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The response of Ne-228 liquid scintillator to 3.5, 5.8 and 10.5 MeV protons
N UCL EAR INSTRUMENTS
AND METHODS
IO4
(1972) 253-256;
© NORTH-HOLLAND
PUBLISHING
CO.
THE R E S P O N S E OF NE-228 L I Q U I D S C I N T I L L A T O R TO 3.5, 5.8 AND 10.5 MeV P R O T O N S
R. MADEY and F. M. WATERMAN Physics Department, Kent State University, Kent, Ohio 44242, U.S.A.
Received 16 June 1972 The response of NE-228 liquid and NE-102 plastic scintillators to 3.5, 5.8 and 10.5 MeV protons has been measured relative to electrons. These proton energies represent the kinetic energies of recoil protons from the elastic scattering of 14 MeV neutrons
at angles of 30°, 40° and 60°. The response of NE-228 relative to that of NE-102 plastic scintillator varies from 78% for 3.5 MeV protons to 91% for 10.5 MeV protons.
1. Introduction The ratio of hydrogen to carbon atoms of 2.11 for NE-228 liquid scintillator is about twice that oi standard scintillators. As specified by the manufacturer*, the numerical densities of hydrogen and carbon in NE-228 are 6.632x 1022 atoms/cm 3 and 3.127× 10 22 atoms/cm 3, respectively. We have used NE-228 for the first detector in a selfcontained time-of-flight spectrometer for energeticneutronsl-3). This spectrometer consists of two primary scintillation counters shown as the D1 and D2 detectors in fig. 1. The principle of the spectrometer requires an incident neutron to scatter elastically from a hydrogen nucleus in the D1 detector. The scattered neutlon must then travel over a known flight path and interact in the D2 detector. The time-of-flight of the scattered neutron is measured by the time interval between the scintillation pulses in the D1 and D2 detectors. Kinematically, the kinetic energy of the incident neutron is specified in terms of the scattering angle, the flight path, and the flight-time of the scattered neutron. The high-hydrogen content of NE-228 makes this scintillator attractive from the point of view of increasing the spectrometer efficiency; more importantly, however, the fact that NE-102 plastic (CHl.los) and NE-228 liquid (CH2.10 scintillator differ significantly in their relative hydrogen-carbon composition allows us to determine the background from neutron-carbon interactions in spectral measurements above 15 MeV by making separate measurements with NE-228 and NE-102 D1 detectors. The need for an energy calibration of the NE-228 D1 detector required a knowledge of the response of NE-228 to protons. In this paper, we report measurements of the response of NE-228 liquid and NE-102
plastic scintillator to 3.5, 5.8 and 10.5 MeV recoil protons from the elastic scattering of 14 MeV neutrons. 2. Experimental arrangement The configuration of the detector and the neutron source are shown in fig. I. The 14 MeV neutrons from the t(d,n)~ reaction were incident on the scintillation detector D1. A second scintillation detector D2 was positioned 175 cm from the D1 detector at an angle 0 with respect to the direction of the incident neutrons. The pulse-height spectra of recoil protons in the D1 scintillator were measured for neutron scattering angles of 30 °, 40 °, and 60 ° which were defined by the position of the D2 detector. For 14 MeV incident neutrons, these angles correspond kinematically to recoil proton energies of 3.5, 5.8 and 10.5 MeV, respectively. For these measurements, the DI detector consisted of a 2½ in. diam. by 2½ in. high NE-228 liquid scintillator; the D2 detector consisted of a 4 in. diam. by 4 in. high NE-102 plastic scintillator. Both scintillators were mounted on Amperex 58DVP photomultipliers. Fig. 2 is a block diagram of the electronic apparatus
DEUTERONS
TRITIUM~
/
~
e 4~
* Supplied by Nuclear Enterprises, Inc., 935 Terminal Way, San Carlos, California 94070, U.S.A. OCTOBER 1972
~
253
~DETECTOR
Fig. 1. Configuration of detectors and the neutron source.
