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Cross-section measurements with a neutron generator
NUCLEAR
INSTRUMENTS
AND
METHODS
II 5
Q974)
345-347;
©
NORTH-HOLLAND
PUBLISHING
CO.
CROSS-SECTION MEASUREMENTS WITH A NEUTRON GENERATOR S. M U B A R A K M A N D ,
M. A H M A D , M. A N W A R and M. S. C H A U D H R Y
Pakistan Institute of Nuclear Science and Technology, Rawalpindi, Pakistan Received 18 June 1973 and in revised form 2 August 1973 A 14.7 MeV neutron beam from a neutron generator has been elastically scattered from a plastic scintillation detector. The scattered neutrons are detected with another detector. The energy of the scattered neutrons can be selected by changing
the angle of scattering in a time-of-flight arrangement. Total cross-section measurements are then possible at all energies between 1.7 MeV and 14.7 MeV.
1. Introduction
being set by the bias on the neutron detectors. The entire set-up is similar to the one used for the measurement of neutron detection efficiency through the study of n - p scattering1).
A neutron generator (Texas Nuclear) with a limited capability for the acceleration of charged particles is usually suitable for fast-neutron cross-section measurements only around 14 MeV. With heavy deuterium targets it may be possible to do cross-section measurements just below 3 MeV. In the present set-up a 14.7 MeV neutron beana from the neutron generator has been primarily scattered from a plastic scintillator which also serves as a zero-time source. The elastically scattered neutrons are then detected in another scintillator which provides the stop pulses for time-of-flight spectroscopy. By varying the angle of scattering through the rotation of the second detector any neutron energy can be selected in the time-of-flight spectrum. Total cross-section measurements are then possible at all energies between 1 MeV and 14.7 MeV, the lower limit --
; deuterons
--
~
.~-de
t
ff target
(iz. k~).
2. Experimental method An accelerated beam of deuterons at 120 keV is bombarded onto a tritium target (fig. 1). The recoil alpha particles are detected on a 2" diam. ZnO(Ga) scintillation detector mounted on a 56 AVP photomultiplier tube. This detector is type NE843 (Nuclear Enterprises), has a decay time of 0.4 ns and is particularly suitable for high-flux counting2). The solid angle subtended by the alpha detector at the tritium target is variable. Neutrons from the tritium target strike a primary neutron detector which is a r ' x 1" plastic scintillator type N E I 0 2 A mounted on a 56 AVP photomultiplier tube. A fast coincidence (resolution 3 ns) between the signals from the alpha-particle detector and the primary neutron detector gives the start signal for the neutron time-of-flight measurement (fig. 2). The stop signal is obtained from the final neutron detector which
shadow bar
,r
start
scatterer
~._~Ld e t
~]
gore (psd.)
Fig. 1. Geometrical layout o f the system for total cross-section measurements using time-of-flight spectroscopy.
345
Fig. 2. Block diagram of the electronics showing time-of-flight arrangement incorporating pulse shape discrimination against gamma rays.
346
s. MUBARAKMAND et al.
is a 2" diam, 2" thick liquid scintillator (type NE213) mounted on a 56 AVP photomultiplier tube. The energy of the scattered neutrons can be selected by varying the angle 0 of the final detector with respect to the direction of the incident neutron beam which is fixed by the alignment between the alpha-particle detector and the primary neutron detector. At large angles the energy spread of the scattered neutrons increases considerably within the solid angle of the final detector. This energy spread can be minimised by increasing the flight path at these large angles. The reduction in scattered neutron flux at greater angles is offset by an increase in the incident deuteron current. Total cross sections are measured by recording time-of-flight spectra at different angles 0 both with samples in and samples out and then taking the ratios of the counts in the neutron peaks for the two cases.
1
TABLE
Total corrected cross sections lbr rzC, 27A1 and 23su at different neutron energies.
