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An effect of scattered neutrons from photomultipliers on the neutron spectrum measured by 6Li glass scintillators
NUCLEAR
INSTRUMENTS
AND
65 (1968) 113-I14; ©
METHODS
NORTH-HOLLAND
PUBLISHING
CO.
A N EFFECT OF S C A T T E R E D N E U T R O N S F R O M P H O T O M U L T I P L I E R S O N T H E N E U T R O N S P E C T R U M M E A S U R E D BY 6Li GLASS S C I N T I L L A T O R S M. OHKUBO Linac Laboratory, Division of Physics, Japan Atomic Energy Research Institute, Tokai-mura, Ibaragi-ken, Japan Received 6 August 1968 The neutron spectrum measured by aLi glass scintillators has been found to have an enhancement peak at E~ = 2.87 keV, due to resonance scattering from 23Na contained in the glass of photomultipliers. Precise knowledge of the detection efficiency of neutron detectors is required, particularly in the keV neutron energy region, in order to fill the need for accurate neutron cross sections and neutron spectra. In the keV neutron energy region, 6Li glass scintillators are often used as neutron detectors. The neutron detection efficiency of 6Li glass scintillators can be easily calculated by using the neutron cross sections of the components of the glass, such as 6Li, O, Si, A1, Mg, Cel). For more precise calculations, multiple scattering of the neutrons in the glass scintillators must be taken into account. In this letter, we describe an effect of the 2.87 keV resonance of 2ZNa in the glass of photomultipliers upon the efficiency of 6Li glass scintillators which are used as a time-of-flight detector. The time spectra of the open neutron beam measured by these detectors have an enhancement peak at E, = 2.87 keV, caused by the resonant scattered neutrons from the sodium contained in the glass of the photomultiplier. Instrumentation of the time-of-flight spectrometer at the J A E R I Linac was described elsewhere2). The timeof-flight detector consists of four sets of 6Li glass scintillators (4~" dia. x ½" NE-905 or N E - 9 0 8 ) a n d a o
o f,,,,..,
o
photomultiplier of EMI9579B. Fig. 1 shows the schematic figure of the 2 sets of the detector. They are placed in the beam hole of the shielding, where the incident neutrons enter perpendicularly into the glass scintillators. The glass scintillators are coated with aluminum on one face, and are polished on the other face. Each polished surface of the glass scintillators is faced to the window glass of the photomultipliers with air gaps of ~ 2 mm. The photomultipliers, the voltage dividers for the dynodes and the emitter follower circuits for the output signal are contained in an aluminum box. Neutron pulses from each emitter follower circuits are mixed and amplified, the pulse height distribution being measured to have an fwhm of ~ 40%. Fig. 2 shows the time spectrum of the open neutron beam measured by these detectors. At E n = 5.91 keY, there was observed an absorption dip caused by the aluminum windows of the evacuated flight tubes (total window thickness ~ 11 mm). At about E, = 2.85 keV, a broad peak is observed which has inverse trend against the ordinary sense. Since a large resonance in 23Na is well known to exist at E, = 2.85 keV, the ob-
o
6000I
[-.-
. .............. ""'"i o d
o',
i
z
~
o
" ,-4%o %
"%~,
.t~o
.
..
:" ~""',"' .'~:.~,.,~"
,ooo
" "" . ' ,
o u
,:,;
z
io
t
2000
.41
o
o o
""'.- ......... -"" o
t
No
E,= 5.91 KeV
o
EN=2.87 KeV
o
,50
Fig. 1. Configuration of time-of-flight detectors. The 6Li glass scintillators are faced on the window glasses of the photomultipliers of EMI 9579B with air gaps of ~ 2 ram.
2bo
CHANNEL
z~o
3bo,
Fig. 2. A time spectrum of the open neutron beam measured by these detectors. The channel width of the time analyzer was 0.25 #see, and the beam pulse width of the linac was 0.4 /~sec.