254
R. M A D E Y A N D F. M. W A T E R M A N
]
°,
DETECTOR
D2 DETECTOR
I
I
TIME-OF-FLIGHT INSERTION DELAY
DISCRIMINATOR
I
U
8, STRETCHER
I
DISCRIMINATOR
r
GATE INPUT
COINCIDENCE II-LF
-V
...9{
ANALYZER
Fig. 2. Block d i a g r a m o f the electronic a p p a r a t u s for m e a s u r i n g recoil-proton pulse-height spectra in the D1 detector.
used in these measurements. The pulse-height of each event in the D1 detector was recorded on the multichannel analyzer whenever there was a coincidence between events in the D1 and D2 detectors. To produce a coincidence, the D2 event had to occur within a preselected time interval following the event in the D I detector. This time interval was defined by the time-of-
flight insertion d e l a S a n d the time duration of the D2 I1~ . . . logic pulse. In thesemeasurements, thls t~me interval was selected to correspond to the expected interval of neutron flight-times. Gamma-rays scattering between the D1 and D2 detectors could not produce the desired coincidences because the short flight-time of gammarays did not fall within the predetermined time-of-flight
5oo 124 +- :5
58.5+2
i °,%
I
i
o'o°,.°°° °.°
400
°,°%° J tu z Z ,,,:I
.,..
2.62
MeV
GAMMA- RAYS
-i- 300 o n,-
1.28 MeV GAMMA- RAYS
"
113 ± 4
I-.D z o u
t
I
=
-....'.'....
200
•• •
I00 • • ".o
, 40
, 60
5.8
% • •== %
MeV
.%
RECOIL PROTONS
lee
=%. • o°
oeo~o I
I
flO
%°
o•
• %.°
el • l
I
i
I00
I
120
I
I~
140
i
160
CHANNEL NUMBER
Fig. 3. The recoil-proton pulse-height s p e c t r u m for a n e u t r o n scattering angle o f 40 °. T h e r a n d o m coincidence b a c k g r o u n d h a s been subtracted f r o m these data. Superposed o n the recoil-proton pulse-height s p e c t r u m are the 1.28 a n d 2.62 M e V g a m m a - r a y C o m p t o n pulse-height spectra.
THE RESPONSE
OF N E - 2 2 8
LIQUID
SCINTILLATOR
window for neutrons. The recoil-proton pulse-height spectrum recorded at a neutron scattering angle of 40 ° is shown in fig. 3. The random coincidence background has been subtracted from these data. Also shown in fig. 3 relative to the recoil-proton pulse-height spectrum are the Compton spectra of the 1.28 MeV gamma-rays from sodium-22 and the 2.62 MeV gamma-rays from the thorium-228. These Compton spectra, which are generated in the D1 scintillator, serve as calibration points for determining the light output of the recoil protons relative to electrons. Since large scintillators are used in this spectrometer, it is necessary to have calibration points, such as the peaks of the Compton spectra, which are insensitive to the pulse height resolution of the detector. The Compton peak is taken to correspond to the maximum Compton electron energy produced by the associated gamma-ray. For a gamma-ray with energy Er, the maximum Compton electron energy T max is given by T max
=
E~/(1 +moc2/2Er),
(1)
where moc2=0.51 MeV. From eq. (1), the maximum electron energies that correspond to 0.51, 1.28 and 2.62 MeV gamma-rays are 0.34, 1.07 and 2.39 MeV, respectively. The peak positions and the estimated uncertainties in determining the peak positions are indicated in fig. 3. To determine the response of NE-228 relative to NE-102, and also to ascertain that a calibration technique based on the Compton peaks would give representative results, we measured the response of a 2½ in. diam. by 2½ in. high NE-102 plastic scintillator to 3.5, 5.8 and 10.5 MeV protons in the same manner as that described for the NE-228 liquid scintillator. The response of NE-102 to protons has been measured by several authors 4-6) and calculated by Gooding and Pugh7). 