- Email: [email protected]
Gamma calibration of organic scintillators
Nuclear Instruments and Methods in Physics Research A281 (1989) 349-352 North-Holland, Amsterdam
349
GAMMA CALIBRATION OF ORGANIC SCINTILLATORS R. CHERUBINI
t) ,
G. MOSCHINI
t,Z),
R. NINO
1 ),
R. POLICRONIADES' ) and A. VARELA ,3
1~ 3)*
INFN, Laboratori Nazionali di Legnaro, 35020 Legnaro (Padova), Italy zt Dipartimento di Fisica "G. Galilei"; Università degli Studi, 35100 Padova, Italy s) ININ, Instituto Nacional de Investigaciones Nucleares, Mexico DF, Mexico ')
Received 3 April 1989 The electron light responses of three organic scintillator detectors - a stilbene (3 .81 1 .27 cm ) and two NE102A (5 .0 2.5 cm and 5 .0 7.6 were systematically studied by means of a set of gamma sources. The edge maximum and half-height point energies of the Compton distributions have been determined by the use of a fast/slow coincidence arrangement. which permits taking adequate spectra of the maximum energy Compton electrons. According to these measurements, the maximum energy Compton electrons are located 21 .1±1 .4% below the half-height Compton edge point for stilbene, and 4.8±1 .4% and 6 .6±1 .5% above the Compton edge maxima for the NE102A scintillators, respectively. X
X
CMZ)
z
X
z
-
1. Introduction In the course of some scintillator absolute proton pulse height and neutron efficiency measurements [1] using the associated particle method - we had the problem of normalizing pulse heights from one run to another, and also the need to have a precise determination of the bias settings (B) imposed on the detector responses. These two parameters are related to each other and may have significant variations during an experiment . Since the relative neutron detection efficiency varies as (1 -B/EJ (see e.g. ref. [2]), where En is the neutron's energy, a precise value of B is important to obtain an adequate calibration . Usually, B is determined calibrating the scintillator's light output by using the Compton edge of a set of gamma sources [3]. Normally, the pulse height associated with the maximum energy recoil electrons is taken at a point in between the edge's maximum and the "half maximum" of the actual Compton distribution [4]. These practices introduce some energy uncertainties that are important if the proton equivalent energies are to be determined from the scintillator's electron response . In this paper we present an experimental arrangement for obtaining an accurate electron calibration of an organic scintillator by means of a fast/slow coincidence system . The precise centroid determinations of the maximum energy Compton electron peaks obtained with this
* On leave : ICTP/IAEA Fellow . 0168-9002/89/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
method permit to establish unambiguously the electron energy relations for an organic scintillator .
2. Experimental arrangement It is a known fact that an organic scintillator's electron light response is linear for electron energies >_ 100 keV, and may be expressed by P(E) = c(Ec Eo), where P(E) is the relative pulse height, E. the Compton electron energy, Eo the energy intercept obtained from the extrapolation of the linear portion and c is a parameter that can be chosen to normalize the pulse height scale. Fig. 1 shows the experimental geometry and the fast/slow coincidence system used . In order to obtain the required information, a 3 .81 cm diameter and 1.27 cm thick stilbene scintillator and a 4.8 cm diameter and 1 .0 cm thick NE104 plastic scintillator were used, at the INFN Laboratori Nazionali di Legnaro, Padova, Italy, as a detector for the Compton electrons and the 180' backscattered photons, respectively . Timing signals were derived from each photomultiplier anode. These fast signals were connected to the start and stop inputs of TAC#1 . The linear signals from both amplifiers were passed through two timing single channel analysers (TSCA) defining the windows shown in fig. 1. At this stage, fast signals were demanding a coincidence in TAC # 2. Single channel analyser (SCA) windows were placed around the coincidence peaks from TAC # 1 and TAC#2. At each measurement, equal counting areas from TAC # 1 and TAC # 2 outputs were imposed . A slow coincidence was required
R. Cherubini et al. / Gamma calibration of organic scintillators
350
EMax
ENERGY
Eii2
Fig. 2. Schematic drawing of a typical Compton distribution and its associated gamma coincidence spectrum, with relevant parameters.
between the TAC outputs. This coincidence was then used to gate a slow linear signal from the stilbene detector .
