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The response of plastic scintillator to stopping protons in the energy range 70 to 100 MeV
NUCLEAR
INSTRUMENTS
AND
METHODS
i33 (1976) 311-314;
if)
NORTH-HOLLAND
PUBLISHING
CO.
T H E R E S P O N S E OF P L A S T I C S C I N T I L L A T O R T O S T O P P I N G P R O T O N S IN THE E N E R G Y RANGE 70 TO 100 MeV B. M. P R E E D O M *
Department ~[ Physics, Unicersity q[' South Carolina, Columbia, South Carolina 29208, U.S.A. M.J. SALTMARSH, C.A. LUDEMANN
Oak Ridge National Laboratoo 't, Oak Ridge, Tennessee 37830, U.S.A. and J. ALSTER
Department 0[" Physics, TeI-Acir Unicersity, TeI-Ac, ic, Is'rael Received 13 November 1975 The response o f plastic scintillator has been measured for monoenergetic proton beams of 70, 80, 90 and 100 MeV. Results are presented for the tail-to-peak ratio as a function of the cutoff energy below the peak.
1. Introduction Plastic scintillator detectors are often used to detect energetic protons. As with all detectors used for quantitative measurements, it is necessary to know how the detector responds to the particles being detected. The response is affected by specific geometrical effects and non-uniformities in the density of the scintillator, the reflection surface, and the photocathode. In addition, the response can be affected by nuclear reactions induced by the detected particle within the detector itself. These reactions in most cases remove the particle from the primary energy peak, thus resulting in a loss of counts in that peak while producing a background that is dependent both on the composition of the detector and the incident energy of the particle. Measurements of this effect of reactions in plastic scintillator have been made up to an incident proton energy of 46 MeV 1). Calculations have been made by Measday and Richard-Serre 2) which fit the existing data and allow one to extrapolate to higher energies. However, in order to make corrections for this effect to the order of 1%, it was decided that the calculations should be checked at higher energies. Thus, we have studied the response of our plastic scintillator detectors to protons of energies 70, 80, 90 and 100 MeV. 2. Experimental procedure The proton beam used in these measurements was * Work performed under Office of Naval Research Contract No. N0014-72-A-0455-0002. + Operated by Union Carbide Corp. for the U.S.E.R.D.A.
produced by the isochronous cyclotron at the University of Maryland Cyclotron Laboratory. A beam of protons with an energy of (100.9+0.1)MeV was extracted from the cyclotron. In order to quickly change the energy, a wedge-shaped absorber was placed between the cyclotron and the first analyzing magnet. The absorber could be moved remotely and the resulting energy degradation of the beam determined by the beam analysis system3). The energies used in the measurements reported here were (100.9+0.1) MeV, (90.0___0.1) MeV, (80.0__.0.1) MeV, and (70.0-1-0.1) MeV. The energy analysis system consisted of two 90 ° bending magnets. A slit was placed at the focal point for the analysis system. A beam passing through this slit then proceeded to a steering magnet. The beam was bent through an angle of 22 ~ and then transported approximately 15 m to our detector. There were no slits in the beam line after the steering magnet. From the dispersion of the steering magnet it was calculated that the spatial dispersion of the beam at our detector was 400 keV/cm, thus insuring essentially a monoenergetic beam. The beam intensity was approximately 102 protons/s. The detectors used in these measurements were of two types. One was in the shape of a cone (front diam. = 1", back diam. -- 3", length -- 6") and the other, a truncated wedge (front face = 3.5" x 5", back face = 5" x 5" and length - 4"). The cone-shaped detector and its response to pions has been described previously4). Both detectors were made of NEI02A plastic scintillator by Nuclear Enterprises, Inc. In order to define the
312
B.M.
'
'~'
3. Results
.....~:17,/~:'~':
Differential spectra obtained at 70, 80, 90, and 100 MeV for the conical detector are shown in fig. 1. The resolution (fwhm) of the primary proton peak varies from 1.5 MeV at 70.0 MeV to 2.0 MeV at 100.0 MeV and the peak-to-valley ratios change from ~350:1 at 7 0 M e V to ~250:1 at 100MeV. The truncated wedge-shape detector had intrinsically worse resolution. The resolution varied from 2.5 MeV at 70 MeV to 3.0 MeV at 100 MeV and the peak to valley ratio varied from 300:1 at 70 MeV to 200:1 at
Z 0 (J
IO'=.