3. Analysis The light output of electrons in organic scintillators is known to be a linear function of energy for electrons above approximately 100 keV; therefore, there is a linear relationship between the Compton electron energies and the associated channel numbers of the Compton pulse-height spectra. The analyzer was calibrated in units of equivalent-electron energy by associating the channel numbers with the electron energies of the peaks of the Compton spectra. The equivalentelectron energy corresponding to the channel number of the pulse-height peak of each recoil-proton spectrum was then given by this calibration. The equivalentelectron energies determined for the 3.5, 5.8 and
TO
3.5,
5.8 A N D
10.5 M e V
PROTONS
255
TABLE 1 T h e response o f NE-228 a n d NE-102 scintillators to protons. P r o t o n energy Tp(MeV)
Equivalent-electron energy, Te(MeV) in NE-228 in NE-102
3.5 + 0.2 5.8 + 0.2 10.5 _ 0.2
0.99 + 0.04 2.14 + 0.08 5.06 + 0.09
1.27 + 0.13 2.53 + 0.21 5.60 + 0.14
10.5 MeV recoil protons in both NE-228 and NE-102 scintillator are listed in table 1. The uncertainty in the equivalent-electron energy represents the uncertainty in determining the position of the peak of the recoilproton spectrum. The uncertainty in the recoil-proton energy represents an estimated uncertainty of _+1° in the neutron scattering angle. The response of both NE-228 and NE-102 are plotted in fig. 4. In this figure, the solid line is the calculated response of Gooding and PughT). 4. Conclusion it is evident from fig. 4 that the measured response of NE-102 plastic scintillator agrees with the calculated response of Gooding and Pugh7). This agreement indicates that energy calibrations of large detectors can be carried-out by using the peaks of the gamma-ray Compton spectra as calibration points. The response
NE-228
/
+ NE-I02 -- CALCULATED RESPONSE
:E
v
F~
.:/ .7"
>_r~ W Z W Z 0 n~ 0 hi -J W
Z Ixl _J
0 /:-E4
°o
i
;,
i
~
i
,2
PROTON ENERGY, Tp (MeV) Fig. 4. T h e response o f b o t h NE-102 plastic a n d Ne-228 liquid scintillator to 3.5, 5.8 a n d 10.5 M e V protons. T h e solid curve is the calculated r e s p o n s e o f G o o d i n g a n d Pugh.
256
R. M A D E Y
AND
o f NE-228 liquid scintillator is less t h a n the response o f NE-102 plastic scintillator. The response o f NE-228 relative to NE-102 for 3.5, 5.8 a n d 10.5 M e V p r o t o n s is 78, 85, a n d 91 percent, respectively. W e are grateful to the N a t i o n a l R e s e a r c h Council, O t t a w a , C a n a d a and to Dr. K. W. Geiger for use o f the 14 M e V n e u t r o n - g e n e r a t o r facility. W e t h a n k Mr. D. Elliott for his c o o p e r a t i o n in c o n n e c t i o n with the o p e r a t i o n o f the n e u t r o n generator. This w o r k was s u p p o r t e d in p a r t by the U.S. A t o m i c E n e r g y C o m mission u n d e r c o n t r a c t AT(30-1)-3914.
F. M. W A T E R M A N
References 1) R. Madey, IEEE Trans. Nucl. Sci. NS-15, no. 3 (1968) 426. 2)D. H. Osborn and R. Madey, CONF-691101, Proc. 2nd Intern. Conf. Accelerator dosimetry and experience (Stanford Linear Accelerator Center, Stanford, Calif., Nov. 5 7, 1969). a) R. Madey, Proc. Intern. Syrup. Nuclear electronics and radioprotection (Toulouse, France, March 1968). 4) G. D. Badhwar, C. L. Deney, B. R. Dennis and M. F. Kaplon, Nucl. Instr. and Meth. 57 (1967) 116. 5) H. C. Evans and E. H. Bellamy, Proc. Phys. Soc. 74 (19.59) 483. 6) D. L. Smith, R~ G. Polk and T. G. Miller, Nucl. Instr. and Meth. 64 (1968) 157. 7) T. J. Gooding and H. G. Pugh, Nucl. Instr. and Meth. 7 (1960) 189; 11 (1961) 365.
