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On the detection of low energy γ-rays with plastic scintillators
Nuclear Instruments and Methods 188 ( 1981 ) 473 474 North-Holland Publishing Company
473
ON THE DETECTION OF LOW ENERGY 7-RAYS WITH PLASTIC SCINTILLATORS M, CARLA *, P. MACCIOTTA and S. SERCI lstituto di 1.Tsico dell'Universitd, Cagliori, Italy, lstituto Nazionale di bTsica Nucleare, Sezione di Torino, Italy
Received 9 March 1981
An energy limit of 3 4 keV is achieved for the detection of 3,-rays with plastic scintillators.
Plastic scintillators are not generally used for the detection of 7-rays due to their low efficiency of conversion and light yield as compared to NaI crystals. Indeed they are still the most appropriate detector in experiments for which high counting rates ( > 1 0 6 / s ) , good timing properties and large surfaces are needed. We encountered these problems during the development of a set-up for time-resolved M6ssbauer spectroscopy in which the time difference from the 122 keV and the subsequent 14.4 keV 7-rays from a STCo source had to be measured with a precision of a few nanoseconds with single counting rates exceeding 106/s. In addition, we were also interested in the detection of 7.3 keV conversion electrons from the same source. Up to ~15 keV the major part (~60%) of the T-rays still interact with the carbon atoms of the scintillator by the photoelectric effect and thus produce lines that allow some energy discrimination. In the literature we found that a limit of ~ 1 0 keV was given for plastic scintillators [1,2] and 6 k e V for anthracene [1]. We have therefore investigated whether the recent improvements o f photomultipliers and fast electronics could give a lower limit. We have studied the properties of NE 102 A, NE 110 and Pilot U plastic scintillators, coupled to EMI 9813 KB or Philips XP 2020 photornultipliers. The scintillators were in the form of discs or parallellepipeds, 5 or 3 mm thick, covered with a thin (20 /am) aluminized Mylar foil. Commercial fast electronics (Lecroy) were used for the signal processing. The best results were obtained with a selected EMI
* Visitor of CERN, Geneva, Switzerland. 0 0 2 9 - 5 5 4 X / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 5 0 © 1981 North-Holland
9813 KB photomultiplier coupled to a disc of NE 102 A. Fig. la shows the pulse height spectrum obtained with the 5.9 keV X-rays from a SSFe source. The mean number of photoelectrons corresponding to the peak is 3.5 + 0.25, as inferred from the fitting of the spectrum to a Poisson distribution with the mean number of photoelectrons left as free parameter and from the position o f the one-photoelectron peak. The latter is in part due to the noise o f the photomultiplier, in part to the long-lived states in the scintillator [1]. Fib. l b shows the spectrum obtained with the same source but with the threshold of the discriminator adjusted to a value corresponding to 1.5 photoelectrons. In these conditions the efficiency for the detection o f the 5.9 keV X-rays in (82 + 3)%, as inferred from the shape o f the peak compared to the above Poisson distribution and from a direct measurement of the strength of the source. Fig. l c finally shows the spectrum obtained with the 8.1 keV X-rays from a 6SZn source, always with a threshold at 1.5 photoelectrons. The mean number of photoelectrons is 5.0 + 0.25 and the efficiency (94 + 2)%. In conclusion we think that we have obtained an improvement of about a factor 2 compared to previous measurements and we can put a limit of ~ 3 - 4 keV for the detection of 7-rays with plastic scintillators and with efficiencies greater than 50%. We thank Prof. T. Bressani for useful guidance and advice and Mr. K. Ratz for his help at CERN, where part of this work was done.
M. Caria ctal. /Detection o / l o w energy ~-rays
474
6x104 [ "
References
5xlO4
[ 1 ] t".T. Porter, M.S. Freedman, F. Wagner, Jr. and J.S. Sherman, Nucl. Instr. and Meth. 39 (1966) 35. [2] Z.M. Cho, C.M. Tsai and L.A. Eriksson, IEH£ Trans. Nucl. Sci. NS-22 (1975) 72.
! I
4xlO4 3xlO4
a)
j.
5,gKeV
2 xlO4 L"
:/"
",%
! Z Z
104 L_ t t ~
i
I LP a~ LLI
o L
U')
2 xlO4 1
2
"..
3
'-.....,.
photoelectrons
"~"'"~
k~
J. . . . . 5 9 KeV
Z 0 (D
I
5;'"~""...,
b)
I04 0
"'~'~---..k .... 8,1 KeY
2x104 !-
f
,04
2':
0 0
".,
C) %
50 LO0 CHANNEL
L5O
Fig. 1. Pulse height spectra obtained with a scintillator NE 102A coupled to an EMI 9813 KB photomultiplier when irradiated (a) with 5.9 keV X-rays, no threshold; (b) with 5.9 keV X-rays, threshold at 1.5 photoelectrons; (c) with 8.1 keV X-rays, threshold at 1.5 photoelectrons.