Ep = 90.0± 0.1MeV
:! i
•
et al.
beam direction to insure that only the centers of the detectors were illuminated, a thin AE detector (0.125" thick, 0.7545" diameter) was placed in front of each stopping detector and a coincidence between the two detectors was required. The threshold for an acceptable AE pulse was varied across the proton peak and no statistically significant change was observed in the response spectrum of the stopping detector,
Ep ; IOO.9 ± 0,I MeV
.i~~ :"
PREEDOM
-..~;',"/*'
406
D, 3:;7. • ..
•
.•7..
M,fi_
m m
"i
405 =
I03
Ep: 80.0 * 0.I MeV
IC~
t:~'r:."
j
"
-
.~ :~!,
/
-
m
i Ep=70.0±0A
E 403
g
tO' -.~.':.
z
j
o
MeV
-
/ Ep,'70.O-',QIMeVi ....,~/%~...l ..~:.
.',;~.; i
.
102 .
.
.
.'.
0
400
200
300
400
500
CHANNEL NUMBER
Fig. 2. Integral spectra for rnonoenergetic protons incident on a plastic scintillation detector. Each channel equals the s u m o f the counts in all lower channels, CHANNEL
NUMBER
Fig. 1. Differential spectra for monoenergetic p r o t o n s incident o n a plastic scintillation detector.
RESPONSE
OF
PLASTIC
100 MeV. The 4.44 MeV state of ~2C is seen as a shoulder on the low energy side of the primary peak. Also characteristic of the spectra is the excitation of the giant dipole states around 18 to 20 MeV excitation in the ~2C nucleusS). In fig. 2, integral spectra are shown for the data at 70.0 MeV and at 100 MeV. Each channel in these spectra is the sum of the counts in all lower channels. From these integral spectra, it is a simple matter to calculate the number of counts above vs the number of counts below any cutoff channel. From the integral spectra, the ratio of the number of counts in the tail to the number of counts in the peak [f/(1 - f ) ] was formed. This ratio is plotted in fig. 3 for different cutoffs below the primary peak at each incident proton energy. The same empirical curve is drawn through the data at each energy. The fact that ~2 .
.
.
.
.
~
[
~0 8
•
E
COUNTER
~
~
~
el
o c COUNTER
SLOPE ~ t % / 4 MeV
/
40
25
45 20 CUTOFF BELOW PEAK (MeV)
Fig. 3. E m p i r i c a l t a i l - t o - p e a k r a t i o f o r d i f f e r e n t i n c i d e n t energies
SCINTILLATOR
313
the same curve fits the data at each energy indicates that the cross-sections for all of the allowed nonelastic reactions have approximately the same energy dependence between 70 and 100 MeV. A favorable feature of these curves is the small slope (~0.25% /MeV). Because of this slope, quite accurate corrections can be made for the protons removed from the primary peak, i.e. if the cutoff below the primary peak is known to better than 2 MeV, then the correction is known to better than 0.5%. Also shown in fig. 3 are ratios obtained for the truncated wedge detector. The difference in slope is attributed to both the worse resolution of this detector and a non-uniform light collection efficiency from the front to the back of the detector. However, it should be noted that the differences between the conical detector and the truncated wedge are always less than 1% in the tail-to-peak ratio. The present data and those of Baker et al.1) are compared with the calculations of Measday and Richard-Serre 2) in fig. 4. There is general agreement between the data and the calculations with the largest discrepancy being 0.6% in the tail-to-peak ratio. However, a disturbing feature in fig. 4 is that the slope of the data is rather different than that of the calculations, indicating that the error one might make in using the calculations above 100 MeV could be substantial. If the data for the truncated wedge is used, this discrepancy is diminished due to the lower value of the 100 MeV point. It is obvious that data must be taken at energies above 100 MeV if one needs to know this tail-to-peak correction to better than a few percent.
as a function of cutoff below the peak. The "E Counter" is the truncated cone and the "C Counter" is the truncated wedge.