AND METHODS
IO4
(1972) 253-256;
© NORTH-HOLLAND
PUBLISHING
CO.
THE R E S P O N S E OF NE-228 L I Q U I D S C I N T I L L A T O R TO 3.5, 5.8 AND 10.5 MeV P R O T O N S
R. MADEY and F. M. WATERMAN Physics Department, Kent State University, Kent, Ohio 44242, U.S.A.
Received 16 June 1972 The response of NE-228 liquid and NE-102 plastic scintillators to 3.5, 5.8 and 10.5 MeV protons has been measured relative to electrons. These proton energies represent the kinetic energies of recoil protons from the elastic scattering of 14 MeV neutrons
at angles of 30°, 40° and 60°. The response of NE-228 relative to that of NE-102 plastic scintillator varies from 78% for 3.5 MeV protons to 91% for 10.5 MeV protons.
1. Introduction The ratio of hydrogen to carbon atoms of 2.11 for NE-228 liquid scintillator is about twice that oi standard scintillators. As specified by the manufacturer*, the numerical densities of hydrogen and carbon in NE-228 are 6.632x 1022 atoms/cm 3 and 3.127× 10 22 atoms/cm 3, respectively. We have used NE-228 for the first detector in a selfcontained time-of-flight spectrometer for energeticneutronsl-3). This spectrometer consists of two primary scintillation counters shown as the D1 and D2 detectors in fig. 1. The principle of the spectrometer requires an incident neutron to scatter elastically from a hydrogen nucleus in the D1 detector. The scattered neutlon must then travel over a known flight path and interact in the D2 detector. The time-of-flight of the scattered neutron is measured by the time interval between the scintillation pulses in the D1 and D2 detectors. Kinematically, the kinetic energy of the incident neutron is specified in terms of the scattering angle, the flight path, and the flight-time of the scattered neutron. The high-hydrogen content of NE-228 makes this scintillator attractive from the point of view of increasing the spectrometer efficiency; more importantly, however, the fact that NE-102 plastic (CHl.los) and NE-228 liquid (CH2.10 scintillator differ significantly in their relative hydrogen-carbon composition allows us to determine the background from neutron-carbon interactions in spectral measurements above 15 MeV by making separate measurements with NE-228 and NE-102 D1 detectors. The need for an energy calibration of the NE-228 D1 detector required a knowledge of the response of NE-228 to protons. In this paper, we report measurements of the response of NE-228 liquid and NE-102
plastic scintillator to 3.5, 5.8 and 10.5 MeV recoil protons from the elastic scattering of 14 MeV neutrons. 2. Experimental arrangement The configuration of the detector and the neutron source are shown in fig. I. The 14 MeV neutrons from the t(d,n)~ reaction were incident on the scintillation detector D1. A second scintillation detector D2 was positioned 175 cm from the D1 detector at an angle 0 with respect to the direction of the incident neutrons. The pulse-height spectra of recoil protons in the D1 scintillator were measured for neutron scattering angles of 30 °, 40 °, and 60 ° which were defined by the position of the D2 detector. For 14 MeV incident neutrons, these angles correspond kinematically to recoil proton energies of 3.5, 5.8 and 10.5 MeV, respectively. For these measurements, the DI detector consisted of a 2½ in. diam. by 2½ in. high NE-228 liquid scintillator; the D2 detector consisted of a 4 in. diam. by 4 in. high NE-102 plastic scintillator. Both scintillators were mounted on Amperex 58DVP photomultipliers. Fig. 2 is a block diagram of the electronic apparatus
DEUTERONS
TRITIUM~
/
~
e 4~
* Supplied by Nuclear Enterprises, Inc., 935 Terminal Way, San Carlos, California 94070, U.S.A. OCTOBER 1972
~
253
~DETECTOR
Fig. 1. Configuration of detectors and the neutron source.