For the NE102A plastic scintillator detectors (2 .5 x 5 .0 cm2 and 5.0 x 7.6 cm,2, diameter x thickness, respectively), a similar coincidence system was used . The information for these detectors was obtained in experi-
Fig. 1. Electronics and experimental setup. TPC: time pickoff control; AMP DDL: double delay amplifier ; Delay AMP: delay amplifier ; LSD: logic shaper and delay; TSCA : timing single channel analyser ; TAC: time to pulse height converter; SLOW COINC L.G . : slow coincidence and linear gate ; ADC: analog to digital converter.
ments performed at the ININ, Tandem Laboratory, Mexico, in which case two NE102A scintillators were used for the experimental arrangement depicted in fig. 1.
Table 1 shows the gamma sources used and the maximum Compton electron energies (Ec = 2E ~2,/(0 .511 + 2EY) E in MeV) . In all cases, easily available gamma sources, constructed for calibration purposes, were used .
Table 1 Gamma sources used for electron light response measurement of the stilbene and NE102A scintillator detectors Source
EY [MeV]
E. [MeV]
24
0.060 0.279 0.344 a~ 0.511 1.275 0.662 0.835 1.253 a>
0.011 0.146 0.197 0.341 1 .062 0.477 0.639 1 .041
'Am 203 Hg 133 22 22
Ba Na
Na 137C s 54
Mn
60 Co a>
60 133B a (used at ININ) the weighted mean For Co and energies were taken as the characteristic energies. 203 Hg, 13 'Cs, As a first step, gamma ray sources of
Na were observed taking the maximum energy recoil electron spectrum as described formerly . The entire Compton distribution was simultaneously recorded on a Silena Cicero 84 multichannel analyser 60
Co and
22
for the stilbene detector and on a Canberra Series 88 multiparameter analyser for the NE102A detectors.
Weighted mean energy .
54M
c
n
' 3 'Cs
0 U
channel
Fig. 3 . Typical results for
54
2°6
'51'
channel
200 -5-
x 7 .6 Mn and 13'Cs gamma sources. The normal Compton and the coincidence spectra, taken with the 5.0 cm2 NE102A detector, are shown.
R. Cherubini et al / Gamma calibration of organic scintillators
351
Table 2 Relevant results for electron light response of stilbene and NE102A detectors (the energies are given in keV) . In each case : AE m =(E,-E m) x 100/E, [%]; AEI/2 =(EI 2 -E~)x100/Ec [%] Source
203 Hg 133 B a 22N a 22 Na 137C s 54
Mn
60 Co
Stilbene
NE102A (2 .5 x 5.0 cm 2 )
Em
E1/2
AE_
AEI /2
Em
327 1071 456 62l 1026
172 422 1295 573 765 1274
20 .0 7.0 0.8 4.5 2.8 1.5
18 .3 23 .9 22 .0 20 .0 19 .7 22 .4
155 323 1028 454 596 1001
1l7
EI /2
227 372 1112 503 650 1l72
Fig. 2 shows a schematic drawing of a Compton distribution and its associated gamma coincidence spectrum, with the parameters involved in the calibration procedure. Typical experimental results for 54 Mn and 137 Cs are reported in fig. 3. The peak pulse heights in the coincidence spectra were fixed taking a weighted
mean between channels containing 80% of the maximum countings . Figs . 4a and 4b show the electron responses obtained with this method for the stilbene and the 2.5 x 5.0 cmZ NE102A detectors, respectively .
In agreement with the symbols depicted in fig. 2, table 2 shows the relevant results, in units of maximum 1200 1000C7 a w z
800-
0
400 -
0 U
200-
W z
0
AE_
AEI /2
Em
E1/2
AE_
AEI / 2
21 .2 5 .1 3 .2 3.9 6.8 3.8
15 .2 9.1 4.7 5.4 1.8 12 .6
319 1014 443 592 -
388 1141 516 681 -
6.5 4.5 7.3 8.1 -
13 .8 7.4 7.9 5.8 -
recoil Compton energies, for the maximum and the one-half values on the Compton distributions for each one of the detectors considered . According to these data, the maximum energy Compton electrons are
located 21 .1 ± 1.4% below the half-height Compton edge point for stilbene, and 4.8 ± 1 .4% and 6.6 ± 1 .5% above the Compton edge maxima for the NE102A scintillators of 2.5
x 5.0
cm 2 and 5.0
x
7.6 cmZ , respectively.