113
114
M. O H K U B O
z 1.0, O 'o "®++
O~
•
•
•
# • ®
% * = ==
o...'•o-o,%~
N z ~°5
0 40
60
80
CHANNEL
Fig. 3. The neutron transmission of a window glass of the photomultiplier of EMI 9579B. A resonance dip due to 23Na is very clearly observed at Ea = 2.87 keV. Both of the channel width of the time analyzer and the beam pulse width were 1 #sec.
served peak is attributed to the resonance scattering of the neutrons from the sodium contained in the glass scintillators or other surrounding materials. According to a catalogue, pyrex glass is used as window glass for the EM[ 9579B. In order to determine the content of sodium, we measured the neutron transmission of a glass scintillator (NE-908 1,, thick) and a window glass of E M [ 9579B (6.5 m m thick) which was cut from a broken photomultiplier tube. The measurements were made by using the J A E R [ Linac time-of-flight spectrometer with a 50-m flight path. A large resonance dip was observed at En = 2.87 keV for the transmission of the window glass of the photomultiplier as shown in fig. 3. On the other hand, no dip was observed for the transmission of the glass scintillator within experimental errors. From the dip area in the transmission curve, the content of the sodium iv the glass was cal-
culated to be (1.66 ___0.11) x 102t N a atoms/cm 3 using an area analysis code2), where resonance parameters were assumed to be F = 424 __ 13 eV and a0 = 350 __ 25 b3). The resonance energy obtained was En = 2.87 __ 0.02 keV. F r o m the content of sodium thus obtained, the peak area in the open neutron beam was estimated and it showed good agreement to the observed one. As a result, it is concluded that the broad peak at E n = 2.87 keV in the open neutron beam is due to back scattered neutrons from 23Na contained in the glass of the photomultiplier tubes. This effect is interpreted as the apparent variation of the detection efficiency of the neutron detectors brought forth by multiple scattering. The scattered neutrons from other materials around the glass scintillators may cause similar effects. But no significant effect except that from sodium in the photomultiplier is observed by these detectors. To obtain the absolute value of the efficiency of the detector by the Monte-Carlo method+), it is necessary to consider the effects of the materials and geometries surrounding the detectors. This letter is presented as an example of the apparent variation of the detection efficiency by the resonance scattering of the neutrons from materials surrounding the neutron detectors. References
l) L. A. Wright, D. H. C. Harris and P. A. Egelstaff,Nucl. Instr. and Meth. 33 (1965) 181. z) A. Asami, T. Fuketa, Y. Kawarasaki, Y. Nakajima, M. Ohkubo, T. Sakuta, T. Takahashi and H. Takekoshi, JAERI1138 (1967). z) M. C. Moxon and N. J. Pattenden, Intern. Conf. Nuclear data for reactors, Paris (Oct. 1966) CN-32/27. 4) H. O. Zetterstr/Sm, S. Schwarz and L. G. Str6mberg, Nucl. Instr. and Meth. 42 (lq66) 277.
INSTRUMENTS
AND
65 (1968) 113-I14; ©
METHODS
NORTH-HOLLAND
PUBLISHING
CO.
A N EFFECT OF S C A T T E R E D N E U T R O N S F R O M P H O T O M U L T I P L I E R S O N T H E N E U T R O N S P E C T R U M M E A S U R E D BY 6Li GLASS S C I N T I L L A T O R S M. OHKUBO Linac Laboratory, Division of Physics, Japan Atomic Energy Research Institute, Tokai-mura, Ibaragi-ken, Japan Received 6 August 1968 The neutron spectrum measured by aLi glass scintillators has been found to have an enhancement peak at E~ = 2.87 keV, due to resonance scattering from 23Na contained in the glass of photomultipliers. Precise knowledge of the detection efficiency of neutron detectors is required, particularly in the keV neutron energy region, in order to fill the need for accurate neutron cross sections and neutron spectra. In the keV neutron energy region, 6Li glass scintillators are often used as neutron detectors. The neutron detection efficiency of 6Li glass scintillators can be easily calculated by using the neutron cross sections of the components of the glass, such as 6Li, O, Si, A1, Mg, Cel). For more precise calculations, multiple scattering of the neutrons in the glass scintillators must be taken into account. In this letter, we describe an effect of the 2.87 keV resonance of 2ZNa in the glass of photomultipliers upon the efficiency of 6Li glass scintillators which are used as a time-of-flight detector. The time spectra of the open neutron beam measured by these detectors have an enhancement peak at E, = 2.87 keV, caused by the resonant scattered neutrons from the sodium contained in the glass of the photomultiplier. Instrumentation of the time-of-flight spectrometer at the J A E R I Linac was described elsewhere2). The timeof-flight detector consists of four sets of 6Li glass scintillators (4~" dia. x ½" NE-905 or N E - 9 0 8 ) a n d a o
o f,,,,..,
o
photomultiplier of EMI9579B. Fig. 1 shows the schematic figure of the 2 sets of the detector. They are placed in the beam hole of the shielding, where the incident neutrons enter perpendicularly into the glass scintillators. The glass scintillators are coated with aluminum on one face, and are polished on the other face. Each polished surface of the glass scintillators is faced to the window glass of the photomultipliers with air gaps of ~ 2 mm. The photomultipliers, the voltage dividers for the dynodes and the emitter follower circuits for the output signal are contained in an aluminum box. Neutron pulses from each emitter follower circuits are mixed and amplified, the pulse height distribution being measured to have an fwhm of ~ 40%. Fig. 2 shows the time spectrum of the open neutron beam measured by these detectors. At E n = 5.91 keY, there was observed an absorption dip caused by the aluminum windows of the evacuated flight tubes (total window thickness ~ 11 mm). At about E, = 2.85 keV, a broad peak is observed which has inverse trend against the ordinary sense. Since a large resonance in 23Na is well known to exist at E, = 2.85 keV, the ob-
o
6000I
[-.-
. .............. ""'"i o d
o',
i
z
~
o
" ,-4%o %
"%~,
.t~o
.