4. Summary
,4,
• C.A. BAKER e t a / . N.I.M. 711;117(f969)(IO-MeV CUTOFF) i • PRESENT DATA (10 MeV CUTOFF) ~2 '[~CALC BY MEASDAY AND RICHARD-SERRE N.I.M 76; ] 45 (tqF, q) (4 44 ]NCI tlBFh IN PF AK USFD I 10 El-
1
-,
I 8 ~
J 6
0
10
20
50 40 50 60 ?0 PROTON ENERGY (MeV)
80
90
t00
Fig. 4. Tail-to -p eak ratio for a 10 MeV cutoff below the peak as a function of the incident energy. The curve is that calculated in ref. 2.
By using a monoenergetic proton beam incident on a plastic scintillator detector, the response of the detector has been measured. These measurements allow one to make corrections for the protons removed from a peak in a spectrum due to all inelastic processes. This correction can be made to better than 1% accuracy. A comparison of the present results with calculations by Measday and Richard-Serre shows generally good agreement with the indication, however, of a need to extend the measurements to higher energies. The authors wish to thank the entire technical and scientific staff of the University of Maryland Cyclotron Laboratory for their hospitality and assistance. Their cooperation and support enabled us to make these measurements with a minimum amount of time lost
314
B.M. PREEDOM et al.
due to our lack of familiarity with their laboratory. Also, we wish to thank J. Dussart of the Los Alamos IVleson Physics Facility for his help in crating and shipping our electronics and detectors from Los Alamos to the University of Maryland. References 1) C.A. Baker, B.E. Bonner, I. M. Blair, F.P. Brady, J . A .
~) ;3) 4) ~)
Edgington and V.J. Howard, Nuch Instr. and Meth. 71 (1969) 117. D. F. Measday and C. Richard-Serre, Nucl. Instr. and Meth. "/6 (1969) 45. R. E. Berg, Univ. of Maryland Cyclotron Laboratory Progress Report, (1974) p. 146 (unpublished). M.J. Saltmarsh, B.M. Preedom, R . D . Edge and C.W. Darden, Nucl. Instr. and Meth. 105 (1972) 311. M. Buenerd, P. de Saintignon, P. Martin and J. M. Loiseaux, Phys. Rev. Lett. 33 (1974) 1233.
INSTRUMENTS
AND
METHODS
i33 (1976) 311-314;
if)
NORTH-HOLLAND
PUBLISHING
CO.
T H E R E S P O N S E OF P L A S T I C S C I N T I L L A T O R T O S T O P P I N G P R O T O N S IN THE E N E R G Y RANGE 70 TO 100 MeV B. M. P R E E D O M *
Department ~[ Physics, Unicersity q[' South Carolina, Columbia, South Carolina 29208, U.S.A. M.J. SALTMARSH, C.A. LUDEMANN
Oak Ridge National Laboratoo 't, Oak Ridge, Tennessee 37830, U.S.A. and J. ALSTER
Department 0[" Physics, TeI-Acir Unicersity, TeI-Ac, ic, Is'rael Received 13 November 1975 The response o f plastic scintillator has been measured for monoenergetic proton beams of 70, 80, 90 and 100 MeV. Results are presented for the tail-to-peak ratio as a function of the cutoff energy below the peak.
1. Introduction Plastic scintillator detectors are often used to detect energetic protons. As with all detectors used for quantitative measurements, it is necessary to know how the detector responds to the particles being detected. The response is affected by specific geometrical effects and non-uniformities in the density of the scintillator, the reflection surface, and the photocathode. In addition, the response can be affected by nuclear reactions induced by the detected particle within the detector itself. These reactions in most cases remove the particle from the primary energy peak, thus resulting in a loss of counts in that peak while producing a background that is dependent both on the composition of the detector and the incident energy of the particle. Measurements of this effect of reactions in plastic scintillator have been made up to an incident proton energy of 46 MeV 1). Calculations have been made by Measday and Richard-Serre 2) which fit the existing data and allow one to extrapolate to higher energies. However, in order to make corrections for this effect to the order of 1%, it was decided that the calculations should be checked at higher energies. Thus, we have studied the response of our plastic scintillator detectors to protons of energies 70, 80, 90 and 100 MeV. 2. Experimental procedure The proton beam used in these measurements was * Work performed under Office of Naval Research Contract No. N0014-72-A-0455-0002. + Operated by Union Carbide Corp. for the U.S.E.R.D.A.