3. Conclusions Considering that the uncertainties here reported are yet significant and dependent on detector resolution and size, unless one is satisfied with an approximate calibration, it is advisable to perform calibration measurements for each particular case.
STILBENE
Another problem arises from the fact that e° Co and 133Ba present double close lines, for which a weighted
600 -
0
NE102A (5 .0 x 7.6 cm2 )
mean energy might not be adequate as an effective parameter in the calibration procedure. For 54 Mn we have neglected corrections concerning its high continuum background . 200 " 400 " 600 . 800 CHANNELS
1000
In practice, the edge maximum or the half-maximum energy point of a Compton distribution is to be chosen
as actual reference, once the calibration procedure has been established as previously described. In other words, the maximum Compton electron energy position can be referred either to the edge maximum or to the half-maximum depending on the more precise definition (cf. ref.
[5]) . In this work we have chosen the half-maximum for the stilbene gamma calibration and the edge maximum of the actual Compton distribution in the case of the NE102A gamma calibrations .
References Fig. 4. Electron responses of the stilbene and NE102A (2 .5 x 5.0 cm2) scintillator detectors .
[Il R. Cherubini, G. Moschini, R. Nino, R. Policroniades and A. Varela, Nucl . Instr. and Meth . A269 (1988) 623,
352
R. Cherubini et al. / Gamma calibration of organic scintillators
[2] E. Finckh, in : Nuclear Spectroscopy and Reactions, part B ed . J. Cerny (Academic Press, New York and London, 1974) chap. VI . A. [3] V.V. Verbinsky, W.R . Burrus, T.A . Love, W. Zobel and N.W . Hill, Nucl. Instr. and Meth . 65 (1968) 8.
[4] G. Dietze and H. Klein, Nucl . Instr . and Meth. 193 (1982) 549. [5] H.H . Knox and F.G . Miller, Nucl . Instr. and Meth . 101 (1972) 519.
349
GAMMA CALIBRATION OF ORGANIC SCINTILLATORS R. CHERUBINI
t) ,
G. MOSCHINI
t,Z),
R. NINO
1 ),
R. POLICRONIADES' ) and A. VARELA ,3
1~ 3)*
INFN, Laboratori Nazionali di Legnaro, 35020 Legnaro (Padova), Italy zt Dipartimento di Fisica "G. Galilei"; Università degli Studi, 35100 Padova, Italy s) ININ, Instituto Nacional de Investigaciones Nucleares, Mexico DF, Mexico ')
Received 3 April 1989 The electron light responses of three organic scintillator detectors - a stilbene (3 .81 1 .27 cm ) and two NE102A (5 .0 2.5 cm and 5 .0 7.6 were systematically studied by means of a set of gamma sources. The edge maximum and half-height point energies of the Compton distributions have been determined by the use of a fast/slow coincidence arrangement. which permits taking adequate spectra of the maximum energy Compton electrons. According to these measurements, the maximum energy Compton electrons are located 21 .1±1 .4% below the half-height Compton edge point for stilbene, and 4.8±1 .4% and 6 .6±1 .5% above the Compton edge maxima for the NE102A scintillators, respectively. X
X
CMZ)
z
X
z
-
1. Introduction In the course of some scintillator absolute proton pulse height and neutron efficiency measurements [1] using the associated particle method - we had the problem of normalizing pulse heights from one run to another, and also the need to have a precise determination of the bias settings (B) imposed on the detector responses. These two parameters are related to each other and may have significant variations during an experiment . Since the relative neutron detection efficiency varies as (1 -B/EJ (see e.g. ref. [2]), where En is the neutron's energy, a precise value of B is important to obtain an adequate calibration . Usually, B is determined calibrating the scintillator's light output by using the Compton edge of a set of gamma sources [3]. Normally, the pulse height associated with the maximum energy recoil electrons is taken at a point in between the edge's maximum and the "half maximum" of the actual Compton distribution [4]. These practices introduce some energy uncertainties that are important if the proton equivalent energies are to be determined from the scintillator's electron response . In this paper we present an experimental arrangement for obtaining an accurate electron calibration of an organic scintillator by means of a fast/slow coincidence system . The precise centroid determinations of the maximum energy Compton electron peaks obtained with this
* On leave : ICTP/IAEA Fellow . 0168-9002/89/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
method permit to establish unambiguously the electron energy relations for an organic scintillator .