473
ON THE DETECTION OF LOW ENERGY 7-RAYS WITH PLASTIC SCINTILLATORS M, CARLA *, P. MACCIOTTA and S. SERCI lstituto di 1.Tsico dell'Universitd, Cagliori, Italy, lstituto Nazionale di bTsica Nucleare, Sezione di Torino, Italy
Received 9 March 1981
An energy limit of 3 4 keV is achieved for the detection of 3,-rays with plastic scintillators.
Plastic scintillators are not generally used for the detection of 7-rays due to their low efficiency of conversion and light yield as compared to NaI crystals. Indeed they are still the most appropriate detector in experiments for which high counting rates ( > 1 0 6 / s ) , good timing properties and large surfaces are needed. We encountered these problems during the development of a set-up for time-resolved M6ssbauer spectroscopy in which the time difference from the 122 keV and the subsequent 14.4 keV 7-rays from a STCo source had to be measured with a precision of a few nanoseconds with single counting rates exceeding 106/s. In addition, we were also interested in the detection of 7.3 keV conversion electrons from the same source. Up to ~15 keV the major part (~60%) of the T-rays still interact with the carbon atoms of the scintillator by the photoelectric effect and thus produce lines that allow some energy discrimination. In the literature we found that a limit of ~ 1 0 keV was given for plastic scintillators [1,2] and 6 k e V for anthracene [1]. We have therefore investigated whether the recent improvements o f photomultipliers and fast electronics could give a lower limit. We have studied the properties of NE 102 A, NE 110 and Pilot U plastic scintillators, coupled to EMI 9813 KB or Philips XP 2020 photornultipliers. The scintillators were in the form of discs or parallellepipeds, 5 or 3 mm thick, covered with a thin (20 /am) aluminized Mylar foil. Commercial fast electronics (Lecroy) were used for the signal processing. The best results were obtained with a selected EMI
* Visitor of CERN, Geneva, Switzerland. 0 0 2 9 - 5 5 4 X / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 5 0 © 1981 North-Holland
9813 KB photomultiplier coupled to a disc of NE 102 A. Fig. la shows the pulse height spectrum obtained with the 5.9 keV X-rays from a SSFe source. The mean number of photoelectrons corresponding to the peak is 3.5 + 0.25, as inferred from the fitting of the spectrum to a Poisson distribution with the mean number of photoelectrons left as free parameter and from the position o f the one-photoelectron peak. The latter is in part due to the noise o f the photomultiplier, in part to the long-lived states in the scintillator [1]. Fib. l b shows the spectrum obtained with the same source but with the threshold of the discriminator adjusted to a value corresponding to 1.5 photoelectrons. In these conditions the efficiency for the detection o f the 5.9 keV X-rays in (82 + 3)%, as inferred from the shape o f the peak compared to the above Poisson distribution and from a direct measurement of the strength of the source. Fig. l c finally shows the spectrum obtained with the 8.1 keV X-rays from a 6SZn source, always with a threshold at 1.5 photoelectrons. The mean number of photoelectrons is 5.0 + 0.25 and the efficiency (94 + 2)%. In conclusion we think that we have obtained an improvement of about a factor 2 compared to previous measurements and we can put a limit of ~ 3 - 4 keV for the detection of 7-rays with plastic scintillators and with efficiencies greater than 50%. We thank Prof. T. Bressani for useful guidance and advice and Mr. K. Ratz for his help at CERN, where part of this work was done.
M. Caria ctal. /Detection o / l o w energy ~-rays
474
6x104 [ "
References
5xlO4
[ 1 ] t".T. Porter, M.S. Freedman, F. Wagner, Jr. and J.S. Sherman, Nucl. Instr. and Meth. 39 (1966) 35. [2] Z.M. Cho, C.M. Tsai and L.A. Eriksson, IEH£ Trans. Nucl. Sci. NS-22 (1975) 72.
! I
4xlO4 3xlO4
a)
j.
5,gKeV
2 xlO4 L"
:/"
",%
! Z Z
104 L_ t t ~
i
I LP a~ LLI
o L
U')
2 xlO4 1
2
"..
3
'-.....,.
photoelectrons
"~"'"~
k~
J. . . . . 5 9 KeV
Z 0 (D
I
5;'"~""...,
b)
I04 0
"'~'~---..k .... 8,1 KeY
2x104 !-
f
,04
2':
0 0
".,
C) %
50 LO0 CHANNEL
L5O
Fig. 1. Pulse height spectra obtained with a scintillator NE 102A coupled to an EMI 9813 KB photomultiplier when irradiated (a) with 5.9 keV X-rays, no threshold; (b) with 5.9 keV X-rays, threshold at 1.5 photoelectrons; (c) with 8.1 keV X-rays, threshold at 1.5 photoelectrons.