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Measurement and parametrization of the response of NE102 to fast neutrons
Nuclear Instruments and Methods 185 (1981) 287-289 North-Holland Publishing Company
287
MEASUREMENT AND PARAMETERIZATION OF THE RESPONSE OF NE102 TO FAST NEUTRONS * S.D. MARTIN, JR., ** H.L. WOOLVERTON, R.L. YORK ***, S. NATH, K.K. SEKItARAN ?, J.C. HIEBERT and L.C. NORTHCLIFFE Cyclotron Institute, Texas A & M University, College Station, Texas 77843, U.S.A.
Received 10 November 1980
Measurements of the relative light output of the plastic scintillator NE102 in response to neutrons with energies 2.6-28 MeV are reported and a smooth parameterization of the results is presented.
1. Introduction An important factor in the use of scintillators for the determination of neutron cross sections is an accurate knowledge of the scintillator response function, i.e., light output vs. recoil proton energy. This is the case whether a Monte Carlo prediction of the detector efficiency is used or, as is done at this laboratory [1], the observed pulse-height distribution is fitted by one calculated from the n-p differential and total cross sections. The scintillator material most used at this laboratory is NE102. Published data on the response function for protons and neutrons in that material are limited [ 2 - 8 ] and not always in good agreement. Since an analytic expression for the response function is desirable in our detector pulseheight distribution calculations, we have assumed that the convenient parameterization given by Masterson [9] for NE218 represented in adequate approximation for the shape of the NE102 response function. One slightly troublesome characteristic of that parameterization is the discontinuities in slope and magnitude encountered at the boundaries of the various energy regions covered. The present work was undertaken with the objectives of measuring the response function over a wide energy region and obtaining a * Work supported in part by the National Science Foundation and the U.S. Department of Energy. ** Present address: Lackland Air Force Base, Texas 78236, U.S.A. *** Present address: Los Alamos Scientific Laboratory, Los Alamos, New Mexico 87545, U.S.A. t Present address: Department of Physics, University of Houston, ttouston, Texas 77004, U.S.A. 0 0 2 9 - 5 5 4 X / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 5 0 © North-Holland
smooth parameterization based on all of the available data.
2. Experimental method In separate runs, deuteron beams of 12.5 and 50 MeV from the Texas A & M 88" cyclotron were used to bombard thick carbon targets, producing continuum distributions of neutrons in the 2.6-9.1 and 6 . 5 - 2 8 . 0 MeV energy regions respectively. The neutrons emerged through a 2.54 cm square collimator at 0 °. The detector was an NE102 cylinder of length 2.54 cm and diameter 5.08 cm with its axis on the collimator axis, viewed through a cylindrical air light pipe of length 5 cm by an RCA 8575 photomultiplier tube. The flight path from center of target to center of scintillator was 6.18 m. The data obtained were twoparameter continuum distributions of neutron time-of-flight t vs. photomultiplier pulse height H. The t-value was given by a time-to-amplitude converter started by a fast discriminator on the detector output pulse and stopped by a zero-crossing discriminator triggered by the cyclotron rf voltage. Micropulse selection was used to widen the separation between cyclotron beam bursts, eliminating frame overlap ambiguity for the slower neutrons. Calibration of the t-scale was simplified by the presence in th spectrum of a peak due to prompt ~,-rays from the target, which shifts by a measured amount upon insertion of a known delay into the stop signal. The neutron energy for each t-value was calculated relativistically. The H-value for that energy was obtained by determining the "end-point" of the pulse-height
S.D. Martin et al. /Response of NE102
288
spectrum, that is, the H-value for which the yield fell to one-half of its plateau value. The linearity of the analog-to-digital converter used in measuring the H-distribution was monitored by sending calibration pulses from a research pulser to the test input of the photomultiplier tube base while the two-parameter data were being taken. Several runs with different amplifier gain settings were made at each of the two beam energies.