2. Experimental arrangement It is a known fact that an organic scintillator's electron light response is linear for electron energies >_ 100 keV, and may be expressed by P(E) = c(Ec Eo), where P(E) is the relative pulse height, E. the Compton electron energy, Eo the energy intercept obtained from the extrapolation of the linear portion and c is a parameter that can be chosen to normalize the pulse height scale. Fig. 1 shows the experimental geometry and the fast/slow coincidence system used . In order to obtain the required information, a 3 .81 cm diameter and 1.27 cm thick stilbene scintillator and a 4.8 cm diameter and 1 .0 cm thick NE104 plastic scintillator were used, at the INFN Laboratori Nazionali di Legnaro, Padova, Italy, as a detector for the Compton electrons and the 180' backscattered photons, respectively . Timing signals were derived from each photomultiplier anode. These fast signals were connected to the start and stop inputs of TAC#1 . The linear signals from both amplifiers were passed through two timing single channel analysers (TSCA) defining the windows shown in fig. 1. At this stage, fast signals were demanding a coincidence in TAC # 2. Single channel analyser (SCA) windows were placed around the coincidence peaks from TAC # 1 and TAC#2. At each measurement, equal counting areas from TAC # 1 and TAC # 2 outputs were imposed . A slow coincidence was required
R. Cherubini et al. / Gamma calibration of organic scintillators
350
EMax
ENERGY
Eii2
Fig. 2. Schematic drawing of a typical Compton distribution and its associated gamma coincidence spectrum, with relevant parameters.
between the TAC outputs. This coincidence was then used to gate a slow linear signal from the stilbene detector .
For the NE102A plastic scintillator detectors (2 .5 x 5 .0 cm2 and 5.0 x 7.6 cm,2, diameter x thickness, respectively), a similar coincidence system was used . The information for these detectors was obtained in experi-
Fig. 1. Electronics and experimental setup. TPC: time pickoff control; AMP DDL: double delay amplifier ; Delay AMP: delay amplifier ; LSD: logic shaper and delay; TSCA : timing single channel analyser ; TAC: time to pulse height converter; SLOW COINC L.G . : slow coincidence and linear gate ; ADC: analog to digital converter.
ments performed at the ININ, Tandem Laboratory, Mexico, in which case two NE102A scintillators were used for the experimental arrangement depicted in fig. 1.
Table 1 shows the gamma sources used and the maximum Compton electron energies (Ec = 2E ~2,/(0 .511 + 2EY) E in MeV) . In all cases, easily available gamma sources, constructed for calibration purposes, were used .
Table 1 Gamma sources used for electron light response measurement of the stilbene and NE102A scintillator detectors Source
EY [MeV]
E. [MeV]
24
0.060 0.279 0.344 a~ 0.511 1.275 0.662 0.835 1.253 a>
0.011 0.146 0.197 0.341 1 .062 0.477 0.639 1 .041
'Am 203 Hg 133 22 22
Ba Na
Na 137C s 54
Mn
60 Co a>
60 133B a (used at ININ) the weighted mean For Co and energies were taken as the characteristic energies. 203 Hg, 13 'Cs, As a first step, gamma ray sources of
Na were observed taking the maximum energy recoil electron spectrum as described formerly . The entire Compton distribution was simultaneously recorded on a Silena Cicero 84 multichannel analyser 60
Co and
22
for the stilbene detector and on a Canberra Series 88 multiparameter analyser for the NE102A detectors.
Weighted mean energy .
54M
c
n
' 3 'Cs
0 U
channel
Fig. 3 . Typical results for
54
2°6
'51'
channel
200 -5-
x 7 .6 Mn and 13'Cs gamma sources. The normal Compton and the coincidence spectra, taken with the 5.0 cm2 NE102A detector, are shown.