..
:" ~""',"' .'~:.~,.,~"
,ooo
" "" . ' ,
o u
,:,;
z
io
t
2000
.41
o
o o
""'.- ......... -"" o
t
No
E,= 5.91 KeV
o
EN=2.87 KeV
o
,50
Fig. 1. Configuration of time-of-flight detectors. The 6Li glass scintillators are faced on the window glasses of the photomultipliers of EMI 9579B with air gaps of ~ 2 ram.
2bo
CHANNEL
z~o
3bo,
Fig. 2. A time spectrum of the open neutron beam measured by these detectors. The channel width of the time analyzer was 0.25 #see, and the beam pulse width of the linac was 0.4 /~sec.
113
114
M. O H K U B O
z 1.0, O 'o "®++
O~
•
•
•
# • ®
% * = ==
o...'•o-o,%~
N z ~°5
0 40
60
80
CHANNEL
Fig. 3. The neutron transmission of a window glass of the photomultiplier of EMI 9579B. A resonance dip due to 23Na is very clearly observed at Ea = 2.87 keV. Both of the channel width of the time analyzer and the beam pulse width were 1 #sec.
served peak is attributed to the resonance scattering of the neutrons from the sodium contained in the glass scintillators or other surrounding materials. According to a catalogue, pyrex glass is used as window glass for the EM[ 9579B. In order to determine the content of sodium, we measured the neutron transmission of a glass scintillator (NE-908 1,, thick) and a window glass of E M [ 9579B (6.5 m m thick) which was cut from a broken photomultiplier tube. The measurements were made by using the J A E R [ Linac time-of-flight spectrometer with a 50-m flight path. A large resonance dip was observed at En = 2.87 keV for the transmission of the window glass of the photomultiplier as shown in fig. 3. On the other hand, no dip was observed for the transmission of the glass scintillator within experimental errors. From the dip area in the transmission curve, the content of the sodium iv the glass was cal-
culated to be (1.66 ___0.11) x 102t N a atoms/cm 3 using an area analysis code2), where resonance parameters were assumed to be F = 424 __ 13 eV and a0 = 350 __ 25 b3). The resonance energy obtained was En = 2.87 __ 0.02 keV. F r o m the content of sodium thus obtained, the peak area in the open neutron beam was estimated and it showed good agreement to the observed one. As a result, it is concluded that the broad peak at E n = 2.87 keV in the open neutron beam is due to back scattered neutrons from 23Na contained in the glass of the photomultiplier tubes. This effect is interpreted as the apparent variation of the detection efficiency of the neutron detectors brought forth by multiple scattering. The scattered neutrons from other materials around the glass scintillators may cause similar effects. But no significant effect except that from sodium in the photomultiplier is observed by these detectors. To obtain the absolute value of the efficiency of the detector by the Monte-Carlo method+), it is necessary to consider the effects of the materials and geometries surrounding the detectors. This letter is presented as an example of the apparent variation of the detection efficiency by the resonance scattering of the neutrons from materials surrounding the neutron detectors. References
l) L. A. Wright, D. H. C. Harris and P. A. Egelstaff,Nucl. Instr. and Meth. 33 (1965) 181. z) A. Asami, T. Fuketa, Y. Kawarasaki, Y. Nakajima, M. Ohkubo, T. Sakuta, T. Takahashi and H. Takekoshi, JAERI1138 (1967). z) M. C. Moxon and N. J. Pattenden, Intern. Conf. Nuclear data for reactors, Paris (Oct. 1966) CN-32/27. 4) H. O. Zetterstr/Sm, S. Schwarz and L. G. Str6mberg, Nucl. Instr. and Meth. 42 (lq66) 277.