•
REF. 2
A REE 6
1
•
• REF. 7
~
9
v REE
8
• THLS
EXP~T
0/ Ld Z UJ 7 Z 0
rr
3. Results and discussion
I--6
_A OA
The results in three energy regions are displayed in figs. 1 - 3 , along with all published data for NE102 known to the authors. It should be noted that the data of refs. 2 and 4 were obtained with direct proton beams and are more likely to be influenced by geometric effects arising from the definiteness of the proton range. All other data were obtained with recoil-proton distributions from neutron irradiation of the scintillator. The pulse-height measurements made in the present experiment were only relative. They were fitted by a smooth, continuous curve, which was then renormalized to have the significance of equivalent electron energy (energy of an electron which would give the same pulse height). The renormalization factor used was the weighted average of the normalization factors required to fit the data of
1.4 •
P Z W _J
5
5 5
I
I
I
5
Io
15
ENERGY (MeV)
Fig. 2. Response of NE102 to neutrons and protons in the energy region 4 - I 8 MeV.
F
I
I
I
2
5
REF. 2
o REF. 5 :~
1.2
>,.(.9 rY 1.0 W Z W
o REF. 5 '~ REE 6 • REE 7 v REE
8
• THIS EXP'T
z 0.8 o nteD i,i 0.6 ~J i,i tz 0.4 m ..J
.5 o.2 LIJ
0
o
I
4
ENERGY (MeV) Fig. 1. Response of NE 102 to neutrons and protons in the energy region 0 - 4 MeV.
289
S.D. Martin et aL /Response of NE102
>~ 24
/"
>.o rY" I.iJ Z L.O Z 0 rr I--(,_) L~ _J LId I-Z Ld d
v REF. 8 • THIS
.'9"./
I
EXPIT
16
8
j./-
>
5 0(~
I 8
I
I 24
16
I 52
ENERGY (MeV) Fig. 3. Response of NE102 to neutrons and protons in the energy region 0-36 MeV. The dashed curve shows the calculation of Gooding and Pugh [4].
Smith et al. [6] and of Madey et al. [7,8] in the energy region from 9.5 to 20 MeV. These data were used because they were obtained with neutrons, while the data of Evans and Bellamy [2] and of Gooding and Pugh [4] were not used for this purpose because they were obtained with direct proton beams. Concerning the accuracy of these data, the error in the neutron energy determination was negligible, while the error in the pulse-height determination was typically less than one-half channel on a 64 channel scale, more or less consistent with the scatter of the points seen in figs. 2 and 3. Some inconsistencies between the various runs were noted. While these were larger than expected and remain unexplained, they were not much larger than the estimated accuracy of the end-point determination. They contribute to the spread of the points seen in the figures. The solid curve shown in the figures is given b y the following parameterization: 0
3 MeV ;
H = 0.058E + 0.072E 2 ; 3
MeV ;
H = - 0 . 3 1 5 + 0.268E + 0.037E 2 ; 5 < En < 7.5 MeV ; H = 3.488 - 1.8287E + 0.4193E 2 - 0.02302E s ;
7.5 < E n < 3 0 MeV ; H = 0.0424 + 2.3604 X %/[1.0 + 0.10432(E - 3.9255) 2] . It is smooth throughout the energy region and agrees well with the bulk of the data. Deviations from the parameterization given by Masterson [9] for NE218 are within the experimental errors.
References
[ 1] L.C. Northcliffe, C.W. Lewis and D.P. Saylor, Nucl. Instr. and Meth. 83 (1970) 93. [2] H.C. Evans and E.H. Bellamy, Proc. Phys. Soc. 74 (1959) 483. [3] M. Gettner and W. Selove, Rev. Sci. Instr. 31 (1960) 450. [4] T.J. Gooding and H.G. Pugh, Nucl. Instr. and Meth. 7 (1960) 189. [5] J.B. Czirr, D.R. Nygren and C.D. Zafiratos, Nucl. Instr. and Meth. 31 (1960) 450. [6] D.L. Smith, R.G. Polk and T.G. Miller, Nucl. Instr. and Meth. 64 (1968) 157. [7] R. Madey and F.M. Waterman, Nucl. Instr. and Meth. 104 (1972) 253. The electron energy values reported in this reference have been reduced by 5%, as recommended in ref. 8. [8] R. Madey, F.M. Waterman, A.R. Baldwin, J.N. Knudson, J.D. Carlson and J. Rappaport, Nucl. Instr. and Meth. 151 (1978) 445. [9] T.G. Masterson, Nucl. Instr. and Meth. 88 (1970) 61.
