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Development of a new type multi-moderator neutron spectrometer
Radiation Physics and Chemistry 61 (2001) 523–526
Development of a new type multi-moderator neutron spectrometer Shingo Taniguchia,*, Masashi Takadab, Takashi Nakamurac a
Beamline Division, Japan Synchrotron Radiation Research Institute, Koto 1-1-1, Mikazuki-cho, Sayo-gun, Hyogo 679-5198, Japan b National Institute of Radiological Sciences, Anagawa 4-9-1, Inage-ku, Chiba-shi, Chiba 263-8555, Japan c Department of Quantum Science and Energy Engineering, Tohoku University, Aza-Aoba, Aramaki, Aoba-ku, Sendai-shi, Miyagi 981-8579, Japan
Abstract A multi-moderator spectrometer on which is mounted a pair of 6Li and 7Li glass scintillators has been developed as a new type neutron spectrometer which can measure the neutron spectrum in a mixed field of neutrons, charged particles and gamma-rays realized in space. The particle identification capability was investigated in neutron-gamma-ray and neutron-proton mixed fields and the neutron response functions of the spectrometer were obtained by calculations and experiments up to 200 MeV. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Mixed field; Neutron measurement; Lithium glass scintillator
1. Introduction
2. Detector
Space activities involving satellites and high altitude airplane flights have increased, leading to the exposure estimation of astronauts and airline crew being an important problem. In space, crew are exposed to the various radiations, neutrons, protons, heavier charged particles, electrons and photons. Although neutrons give rise to a large fraction of the total doses, few neutron measurements have been made due to the difficulty of separating neutrons from the mixed radiation in space. In order to measure neutrons in the mixed field we have developed a spherical multi-moderator spectrometer on which is mounted a pair of 6Li and 7Li glass scintillators. This spectrometer aims to discriminate neutrons from other particles by subtracting the light outputs of the 7Li glass scintillator from those of the 6Li glass scintillator.
A pair of NE912 and NE913 glass scintillators, 2.54 cm diameter 2.54 cm long, has been used. The NE912 is a glass scintillator in which 7.7 wt% of 95% 6 Li-enriched lithium is doped, and in the NE913 8.3 wt% of 99.99% 7Li-enriched lithium is doped. Although 7Li has a low sensitivity to neutrons, 6Li has a high sensitivity to low energy neutrons through the 6 Li(n, a) reaction. The difference of light outputs of NE912 and NE913 is thus considered to be only due to neutrons. Each of these two scintillators is coupled with a photo-multiplier of R1924 (Hamamatsu Photonics) mounted on the outside of the moderator through the acrylic light-guide. The spherical moderators are made of polyethylene and their thicknesses are 1.5, 3.0, 5.0 and 9.0 cm. A schematic view of the spectrometer is shown in Fig. 1.
3. Response functions *Corresponding author. Fax: +81-791-58-0830. E-mail address: [email protected] (S. Taniguchi).
The response functions to neutrons were obtained by calculations. The MCNPX Monte Carlo code (Waters,
0969-806X/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 0 1 ) 0 0 3 2 1 - 8
524
S. Taniguchi et al. / Radiation Physics and Chemistry 61 (2001) 523–526
Fig. 1. Schematic view of the multi-moderator spectrometer with a pair of lithium glass scintillators. Fig. 2. Response functions from thermal up to 200 MeV, calculated by the MCNPX code.
1999) and ENDF/B-VI neutron cross section data library (Rose, 1991) were employed to calculate the response functions. The calculations were done with a mono-energetic parallel neutron beam of 100 different energies from 10@9 to 200 MeV for each moderator thickness. The response functions were also measured using 0.25, 0.55, 1.0, 5.0, 15.0 and 22.0 MeV neutrons in the mono-energetic neutron fields at the Fast Neutron Laboratory (FNL) and the Cyclotron and Radioisotope Center (CYRIC) of Tohoku University, Japan (Baba et al., 1996; Takada et al., 1996). Fig. 2 shows the calculated response functions, while in Table 1, the C/E values and the ratios of calculated and measured neutron responses are listed. The agreement between experiment and calculation is rather good, being within 50%, except for that of the thin moderator of 1.5 cm thickness.