produced by the isochronous cyclotron at the University of Maryland Cyclotron Laboratory. A beam of protons with an energy of (100.9+0.1)MeV was extracted from the cyclotron. In order to quickly change the energy, a wedge-shaped absorber was placed between the cyclotron and the first analyzing magnet. The absorber could be moved remotely and the resulting energy degradation of the beam determined by the beam analysis system3). The energies used in the measurements reported here were (100.9+0.1) MeV, (90.0___0.1) MeV, (80.0__.0.1) MeV, and (70.0-1-0.1) MeV. The energy analysis system consisted of two 90 ° bending magnets. A slit was placed at the focal point for the analysis system. A beam passing through this slit then proceeded to a steering magnet. The beam was bent through an angle of 22 ~ and then transported approximately 15 m to our detector. There were no slits in the beam line after the steering magnet. From the dispersion of the steering magnet it was calculated that the spatial dispersion of the beam at our detector was 400 keV/cm, thus insuring essentially a monoenergetic beam. The beam intensity was approximately 102 protons/s. The detectors used in these measurements were of two types. One was in the shape of a cone (front diam. = 1", back diam. -- 3", length -- 6") and the other, a truncated wedge (front face = 3.5" x 5", back face = 5" x 5" and length - 4"). The cone-shaped detector and its response to pions has been described previously4). Both detectors were made of NEI02A plastic scintillator by Nuclear Enterprises, Inc. In order to define the
312
B.M.
'
'~'
3. Results
.....~:17,/~:'~':
Differential spectra obtained at 70, 80, 90, and 100 MeV for the conical detector are shown in fig. 1. The resolution (fwhm) of the primary proton peak varies from 1.5 MeV at 70.0 MeV to 2.0 MeV at 100.0 MeV and the peak-to-valley ratios change from ~350:1 at 7 0 M e V to ~250:1 at 100MeV. The truncated wedge-shape detector had intrinsically worse resolution. The resolution varied from 2.5 MeV at 70 MeV to 3.0 MeV at 100 MeV and the peak to valley ratio varied from 300:1 at 70 MeV to 200:1 at
Z 0 (J
IO'=.
Ep = 90.0± 0.1MeV
:! i
•
et al.
beam direction to insure that only the centers of the detectors were illuminated, a thin AE detector (0.125" thick, 0.7545" diameter) was placed in front of each stopping detector and a coincidence between the two detectors was required. The threshold for an acceptable AE pulse was varied across the proton peak and no statistically significant change was observed in the response spectrum of the stopping detector,
Ep ; IOO.9 ± 0,I MeV
.i~~ :"
PREEDOM
-..~;',"/*'
406
D, 3:;7. • ..
•
.•7..
M,fi_
m m
"i
405 =
I03
Ep: 80.0 * 0.I MeV
IC~
t:~'r:."
j
"
-
.~ :~!,
/
-
m
i Ep=70.0±0A
E 403
g
tO' -.~.':.
z
j
o
MeV
-
/ Ep,'70.O-',QIMeVi ....,~/%~...l ..~:.
.',;~.; i
.
102 .
.
.
.'.
0
400
200
300
400
500
CHANNEL NUMBER
Fig. 2. Integral spectra for rnonoenergetic protons incident on a plastic scintillation detector. Each channel equals the s u m o f the counts in all lower channels, CHANNEL
NUMBER
Fig. 1. Differential spectra for monoenergetic p r o t o n s incident o n a plastic scintillation detector.
RESPONSE
OF
PLASTIC
100 MeV. The 4.44 MeV state of ~2C is seen as a shoulder on the low energy side of the primary peak. Also characteristic of the spectra is the excitation of the giant dipole states around 18 to 20 MeV excitation in the ~2C nucleusS). In fig. 2, integral spectra are shown for the data at 70.0 MeV and at 100 MeV. Each channel in these spectra is the sum of the counts in all lower channels. From these integral spectra, it is a simple matter to calculate the number of counts above vs the number of counts below any cutoff channel. From the integral spectra, the ratio of the number of counts in the tail to the number of counts in the peak [f/(1 - f ) ] was formed. This ratio is plotted in fig. 3 for different cutoffs below the primary peak at each incident proton energy. The same empirical curve is drawn through the data at each energy. The fact that ~2 .