El3 (MeV)
~Tt (barn)
1.7 2.6 3.? 4.8 6.1 7.3 8.6 9.9 11.0 12.1 13.0 14.3
t2 C
°7Al
'):~SU
2.15±0.02 1.75 ± 0 . 0 2 2.51 ± 0 . 0 2 5 1.41i0.022 1.21±0.02 1.90±0.022 1.12&t~.022 1.304-0.024 1.50±0.025 1.50±0.024 1.454-0.024 1.31 ± 0 . 0 2 4
3.7 :I_0.02 3.2 4-0.02 3.0 4-0.02 2.3 ± 0 . 0 2 2.2 ± 0 £ 2 5 1.8 ± 0.025 1.8 ±0.025 1.8 ± 0.025 1.75±0.03 1.81 ± 0 . 0 3 1.8 ± 0 . 0 3 1.78±0.03
?.2 4-0.04 7.55±0.04 7.7 ± 0 . 0 4 7.5 ± 0.045 7.0 ± 0 . 0 4 5 6.4 ± 0.645 5.8 ± 0 . 0 4 5 5.3 4-0.05 5.0 ± 0 . 0 5 5.15±0.05 5.4 4-0.05 5.9 ± 0 . 0 5
3. Results Neutron time-of-flight spectra for 8.75 MeV scattered neutrons at an angle 0 = 40 ° are shown in fig. 3. i
i Neutrons 1800i
Flight
path elab
=
out
40 °
1 ch=0.472 ~1200
Sample
5m
ns
~
<
The upper spectrum is taken directly, while the lower one is taken with a sample in. The sample is a 1" diam. 2" high right circular cylinder of graphite. The experiment is done in a good geometry arrangement. The peak-to-background ratio is better than 20:1. Random gamma-ray background has been suppressed by the use of pulse shape discrimination against gamma rays. Similar measurements have been made at several energies between 1.7 MeV and 14.7 MeV for targets of 12C,
2c o U 6©O
8.0~
Gammas
I
U 238 °~
"'0. "0
6.0' i •
0 i 110
*
2 ~
h
130
150
L
£~
[
ref. 5
I
. . . . . . . .
.
l
[
]
300 320 Channel
--
340 no
]
~
360
I
2
380
~
.
i Neutrons
(3-
~
~ 2.0i
X2 U
Sample
"0.0
..... r e f . 4
3.o!
600 =
i
Gammas
" 2.0'
i i
120
140
160
4L
J
,
:
300 320 Channel
,
J
340 no
•
J
360
,~ ?,I ~
b
'"J
,,
380
c '2
i'
"
'd' "'
',
!',
',',
Z ;t o;:
1.0 L
','I ,
i
{ •
"-o--_ e
in
c
.
, A[27
! ;
,,
~
t
400 6'01
1200
L
"-o_ ~c~_ .o_~-°~
/
4.0 L
ref. 4
,
,,b ,,-, o,/
y..,'o-o ....
o
',,'
J
400
Fig. 3. Time-of-flight spectra at a scattering angle o f 40' yielding 8.75 MeV neutrons. The upper spectrum is direct and the lower one is with the sample in.
0
)
4
6
8 E n (IMeV)
10
1'2
1'4
Pig. 4. Total cross sections for 1 2 C , 27Al and 23su (depleted uranium) in the range 1.7 MeV to 14.25 MeV.
CROSS-SECTION
MEASUREMENTS
and 238U (depleted uranium). Total cross sections have been derived from measured transmissions from the expression 3) 27A1
a, = In (l/T)/n,
WITH A NEUTRON
GENERATOR
347
The energy spread at various angles within the solid angle of the final neutron detector lies between 100 and 150 keV. The results are shown in table 1. The deviations represent only the statistical errors. There is very good agreement with the already published data 4' 5).
