq’s. Res. A 405 (I 998) 176--I 78
300
400
500
600
700
Fig. 3. The decay time for BGO with a gate.
from Fig. 10 in the RESCEU No.37/96 [3]. The BGO crystal has a ten nanosecond component and a 250 ns component by fitting Fig. 3. But there is a 1 ns fast component by fitting Fig. 2. The fast component is not the decay time of the BGO crystal. It is contributed by Cherenkov photon. So the Cherenkov background can be suppressed indeed by a coincidence gate. The random coincidence background is also suppressed greatly.
References
[I]
L.M. Bol1inger.G.E. Thomas, Rev. Sci. Ins&. 32(1961) 1044. [2] Xu zizong et al., Nucl. Instr. and Meth. A 322 (1992) 239. 133 N. Tsuchida et al.. RESCEU No.37/96. Nucl. Instr. and Meth. A, to be published.