4. Particle discrimination Because of the sensitivities of the lithium scintillators to gamma rays, detector performance in discriminating neutron events from gamma-ray events was first tested by using point radioisotope sources of 252Cf and 60Co. These measurements were performed with a pair of lithium glass scintillators without using a moderator. Neutron and gamma-ray dose rates were monitored by a neutron rem counter and an NaI scintillation survey meter, respectively. It was confirmed that neutron events were buried within gamma-ray events for gamma-ray fluxes ten times higher than the neutron flux under the rough approximation of a neutron quality factor of 10.
Table 1 C/E values of response functions Neutron energy (MeV)
0.25 0.55 1.0 5.0 15.0 22.0
Moderator thickness (cm) 1.5
3.0
5.0
9.0
0.784 0.581 0.346 0.369 0.327 0.231
0.989 0.715 0.581 0.471 0.456 0.383
1.23 0.923 0.749 0.558 0.509 0.499
1.68 1.37 1.18 0.705 0.660 0.851
Conversely, for a ratio of neutron flux to gamma-ray flux higher than 0.3, the neutron events could be clearly separated from the gamma-ray events. Neutron discrimination from charged particles for the detector without moderator was then investigated in a neutron and proton mixed field that was realized by bombarding a 35 MeV proton beam with a 2 mm thick lithium target at the CYRIC. The neutron events were clearly distinguished from proton events in this field in which the neutron flux was approximately equal to the proton flux.
5. Neutron spectrum measurement The neutron spectrum measurement was performed in the proton and neutron mixed field using the AVF
S. Taniguchi et al. / Radiation Physics and Chemistry 61 (2001) 523–526
525
Fig. 3. Experimental geometry in the neutron-proton mixed field at the AVF cyclotron of NIRS.
cyclotron at the National Institute of Radiological Sciences (NIRS), Japan. The protons which were accelerated up to 70 MeV irradiated a 2 mm thick beryllium target. The experimental geometry is shown in Fig. 3. The detector was placed at 451 to the proton beam axis 350 cm behind the target. The protons passing through the target were stopped in an aluminum beam dump and the polyethylene blocks were put between the beam dump and the detector for neutron shielding from the dump. Counts were measured by using five different moderators and were then unfolded with the SAND-II code to get the neutron spectrum in this field. In this
unfolding the 1=E spectrum was used as an initial guess spectrum. The unfolded spectrum is shown in Fig. 4 with the spectrum calculated by the MCNPX code for comparison. The spectrum measured with the detector gives rather good agreement with the calculation both in spectral shape and in absolute values.
6. Conclusion A new type multi-moderator neutron spectrometer using a pair of 6Li and 7Li glass scintillators has
526
S. Taniguchi et al. / Radiation Physics and Chemistry 61 (2001) 523–526
been developed. Neutron discrimination performance of this detector, from gamma rays and protons, was investigated and it was verified that this spectrometer can be used to measure the neutron spectrum in a mixed field of neutrons and protons.
References
Fig. 4. Comparison of the unfolded neutron spectrum with the SAND-II code and the calculated one.
Baba, M., et al., 1996. Development of monoenergetic neutron calibration fields between 8 keV and 15 MeV. Nucl. Instrum. Methods A 376, 115–123. Rose, P.F., 1991. ENDF-201, ENDF/B-VI summary documentation. BNL-NCS-17541, 4th edition. Takada, M., et al., 1996. Characterization of 22 and 33 MeV quasi-monoenergetic neutron fields for detector calibration at CYRIC. Nucl. Instrum. Methods A 372, 253–261. Waters, L.S. (Ed.), 1999. MCNPXt USER’S MANUAL. TPO-E83-G-UG-X-00001.