where T = transmission, and n = number of target atoms/cm 3. The data have been corrected for in-scattering. The correction has been calculated from the expression 3)
AL 2 A°'t --
2 2 °'n(0°)' L1 L2
where d = cross-sectional area of the sample perpendicular to the neutron beam; L = distance between the two neutron detectors; L1 = distance of the sample from the primary neutron detector; L 2 = distance of the sample from the final neutron detector; and ao(0 °) = differential neutron elastic scattering cross section for sample material at 0 ° in the lab. system. The in-scattering correction is kept low by placing the sample midway between the two neutron detectors. In addition to the in-scattering correction, the standard statistical errors have been calculated from the equation 3) Ao-t = (l/in T) [(1 +T)/NoT] ~, where T = transmission through the sample; No = number of counts recorded with the sample out.
4. Conclusions
A neutron generator has been adapted as an instrument for total cross-section measurements in the MeV region up to 14.7 MeV. The present method incorporating three detectors in a time-of-flight mode keeps the random gamma-ray background to a minimum. The machine can be used for cross-section measurements over a range of energies and the limitation imposed by the fact that the incident neutron beam is mono-energetic is removed. The energy spread at each angle cannot be reduced beyond a certain point. This limitation is imposed by the availability of the maximum neutron flux from the neutron generator. This method, therefore, may not be suitable for the study of narrow resonances in total cross sections, particularly below 3 MeV. The authors wish to express their gratitude to Messrs P. K. Maher, A. Sabir and J. Ahmad for their assistance in the present work. References 1) S. M u b a r a k m a n d and M. Anwar, Nucl. Instr. and Meth. 93 (1971) 515. 2) D. Luckey, Nucl. Instr. a n d Moth. 62 (1968) 119. 3) D. W. Miller, Fast neutro, physics, Vol. 2 (1960) pp. 987-1000. 4) S. Cierjacks, P. Forti, D. K o p s c h , L. K r o p p , J. Nebe a n d H. Unseld, K e r n f o r s c h u n g s z e n t r u m K a r l s r u h e report K F K 1000 (1968). 5) N. Nereson and S. D a r d e n , Phys. Rev. 89 (1953) 775.
INSTRUMENTS
AND
METHODS
II 5
Q974)
345-347;
©
NORTH-HOLLAND
PUBLISHING
CO.
CROSS-SECTION MEASUREMENTS WITH A NEUTRON GENERATOR S. M U B A R A K M A N D ,
M. A H M A D , M. A N W A R and M. S. C H A U D H R Y
Pakistan Institute of Nuclear Science and Technology, Rawalpindi, Pakistan Received 18 June 1973 and in revised form 2 August 1973 A 14.7 MeV neutron beam from a neutron generator has been elastically scattered from a plastic scintillation detector. The scattered neutrons are detected with another detector. The energy of the scattered neutrons can be selected by changing
the angle of scattering in a time-of-flight arrangement. Total cross-section measurements are then possible at all energies between 1.7 MeV and 14.7 MeV.
1. Introduction
being set by the bias on the neutron detectors. The entire set-up is similar to the one used for the measurement of neutron detection efficiency through the study of n - p scattering1).
A neutron generator (Texas Nuclear) with a limited capability for the acceleration of charged particles is usually suitable for fast-neutron cross-section measurements only around 14 MeV. With heavy deuterium targets it may be possible to do cross-section measurements just below 3 MeV. In the present set-up a 14.7 MeV neutron beana from the neutron generator has been primarily scattered from a plastic scintillator which also serves as a zero-time source. The elastically scattered neutrons are then detected in another scintillator which provides the stop pulses for time-of-flight spectroscopy. By varying the angle of scattering through the rotation of the second detector any neutron energy can be selected in the time-of-flight spectrum. Total cross-section measurements are then possible at all energies between 1 MeV and 14.7 MeV, the lower limit --
; deuterons
--
~
.~-de
t
ff target
(iz. k~).