287
MEASUREMENT AND PARAMETERIZATION OF THE RESPONSE OF NE102 TO FAST NEUTRONS * S.D. MARTIN, JR., ** H.L. WOOLVERTON, R.L. YORK ***, S. NATH, K.K. SEKItARAN ?, J.C. HIEBERT and L.C. NORTHCLIFFE Cyclotron Institute, Texas A & M University, College Station, Texas 77843, U.S.A.
Received 10 November 1980
Measurements of the relative light output of the plastic scintillator NE102 in response to neutrons with energies 2.6-28 MeV are reported and a smooth parameterization of the results is presented.
1. Introduction An important factor in the use of scintillators for the determination of neutron cross sections is an accurate knowledge of the scintillator response function, i.e., light output vs. recoil proton energy. This is the case whether a Monte Carlo prediction of the detector efficiency is used or, as is done at this laboratory [1], the observed pulse-height distribution is fitted by one calculated from the n-p differential and total cross sections. The scintillator material most used at this laboratory is NE102. Published data on the response function for protons and neutrons in that material are limited [ 2 - 8 ] and not always in good agreement. Since an analytic expression for the response function is desirable in our detector pulseheight distribution calculations, we have assumed that the convenient parameterization given by Masterson [9] for NE218 represented in adequate approximation for the shape of the NE102 response function. One slightly troublesome characteristic of that parameterization is the discontinuities in slope and magnitude encountered at the boundaries of the various energy regions covered. The present work was undertaken with the objectives of measuring the response function over a wide energy region and obtaining a * Work supported in part by the National Science Foundation and the U.S. Department of Energy. ** Present address: Lackland Air Force Base, Texas 78236, U.S.A. *** Present address: Los Alamos Scientific Laboratory, Los Alamos, New Mexico 87545, U.S.A. t Present address: Department of Physics, University of Houston, ttouston, Texas 77004, U.S.A. 0 0 2 9 - 5 5 4 X / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 5 0 © North-Holland
smooth parameterization based on all of the available data.
2. Experimental method In separate runs, deuteron beams of 12.5 and 50 MeV from the Texas A & M 88" cyclotron were used to bombard thick carbon targets, producing continuum distributions of neutrons in the 2.6-9.1 and 6 . 5 - 2 8 . 0 MeV energy regions respectively. The neutrons emerged through a 2.54 cm square collimator at 0 °. The detector was an NE102 cylinder of length 2.54 cm and diameter 5.08 cm with its axis on the collimator axis, viewed through a cylindrical air light pipe of length 5 cm by an RCA 8575 photomultiplier tube. The flight path from center of target to center of scintillator was 6.18 m. The data obtained were twoparameter continuum distributions of neutron time-of-flight t vs. photomultiplier pulse height H. The t-value was given by a time-to-amplitude converter started by a fast discriminator on the detector output pulse and stopped by a zero-crossing discriminator triggered by the cyclotron rf voltage. Micropulse selection was used to widen the separation between cyclotron beam bursts, eliminating frame overlap ambiguity for the slower neutrons. Calibration of the t-scale was simplified by the presence in th spectrum of a peak due to prompt ~,-rays from the target, which shifts by a measured amount upon insertion of a known delay into the stop signal. The neutron energy for each t-value was calculated relativistically. The H-value for that energy was obtained by determining the "end-point" of the pulse-height
S.D. Martin et al. /Response of NE102
288
spectrum, that is, the H-value for which the yield fell to one-half of its plateau value. The linearity of the analog-to-digital converter used in measuring the H-distribution was monitored by sending calibration pulses from a research pulser to the test input of the photomultiplier tube base while the two-parameter data were being taken. Several runs with different amplifier gain settings were made at each of the two beam energies.