254
R. M A D E Y A N D F. M. W A T E R M A N
]
°,
DETECTOR
D2 DETECTOR
I
I
TIME-OF-FLIGHT INSERTION DELAY
DISCRIMINATOR
I
U
8, STRETCHER
I
DISCRIMINATOR
r
GATE INPUT
COINCIDENCE II-LF
-V
...9{
ANALYZER
Fig. 2. Block d i a g r a m o f the electronic a p p a r a t u s for m e a s u r i n g recoil-proton pulse-height spectra in the D1 detector.
used in these measurements. The pulse-height of each event in the D1 detector was recorded on the multichannel analyzer whenever there was a coincidence between events in the D1 and D2 detectors. To produce a coincidence, the D2 event had to occur within a preselected time interval following the event in the D I detector. This time interval was defined by the time-of-
flight insertion d e l a S a n d the time duration of the D2 I1~ . . . logic pulse. In thesemeasurements, thls t~me interval was selected to correspond to the expected interval of neutron flight-times. Gamma-rays scattering between the D1 and D2 detectors could not produce the desired coincidences because the short flight-time of gammarays did not fall within the predetermined time-of-flight
5oo 124 +- :5
58.5+2
i °,%
I
i
o'o°,.°°° °.°
400
°,°%° J tu z Z ,,,:I
.,..
2.62
MeV
GAMMA- RAYS
-i- 300 o n,-
1.28 MeV GAMMA- RAYS
"
113 ± 4
I-.D z o u
t
I
=
-....'.'....
200
•• •
I00 • • ".o
, 40
, 60
5.8
% • •== %
MeV
.%
RECOIL PROTONS
lee
=%. • o°
oeo~o I
I
flO
%°
o•
• %.°
el • l
I
i
I00
I
120
I
I~
140
i
160
CHANNEL NUMBER
Fig. 3. The recoil-proton pulse-height s p e c t r u m for a n e u t r o n scattering angle o f 40 °. T h e r a n d o m coincidence b a c k g r o u n d h a s been subtracted f r o m these data. Superposed o n the recoil-proton pulse-height s p e c t r u m are the 1.28 a n d 2.62 M e V g a m m a - r a y C o m p t o n pulse-height spectra.
THE RESPONSE
OF N E - 2 2 8
LIQUID
SCINTILLATOR
window for neutrons. The recoil-proton pulse-height spectrum recorded at a neutron scattering angle of 40 ° is shown in fig. 3. The random coincidence background has been subtracted from these data. Also shown in fig. 3 relative to the recoil-proton pulse-height spectrum are the Compton spectra of the 1.28 MeV gamma-rays from sodium-22 and the 2.62 MeV gamma-rays from the thorium-228. These Compton spectra, which are generated in the D1 scintillator, serve as calibration points for determining the light output of the recoil protons relative to electrons. Since large scintillators are used in this spectrometer, it is necessary to have calibration points, such as the peaks of the Compton spectra, which are insensitive to the pulse height resolution of the detector. The Compton peak is taken to correspond to the maximum Compton electron energy produced by the associated gamma-ray. For a gamma-ray with energy Er, the maximum Compton electron energy T max is given by T max
=
E~/(1 +moc2/2Er),
(1)
where moc2=0.51 MeV. From eq. (1), the maximum electron energies that correspond to 0.51, 1.28 and 2.62 MeV gamma-rays are 0.34, 1.07 and 2.39 MeV, respectively. The peak positions and the estimated uncertainties in determining the peak positions are indicated in fig. 3. To determine the response of NE-228 relative to NE-102, and also to ascertain that a calibration technique based on the Compton peaks would give representative results, we measured the response of a 2½ in. diam. by 2½ in. high NE-102 plastic scintillator to 3.5, 5.8 and 10.5 MeV protons in the same manner as that described for the NE-228 liquid scintillator. The response of NE-102 to protons has been measured by several authors 4-6) and calculated by Gooding and Pugh7). 