2. Experimental method An accelerated beam of deuterons at 120 keV is bombarded onto a tritium target (fig. 1). The recoil alpha particles are detected on a 2" diam. ZnO(Ga) scintillation detector mounted on a 56 AVP photomultiplier tube. This detector is type NE843 (Nuclear Enterprises), has a decay time of 0.4 ns and is particularly suitable for high-flux counting2). The solid angle subtended by the alpha detector at the tritium target is variable. Neutrons from the tritium target strike a primary neutron detector which is a r ' x 1" plastic scintillator type N E I 0 2 A mounted on a 56 AVP photomultiplier tube. A fast coincidence (resolution 3 ns) between the signals from the alpha-particle detector and the primary neutron detector gives the start signal for the neutron time-of-flight measurement (fig. 2). The stop signal is obtained from the final neutron detector which
shadow bar
,r
start
scatterer
~._~Ld e t
~]
gore (psd.)
Fig. 1. Geometrical layout o f the system for total cross-section measurements using time-of-flight spectroscopy.
345
Fig. 2. Block diagram of the electronics showing time-of-flight arrangement incorporating pulse shape discrimination against gamma rays.
346
s. MUBARAKMAND et al.
is a 2" diam, 2" thick liquid scintillator (type NE213) mounted on a 56 AVP photomultiplier tube. The energy of the scattered neutrons can be selected by varying the angle 0 of the final detector with respect to the direction of the incident neutron beam which is fixed by the alignment between the alpha-particle detector and the primary neutron detector. At large angles the energy spread of the scattered neutrons increases considerably within the solid angle of the final detector. This energy spread can be minimised by increasing the flight path at these large angles. The reduction in scattered neutron flux at greater angles is offset by an increase in the incident deuteron current. Total cross sections are measured by recording time-of-flight spectra at different angles 0 both with samples in and samples out and then taking the ratios of the counts in the neutron peaks for the two cases.
1
TABLE
Total corrected cross sections lbr rzC, 27A1 and 23su at different neutron energies.
El3 (MeV)
~Tt (barn)
1.7 2.6 3.? 4.8 6.1 7.3 8.6 9.9 11.0 12.1 13.0 14.3
t2 C
°7Al
'):~SU
2.15±0.02 1.75 ± 0 . 0 2 2.51 ± 0 . 0 2 5 1.41i0.022 1.21±0.02 1.90±0.022 1.12&t~.022 1.304-0.024 1.50±0.025 1.50±0.024 1.454-0.024 1.31 ± 0 . 0 2 4
3.7 :I_0.02 3.2 4-0.02 3.0 4-0.02 2.3 ± 0 . 0 2 2.2 ± 0 £ 2 5 1.8 ± 0.025 1.8 ±0.025 1.8 ± 0.025 1.75±0.03 1.81 ± 0 . 0 3 1.8 ± 0 . 0 3 1.78±0.03
?.2 4-0.04 7.55±0.04 7.7 ± 0 . 0 4 7.5 ± 0.045 7.0 ± 0 . 0 4 5 6.4 ± 0.645 5.8 ± 0 . 0 4 5 5.3 4-0.05 5.0 ± 0 . 0 5 5.15±0.05 5.4 4-0.05 5.9 ± 0 . 0 5
3. Results Neutron time-of-flight spectra for 8.75 MeV scattered neutrons at an angle 0 = 40 ° are shown in fig. 3. i
i Neutrons 1800i
Flight
path elab
=
out
40 °
1 ch=0.472 ~1200
Sample
5m
ns
~
<
The upper spectrum is taken directly, while the lower one is taken with a sample in. The sample is a 1" diam. 2" high right circular cylinder of graphite. The experiment is done in a good geometry arrangement. The peak-to-background ratio is better than 20:1. Random gamma-ray background has been suppressed by the use of pulse shape discrimination against gamma rays. Similar measurements have been made at several energies between 1.7 MeV and 14.7 MeV for targets of 12C,
2c o U 6©O
8.0~
Gammas
I
U 238 °~
"'0. "0
6.0' i •
0 i 110
*
2 ~
h
130
150
L
£~
[
ref. 5
I
. . . . . . . .