•
REF. 2
A REE 6
1
•
• REF. 7
~
9
v REE
8
• THLS
EXP~T
0/ Ld Z UJ 7 Z 0
rr
3. Results and discussion
I--6
_A OA
The results in three energy regions are displayed in figs. 1 - 3 , along with all published data for NE102 known to the authors. It should be noted that the data of refs. 2 and 4 were obtained with direct proton beams and are more likely to be influenced by geometric effects arising from the definiteness of the proton range. All other data were obtained with recoil-proton distributions from neutron irradiation of the scintillator. The pulse-height measurements made in the present experiment were only relative. They were fitted by a smooth, continuous curve, which was then renormalized to have the significance of equivalent electron energy (energy of an electron which would give the same pulse height). The renormalization factor used was the weighted average of the normalization factors required to fit the data of
1.4 •
P Z W _J
5
5 5
I
I
I
5
Io
15
ENERGY (MeV)
Fig. 2. Response of NE102 to neutrons and protons in the energy region 4 - I 8 MeV.
F
I
I
I
2
5
REF. 2
o REF. 5 :~
1.2
>,.(.9 rY 1.0 W Z W
o REF. 5 '~ REE 6 • REE 7 v REE
8
• THIS EXP'T
z 0.8 o nteD i,i 0.6 ~J i,i tz 0.4 m ..J
.5 o.2 LIJ
0
o
I
4
ENERGY (MeV) Fig. 1. Response of NE 102 to neutrons and protons in the energy region 0 - 4 MeV.
289
S.D. Martin et aL /Response of NE102
>~ 24
/"
>.o rY" I.iJ Z L.O Z 0 rr I--(,_) L~ _J LId I-Z Ld d
v REF. 8 • THIS
.'9"./
I
EXPIT
16
8
j./-
>
5 0(~
I 8
I
I 24
16
I 52
ENERGY (MeV) Fig. 3. Response of NE102 to neutrons and protons in the energy region 0-36 MeV. The dashed curve shows the calculation of Gooding and Pugh [4].
Smith et al. [6] and of Madey et al. [7,8] in the energy region from 9.5 to 20 MeV. These data were used because they were obtained with neutrons, while the data of Evans and Bellamy [2] and of Gooding and Pugh [4] were not used for this purpose because they were obtained with direct proton beams. Concerning the accuracy of these data, the error in the neutron energy determination was negligible, while the error in the pulse-height determination was typically less than one-half channel on a 64 channel scale, more or less consistent with the scatter of the points seen in figs. 2 and 3. Some inconsistencies between the various runs were noted. While these were larger than expected and remain unexplained, they were not much larger than the estimated accuracy of the end-point determination. They contribute to the spread of the points seen in the figures. The solid curve shown in the figures is given b y the following parameterization: 0
3 MeV ;
H = 0.058E + 0.072E 2 ; 3
MeV ;
H = - 0 . 3 1 5 + 0.268E + 0.037E 2 ; 5 < En < 7.5 MeV ; H = 3.488 - 1.8287E + 0.4193E 2 - 0.02302E s ;
7.5 < E n < 3 0 MeV ; H = 0.0424 + 2.3604 X %/[1.0 + 0.10432(E - 3.9255) 2] . It is smooth throughout the energy region and agrees well with the bulk of the data. Deviations from the parameterization given by Masterson [9] for NE218 are within the experimental errors.
References
[ 1] L.C. Northcliffe, C.W. Lewis and D.P. Saylor, Nucl. Instr. and Meth. 83 (1970) 93. [2] H.C. Evans and E.H. Bellamy, Proc. Phys. Soc. 74 (1959) 483. [3] M. Gettner and W. Selove, Rev. Sci. Instr. 31 (1960) 450. [4] T.J. Gooding and H.G. Pugh, Nucl. Instr. and Meth. 7 (1960) 189. [5] J.B. Czirr, D.R. Nygren and C.D. Zafiratos, Nucl. Instr. and Meth. 31 (1960) 450. [6] D.L. Smith, R.G. Polk and T.G. Miller, Nucl. Instr. and Meth. 64 (1968) 157. [7] R. Madey and F.M. Waterman, Nucl. Instr. and Meth. 104 (1972) 253. The electron energy values reported in this reference have been reduced by 5%, as recommended in ref. 8. [8] R. Madey, F.M. Waterman, A.R. Baldwin, J.N. Knudson, J.D. Carlson and J. Rappaport, Nucl. Instr. and Meth. 151 (1978) 445. [9] T.G. Masterson, Nucl. Instr. and Meth. 88 (1970) 61.