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Measurement of absolute efficiency of solid state track detectors for 14 MeV neutrons
Nuclear Instruments and Methods in Physics Research 220 (1984) 547-548 North-Holland, Amsterdam
MEASUREMENT
OF ABSOLUTE
EFFICIENCY
547
OF SOLID STATE TRACK DETECTORS
F O R 14 MeV N E U T R O N S Aruna BOSE and Arun Kumar CHATTERJEE Physics Department, Bose Institute, Calcutta-9, India Received 27 May 1983 and in revised form 28 October 1983
Efficiencies of solid state track detectors were measured with 14 MeV neutrons. Detectors were etched by following the conventional method of etching. Tracks were counted by a scanning electron microscope.
The application of solid state track detectors is increasing in neutron dosimetry or cross section measurements due mainly to their simple operational principle and insensitiveness to fl and 3, rays. These detectors are used in two ways, either as an internal radiator, or along with an external radiator. In the first case, neutrons are detected by means of the ionizing products of a number of nuclear reactions between the neutrons and the consituents of detector material. In the second type, charged particles produced in an adjacent foil containing fissile or alphagenic material are recorded by the detector. This growing applicability demands optimum procedure for processing, evaluation and calibration of these detectors. Although there exist a number of measurements on cellulose nitrate [1-4], other track detectors have yet to draw the attention of workers working in this field.
,
11
I i i _ _
--
/
D
Forced air for target cooling
~
SSTD
Tritium target
Some work is going on with track detectors such as cellulose triacetate or special glasses, but no systematic study of efficiency determination has been reported. With this in mind, we in this report present an accurate determination of track registration efficiency of solid state track detectors such as Makrofol-N [5], cellulose acetate [6] and CR-39 [7]. In the present investigation we use the detectors in internal radiation mode. Detector foils of area 2.25 cm 2 were irradiated with a beam of neutrons of 14.2 MeV energy generated by a (t, d ) neutron generator at our Institute. The experimental set-up is shown in fig. 1. The plastic detectors used were square shaped and were held at fight angles to the projectile deuteron beam. The thicknesses of these detectors are shown in table 1. They were placed at a distance of 3 cm from the focussed spot on the target. Plastic track detectors were irradiated by a monoenergetic neutron beam for a period of five hours. The neutron flux was measured by a multi-biased plastic scintillation detector using a method developed in our laboratory [8], The detector was placed in the forward direction at a distance of 30 cm from the target. In the 3H (2H, n) 4He reaction the neutron distribution is isotropic. A neutron flux of 107 n / c m 2" s was measured with an estimated accuracy of + 3%.
Table 1 Efficiency values of the SST Detector. Detector
I~1
Plastic
Scintillator
P. M . t u b t
Fig. 1. Schematic diagram of experimental arrangement. 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Makrofol-N Cellulose acetate (C1oH1407) CR-39
Thickness
Efficiency × 106
[/~m]
Measured
Calculated
200
10.3
11.1
140 1600
18.1 28.0
548
A. Bose / Efficiency of solid state track detectors
After irradiation, the foils were chemically etched under etching conditions which have already been established as a standard method [9]. Makrofol-N and CR-39 foils were etched in 6N K O H solution at 60°C for 4 h and the cellulose acetate foil was etched in 6N N a O H solution at the same temperature and period. The foils were then washed under running tap water and Mr-dried at room-temperature. The pits revealed in the SSTDs by the process of chemical etching were observed under a scanning electron microscope with a magnification of 1600. Since the number of tracks recorded in the detector gives a direct measure of the neutron fluence, the efficiency of these detectors can be defined as the number of tracks produced per incident neutron. For MakrofolN and CR-39, the pits observed were regular and most of them had nearly circular sectional area, while pits formed in cellulose acetate were irregular in shape and varied in dimension. The efficiencies found are shown in table 1. Background counts were subtracted from the total recorded counts and the statistical error in counting pits is of the order of + 5%. The etching time was made sufficiently long so that all the pits formed in the detectors were revealed and the number was the same when observed from either side of the foil [1]. From a recently published theoretical calculation of the neutron detection efficiency of the cellulose tracetate track detector [10], we find our efficiency value of Makrofol-N in close agreement with the value calculated for cellulose triacetate in the energy region considered. To the best of our knowledge no such theoretical calculations exist for other track detectors: hence we could not compare our experimentally determined efficiencies. On the other hat~d, there might be a variation in sensitivity in the same type of detector procured from different sources and thereby giving rise to different efficiencies. These detectors provide a convenient means of detecting neutrons and of measuring neutron fluence by well defined tracks produced by recoil particles gener-
ated in reaction with constituents of organic solid state detectors. The specimen used for efficiency calibration and experimental sample should be of the same type and processed in the same manner. A solid state track detector may be used in cross section measurements and dosimetry applications. The efficiency value calculated theoretically for cellulose triacetate at 14 MeV agrees with the experimentally determined efficiency of Makrofol-N at the same energy. The authors feel that there exists the need for experimentally determined efficiency values at high and low energies of neutrons. It is to be hoped that future experiments will provide us with sufficient data so that the gaps which are present now can be filled up in an appropriate manner. The authors are grateful to Professor S.C. Bhattacharyya, Director, Bose Institute, Calcutta for allowing them to carry on the work.