.
.
.
.
~
[
~0 8
•
E
COUNTER
~
~
~
el
o c COUNTER
SLOPE ~ t % / 4 MeV
/
40
25
45 20 CUTOFF BELOW PEAK (MeV)
Fig. 3. E m p i r i c a l t a i l - t o - p e a k r a t i o f o r d i f f e r e n t i n c i d e n t energies
SCINTILLATOR
313
the same curve fits the data at each energy indicates that the cross-sections for all of the allowed nonelastic reactions have approximately the same energy dependence between 70 and 100 MeV. A favorable feature of these curves is the small slope (~0.25% /MeV). Because of this slope, quite accurate corrections can be made for the protons removed from the primary peak, i.e. if the cutoff below the primary peak is known to better than 2 MeV, then the correction is known to better than 0.5%. Also shown in fig. 3 are ratios obtained for the truncated wedge detector. The difference in slope is attributed to both the worse resolution of this detector and a non-uniform light collection efficiency from the front to the back of the detector. However, it should be noted that the differences between the conical detector and the truncated wedge are always less than 1% in the tail-to-peak ratio. The present data and those of Baker et al.1) are compared with the calculations of Measday and Richard-Serre 2) in fig. 4. There is general agreement between the data and the calculations with the largest discrepancy being 0.6% in the tail-to-peak ratio. However, a disturbing feature in fig. 4 is that the slope of the data is rather different than that of the calculations, indicating that the error one might make in using the calculations above 100 MeV could be substantial. If the data for the truncated wedge is used, this discrepancy is diminished due to the lower value of the 100 MeV point. It is obvious that data must be taken at energies above 100 MeV if one needs to know this tail-to-peak correction to better than a few percent.
as a function of cutoff below the peak. The "E Counter" is the truncated cone and the "C Counter" is the truncated wedge.
4. Summary
,4,
• C.A. BAKER e t a / . N.I.M. 711;117(f969)(IO-MeV CUTOFF) i • PRESENT DATA (10 MeV CUTOFF) ~2 '[~CALC BY MEASDAY AND RICHARD-SERRE N.I.M 76; ] 45 (tqF, q) (4 44 ]NCI tlBFh IN PF AK USFD I 10 El-
1
-,
I 8 ~
J 6
0
10
20
50 40 50 60 ?0 PROTON ENERGY (MeV)
80
90
t00
Fig. 4. Tail-to -p eak ratio for a 10 MeV cutoff below the peak as a function of the incident energy. The curve is that calculated in ref. 2.
By using a monoenergetic proton beam incident on a plastic scintillator detector, the response of the detector has been measured. These measurements allow one to make corrections for the protons removed from a peak in a spectrum due to all inelastic processes. This correction can be made to better than 1% accuracy. A comparison of the present results with calculations by Measday and Richard-Serre shows generally good agreement with the indication, however, of a need to extend the measurements to higher energies. The authors wish to thank the entire technical and scientific staff of the University of Maryland Cyclotron Laboratory for their hospitality and assistance. Their cooperation and support enabled us to make these measurements with a minimum amount of time lost
314
B.M. PREEDOM et al.
due to our lack of familiarity with their laboratory. Also, we wish to thank J. Dussart of the Los Alamos IVleson Physics Facility for his help in crating and shipping our electronics and detectors from Los Alamos to the University of Maryland. References 1) C.A. Baker, B.E. Bonner, I. M. Blair, F.P. Brady, J . A .
~) ;3) 4) ~)
Edgington and V.J. Howard, Nuch Instr. and Meth. 71 (1969) 117. D. F. Measday and C. Richard-Serre, Nucl. Instr. and Meth. "/6 (1969) 45. R. E. Berg, Univ. of Maryland Cyclotron Laboratory Progress Report, (1974) p. 146 (unpublished). M.J. Saltmarsh, B.M. Preedom, R . D . Edge and C.W. Darden, Nucl. Instr. and Meth. 105 (1972) 311. M. Buenerd, P. de Saintignon, P. Martin and J. M. Loiseaux, Phys. Rev. Lett. 33 (1974) 1233.