3. Analysis The light output of electrons in organic scintillators is known to be a linear function of energy for electrons above approximately 100 keV; therefore, there is a linear relationship between the Compton electron energies and the associated channel numbers of the Compton pulse-height spectra. The analyzer was calibrated in units of equivalent-electron energy by associating the channel numbers with the electron energies of the peaks of the Compton spectra. The equivalentelectron energy corresponding to the channel number of the pulse-height peak of each recoil-proton spectrum was then given by this calibration. The equivalentelectron energies determined for the 3.5, 5.8 and
TO
3.5,
5.8 A N D
10.5 M e V
PROTONS
255
TABLE 1 T h e response o f NE-228 a n d NE-102 scintillators to protons. P r o t o n energy Tp(MeV)
Equivalent-electron energy, Te(MeV) in NE-228 in NE-102
3.5 + 0.2 5.8 + 0.2 10.5 _ 0.2
0.99 + 0.04 2.14 + 0.08 5.06 + 0.09
1.27 + 0.13 2.53 + 0.21 5.60 + 0.14
10.5 MeV recoil protons in both NE-228 and NE-102 scintillator are listed in table 1. The uncertainty in the equivalent-electron energy represents the uncertainty in determining the position of the peak of the recoilproton spectrum. The uncertainty in the recoil-proton energy represents an estimated uncertainty of _+1° in the neutron scattering angle. The response of both NE-228 and NE-102 are plotted in fig. 4. In this figure, the solid line is the calculated response of Gooding and PughT). 4. Conclusion it is evident from fig. 4 that the measured response of NE-102 plastic scintillator agrees with the calculated response of Gooding and Pugh7). This agreement indicates that energy calibrations of large detectors can be carried-out by using the peaks of the gamma-ray Compton spectra as calibration points. The response
NE-228
/
+ NE-I02 -- CALCULATED RESPONSE
:E
v
F~
.:/ .7"
>_r~ W Z W Z 0 n~ 0 hi -J W
Z Ixl _J
0 /:-E4
°o
i
;,
i
~
i
,2
PROTON ENERGY, Tp (MeV) Fig. 4. T h e response o f b o t h NE-102 plastic a n d Ne-228 liquid scintillator to 3.5, 5.8 a n d 10.5 M e V protons. T h e solid curve is the calculated r e s p o n s e o f G o o d i n g a n d Pugh.
256
R. M A D E Y
AND
o f NE-228 liquid scintillator is less t h a n the response o f NE-102 plastic scintillator. The response o f NE-228 relative to NE-102 for 3.5, 5.8 a n d 10.5 M e V p r o t o n s is 78, 85, a n d 91 percent, respectively. W e are grateful to the N a t i o n a l R e s e a r c h Council, O t t a w a , C a n a d a and to Dr. K. W. Geiger for use o f the 14 M e V n e u t r o n - g e n e r a t o r facility. W e t h a n k Mr. D. Elliott for his c o o p e r a t i o n in c o n n e c t i o n with the o p e r a t i o n o f the n e u t r o n generator. This w o r k was s u p p o r t e d in p a r t by the U.S. A t o m i c E n e r g y C o m mission u n d e r c o n t r a c t AT(30-1)-3914.
F. M. W A T E R M A N
References 1) R. Madey, IEEE Trans. Nucl. Sci. NS-15, no. 3 (1968) 426. 2)D. H. Osborn and R. Madey, CONF-691101, Proc. 2nd Intern. Conf. Accelerator dosimetry and experience (Stanford Linear Accelerator Center, Stanford, Calif., Nov. 5 7, 1969). a) R. Madey, Proc. Intern. Syrup. Nuclear electronics and radioprotection (Toulouse, France, March 1968). 4) G. D. Badhwar, C. L. Deney, B. R. Dennis and M. F. Kaplon, Nucl. Instr. and Meth. 57 (1967) 116. 5) H. C. Evans and E. H. Bellamy, Proc. Phys. Soc. 74 (19.59) 483. 6) D. L. Smith, R~ G. Polk and T. G. Miller, Nucl. Instr. and Meth. 64 (1968) 157. 7) T. J. Gooding and H. G. Pugh, Nucl. Instr. and Meth. 7 (1960) 189; 11 (1961) 365.