References [1] M. Balcazar-Garcia, R.K. Bull, I.D. Fall and S.,~. Durani, Nucl. Instr. and Meth. 161 (1979) 91. [2] G. Somagyi, I. Hunyadi and Zs. Varga, Nuclear Track Detection 2 (1978) 191. [3] S.R. Hashemi-Nezhad, P.F. Green, S.A. Durani and R.K. Bull, Nucl. Instr. and Meth. 200 (1982) 525. [4] R.V. Griffith, J.H. Thorngate, K.J. Davidson, D.W. Ruepo pel, J.C. Fisher, L. Tommasino and G. Zapparolli, Radiation Proctection Dosimetry 1 (1981) 61. [5] Bayer AG, West Germany. [6] Kodak Photographic Film. [7] Polytech. Inc. Owensville, Mo. Also available from SGL Homalite, 11 Brookside Drive, USA [8] A.K. Chatterjee and A.M. Ghose, Nucl Instr. and Meth. 49 (1967) 101. [9] K. Becker, Dosimetric application of track etching in topics in radiation dosimetry. [10] G. Pretzsch and B. Dorschel, Nucl. Instr. and Meth. 195 (1982) 551.
MEASUREMENT
OF ABSOLUTE
EFFICIENCY
547
OF SOLID STATE TRACK DETECTORS
F O R 14 MeV N E U T R O N S Aruna BOSE and Arun Kumar CHATTERJEE Physics Department, Bose Institute, Calcutta-9, India Received 27 May 1983 and in revised form 28 October 1983
Efficiencies of solid state track detectors were measured with 14 MeV neutrons. Detectors were etched by following the conventional method of etching. Tracks were counted by a scanning electron microscope.
The application of solid state track detectors is increasing in neutron dosimetry or cross section measurements due mainly to their simple operational principle and insensitiveness to fl and 3, rays. These detectors are used in two ways, either as an internal radiator, or along with an external radiator. In the first case, neutrons are detected by means of the ionizing products of a number of nuclear reactions between the neutrons and the consituents of detector material. In the second type, charged particles produced in an adjacent foil containing fissile or alphagenic material are recorded by the detector. This growing applicability demands optimum procedure for processing, evaluation and calibration of these detectors. Although there exist a number of measurements on cellulose nitrate [1-4], other track detectors have yet to draw the attention of workers working in this field.
,
11
I i i _ _
--
/
D
Forced air for target cooling
~
SSTD
Tritium target
Some work is going on with track detectors such as cellulose triacetate or special glasses, but no systematic study of efficiency determination has been reported. With this in mind, we in this report present an accurate determination of track registration efficiency of solid state track detectors such as Makrofol-N [5], cellulose acetate [6] and CR-39 [7]. In the present investigation we use the detectors in internal radiation mode. Detector foils of area 2.25 cm 2 were irradiated with a beam of neutrons of 14.2 MeV energy generated by a (t, d ) neutron generator at our Institute. The experimental set-up is shown in fig. 1. The plastic detectors used were square shaped and were held at fight angles to the projectile deuteron beam. The thicknesses of these detectors are shown in table 1. They were placed at a distance of 3 cm from the focussed spot on the target. Plastic track detectors were irradiated by a monoenergetic neutron beam for a period of five hours. The neutron flux was measured by a multi-biased plastic scintillation detector using a method developed in our laboratory [8], The detector was placed in the forward direction at a distance of 30 cm from the target. In the 3H (2H, n) 4He reaction the neutron distribution is isotropic. A neutron flux of 107 n / c m 2" s was measured with an estimated accuracy of + 3%.