R. Cherubini et al / Gamma calibration of organic scintillators
351
Table 2 Relevant results for electron light response of stilbene and NE102A detectors (the energies are given in keV) . In each case : AE m =(E,-E m) x 100/E, [%]; AEI/2 =(EI 2 -E~)x100/Ec [%] Source
203 Hg 133 B a 22N a 22 Na 137C s 54
Mn
60 Co
Stilbene
NE102A (2 .5 x 5.0 cm 2 )
Em
E1/2
AE_
AEI /2
Em
327 1071 456 62l 1026
172 422 1295 573 765 1274
20 .0 7.0 0.8 4.5 2.8 1.5
18 .3 23 .9 22 .0 20 .0 19 .7 22 .4
155 323 1028 454 596 1001
1l7
EI /2
227 372 1112 503 650 1l72
Fig. 2 shows a schematic drawing of a Compton distribution and its associated gamma coincidence spectrum, with the parameters involved in the calibration procedure. Typical experimental results for 54 Mn and 137 Cs are reported in fig. 3. The peak pulse heights in the coincidence spectra were fixed taking a weighted
mean between channels containing 80% of the maximum countings . Figs . 4a and 4b show the electron responses obtained with this method for the stilbene and the 2.5 x 5.0 cmZ NE102A detectors, respectively .
In agreement with the symbols depicted in fig. 2, table 2 shows the relevant results, in units of maximum 1200 1000C7 a w z
800-
0
400 -
0 U
200-
W z
0
AE_
AEI /2
Em
E1/2
AE_
AEI / 2
21 .2 5 .1 3 .2 3.9 6.8 3.8
15 .2 9.1 4.7 5.4 1.8 12 .6
319 1014 443 592 -
388 1141 516 681 -
6.5 4.5 7.3 8.1 -
13 .8 7.4 7.9 5.8 -
recoil Compton energies, for the maximum and the one-half values on the Compton distributions for each one of the detectors considered . According to these data, the maximum energy Compton electrons are
located 21 .1 ± 1.4% below the half-height Compton edge point for stilbene, and 4.8 ± 1 .4% and 6.6 ± 1 .5% above the Compton edge maxima for the NE102A scintillators of 2.5
x 5.0
cm 2 and 5.0
x
7.6 cmZ , respectively.
3. Conclusions Considering that the uncertainties here reported are yet significant and dependent on detector resolution and size, unless one is satisfied with an approximate calibration, it is advisable to perform calibration measurements for each particular case.
STILBENE
Another problem arises from the fact that e° Co and 133Ba present double close lines, for which a weighted
600 -
0
NE102A (5 .0 x 7.6 cm2 )
mean energy might not be adequate as an effective parameter in the calibration procedure. For 54 Mn we have neglected corrections concerning its high continuum background . 200 " 400 " 600 . 800 CHANNELS
1000
In practice, the edge maximum or the half-maximum energy point of a Compton distribution is to be chosen
as actual reference, once the calibration procedure has been established as previously described. In other words, the maximum Compton electron energy position can be referred either to the edge maximum or to the half-maximum depending on the more precise definition (cf. ref.
[5]) . In this work we have chosen the half-maximum for the stilbene gamma calibration and the edge maximum of the actual Compton distribution in the case of the NE102A gamma calibrations .
References Fig. 4. Electron responses of the stilbene and NE102A (2 .5 x 5.0 cm2) scintillator detectors .
[Il R. Cherubini, G. Moschini, R. Nino, R. Policroniades and A. Varela, Nucl . Instr. and Meth . A269 (1988) 623,
352
R. Cherubini et al. / Gamma calibration of organic scintillators
[2] E. Finckh, in : Nuclear Spectroscopy and Reactions, part B ed . J. Cerny (Academic Press, New York and London, 1974) chap. VI . A. [3] V.V. Verbinsky, W.R . Burrus, T.A . Love, W. Zobel and N.W . Hill, Nucl. Instr. and Meth . 65 (1968) 8.
[4] G. Dietze and H. Klein, Nucl . Instr . and Meth. 193 (1982) 549. [5] H.H . Knox and F.G . Miller, Nucl . Instr. and Meth . 101 (1972) 519.