.
l
[
]
300 320 Channel
--
340 no
]
~
360
I
2
380
~
.
i Neutrons
(3-
~
~ 2.0i
X2 U
Sample
"0.0
..... r e f . 4
3.o!
600 =
i
Gammas
" 2.0'
i i
120
140
160
4L
J
,
:
300 320 Channel
,
J
340 no
•
J
360
,~ ?,I ~
b
'"J
,,
380
c '2
i'
"
'd' "'
',
!',
',',
Z ;t o;:
1.0 L
','I ,
i
{ •
"-o--_ e
in
c
.
, A[27
! ;
,,
~
t
400 6'01
1200
L
"-o_ ~c~_ .o_~-°~
/
4.0 L
ref. 4
,
,,b ,,-, o,/
y..,'o-o ....
o
',,'
J
400
Fig. 3. Time-of-flight spectra at a scattering angle o f 40' yielding 8.75 MeV neutrons. The upper spectrum is direct and the lower one is with the sample in.
0
)
4
6
8 E n (IMeV)
10
1'2
1'4
Pig. 4. Total cross sections for 1 2 C , 27Al and 23su (depleted uranium) in the range 1.7 MeV to 14.25 MeV.
CROSS-SECTION
MEASUREMENTS
and 238U (depleted uranium). Total cross sections have been derived from measured transmissions from the expression 3) 27A1
a, = In (l/T)/n,
WITH A NEUTRON
GENERATOR
347
The energy spread at various angles within the solid angle of the final neutron detector lies between 100 and 150 keV. The results are shown in table 1. The deviations represent only the statistical errors. There is very good agreement with the already published data 4' 5).
where T = transmission, and n = number of target atoms/cm 3. The data have been corrected for in-scattering. The correction has been calculated from the expression 3)
AL 2 A°'t --
2 2 °'n(0°)' L1 L2
where d = cross-sectional area of the sample perpendicular to the neutron beam; L = distance between the two neutron detectors; L1 = distance of the sample from the primary neutron detector; L 2 = distance of the sample from the final neutron detector; and ao(0 °) = differential neutron elastic scattering cross section for sample material at 0 ° in the lab. system. The in-scattering correction is kept low by placing the sample midway between the two neutron detectors. In addition to the in-scattering correction, the standard statistical errors have been calculated from the equation 3) Ao-t = (l/in T) [(1 +T)/NoT] ~, where T = transmission through the sample; No = number of counts recorded with the sample out.
4. Conclusions
A neutron generator has been adapted as an instrument for total cross-section measurements in the MeV region up to 14.7 MeV. The present method incorporating three detectors in a time-of-flight mode keeps the random gamma-ray background to a minimum. The machine can be used for cross-section measurements over a range of energies and the limitation imposed by the fact that the incident neutron beam is mono-energetic is removed. The energy spread at each angle cannot be reduced beyond a certain point. This limitation is imposed by the availability of the maximum neutron flux from the neutron generator. This method, therefore, may not be suitable for the study of narrow resonances in total cross sections, particularly below 3 MeV. The authors wish to express their gratitude to Messrs P. K. Maher, A. Sabir and J. Ahmad for their assistance in the present work. References 1) S. M u b a r a k m a n d and M. Anwar, Nucl. Instr. and Meth. 93 (1971) 515. 2) D. Luckey, Nucl. Instr. a n d Moth. 62 (1968) 119. 3) D. W. Miller, Fast neutro, physics, Vol. 2 (1960) pp. 987-1000. 4) S. Cierjacks, P. Forti, D. K o p s c h , L. K r o p p , J. Nebe a n d H. Unseld, K e r n f o r s c h u n g s z e n t r u m K a r l s r u h e report K F K 1000 (1968). 5) N. Nereson and S. D a r d e n , Phys. Rev. 89 (1953) 775.