Table 1 Efficiency values of the SST Detector. Detector
I~1
Plastic
Scintillator
P. M . t u b t
Fig. 1. Schematic diagram of experimental arrangement. 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Makrofol-N Cellulose acetate (C1oH1407) CR-39
Thickness
Efficiency × 106
[/~m]
Measured
Calculated
200
10.3
11.1
140 1600
18.1 28.0
548
A. Bose / Efficiency of solid state track detectors
After irradiation, the foils were chemically etched under etching conditions which have already been established as a standard method [9]. Makrofol-N and CR-39 foils were etched in 6N K O H solution at 60°C for 4 h and the cellulose acetate foil was etched in 6N N a O H solution at the same temperature and period. The foils were then washed under running tap water and Mr-dried at room-temperature. The pits revealed in the SSTDs by the process of chemical etching were observed under a scanning electron microscope with a magnification of 1600. Since the number of tracks recorded in the detector gives a direct measure of the neutron fluence, the efficiency of these detectors can be defined as the number of tracks produced per incident neutron. For MakrofolN and CR-39, the pits observed were regular and most of them had nearly circular sectional area, while pits formed in cellulose acetate were irregular in shape and varied in dimension. The efficiencies found are shown in table 1. Background counts were subtracted from the total recorded counts and the statistical error in counting pits is of the order of + 5%. The etching time was made sufficiently long so that all the pits formed in the detectors were revealed and the number was the same when observed from either side of the foil [1]. From a recently published theoretical calculation of the neutron detection efficiency of the cellulose tracetate track detector [10], we find our efficiency value of Makrofol-N in close agreement with the value calculated for cellulose triacetate in the energy region considered. To the best of our knowledge no such theoretical calculations exist for other track detectors: hence we could not compare our experimentally determined efficiencies. On the other hat~d, there might be a variation in sensitivity in the same type of detector procured from different sources and thereby giving rise to different efficiencies. These detectors provide a convenient means of detecting neutrons and of measuring neutron fluence by well defined tracks produced by recoil particles gener-
ated in reaction with constituents of organic solid state detectors. The specimen used for efficiency calibration and experimental sample should be of the same type and processed in the same manner. A solid state track detector may be used in cross section measurements and dosimetry applications. The efficiency value calculated theoretically for cellulose triacetate at 14 MeV agrees with the experimentally determined efficiency of Makrofol-N at the same energy. The authors feel that there exists the need for experimentally determined efficiency values at high and low energies of neutrons. It is to be hoped that future experiments will provide us with sufficient data so that the gaps which are present now can be filled up in an appropriate manner. The authors are grateful to Professor S.C. Bhattacharyya, Director, Bose Institute, Calcutta for allowing them to carry on the work.
References [1] M. Balcazar-Garcia, R.K. Bull, I.D. Fall and S.,~. Durani, Nucl. Instr. and Meth. 161 (1979) 91. [2] G. Somagyi, I. Hunyadi and Zs. Varga, Nuclear Track Detection 2 (1978) 191. [3] S.R. Hashemi-Nezhad, P.F. Green, S.A. Durani and R.K. Bull, Nucl. Instr. and Meth. 200 (1982) 525. [4] R.V. Griffith, J.H. Thorngate, K.J. Davidson, D.W. Ruepo pel, J.C. Fisher, L. Tommasino and G. Zapparolli, Radiation Proctection Dosimetry 1 (1981) 61. [5] Bayer AG, West Germany. [6] Kodak Photographic Film. [7] Polytech. Inc. Owensville, Mo. Also available from SGL Homalite, 11 Brookside Drive, USA [8] A.K. Chatterjee and A.M. Ghose, Nucl Instr. and Meth. 49 (1967) 101. [9] K. Becker, Dosimetric application of track etching in topics in radiation dosimetry. [10] G. Pretzsch and B. Dorschel, Nucl. Instr. and Meth. 195 (1982) 551.