Development of a new type multi-moderator neutron spectrometer Shingo Taniguchia,*, Masashi Takadab, Takashi Nakamurac a
Beamline Division, Japan Synchrotron Radiation Research Institute, Koto 1-1-1, Mikazuki-cho, Sayo-gun, Hyogo 679-5198, Japan b National Institute of Radiological Sciences, Anagawa 4-9-1, Inage-ku, Chiba-shi, Chiba 263-8555, Japan c Department of Quantum Science and Energy Engineering, Tohoku University, Aza-Aoba, Aramaki, Aoba-ku, Sendai-shi, Miyagi 981-8579, Japan
Abstract A multi-moderator spectrometer on which is mounted a pair of 6Li and 7Li glass scintillators has been developed as a new type neutron spectrometer which can measure the neutron spectrum in a mixed field of neutrons, charged particles and gamma-rays realized in space. The particle identification capability was investigated in neutron-gamma-ray and neutron-proton mixed fields and the neutron response functions of the spectrometer were obtained by calculations and experiments up to 200 MeV. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Mixed field; Neutron measurement; Lithium glass scintillator
1. Introduction
2. Detector
Space activities involving satellites and high altitude airplane flights have increased, leading to the exposure estimation of astronauts and airline crew being an important problem. In space, crew are exposed to the various radiations, neutrons, protons, heavier charged particles, electrons and photons. Although neutrons give rise to a large fraction of the total doses, few neutron measurements have been made due to the difficulty of separating neutrons from the mixed radiation in space. In order to measure neutrons in the mixed field we have developed a spherical multi-moderator spectrometer on which is mounted a pair of 6Li and 7Li glass scintillators. This spectrometer aims to discriminate neutrons from other particles by subtracting the light outputs of the 7Li glass scintillator from those of the 6Li glass scintillator.
A pair of NE912 and NE913 glass scintillators, 2.54 cm diameter 2.54 cm long, has been used. The NE912 is a glass scintillator in which 7.7 wt% of 95% 6 Li-enriched lithium is doped, and in the NE913 8.3 wt% of 99.99% 7Li-enriched lithium is doped. Although 7Li has a low sensitivity to neutrons, 6Li has a high sensitivity to low energy neutrons through the 6 Li(n, a) reaction. The difference of light outputs of NE912 and NE913 is thus considered to be only due to neutrons. Each of these two scintillators is coupled with a photo-multiplier of R1924 (Hamamatsu Photonics) mounted on the outside of the moderator through the acrylic light-guide. The spherical moderators are made of polyethylene and their thicknesses are 1.5, 3.0, 5.0 and 9.0 cm. A schematic view of the spectrometer is shown in Fig. 1.
3. Response functions *Corresponding author. Fax: +81-791-58-0830. E-mail address: [email protected] (S. Taniguchi).
The response functions to neutrons were obtained by calculations. The MCNPX Monte Carlo code (Waters,
0969-806X/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 0 1 ) 0 0 3 2 1 - 8
524
S. Taniguchi et al. / Radiation Physics and Chemistry 61 (2001) 523–526
Fig. 1. Schematic view of the multi-moderator spectrometer with a pair of lithium glass scintillators. Fig. 2. Response functions from thermal up to 200 MeV, calculated by the MCNPX code.
1999) and ENDF/B-VI neutron cross section data library (Rose, 1991) were employed to calculate the response functions. The calculations were done with a mono-energetic parallel neutron beam of 100 different energies from 10@9 to 200 MeV for each moderator thickness. The response functions were also measured using 0.25, 0.55, 1.0, 5.0, 15.0 and 22.0 MeV neutrons in the mono-energetic neutron fields at the Fast Neutron Laboratory (FNL) and the Cyclotron and Radioisotope Center (CYRIC) of Tohoku University, Japan (Baba et al., 1996; Takada et al., 1996). Fig. 2 shows the calculated response functions, while in Table 1, the C/E values and the ratios of calculated and measured neutron responses are listed. The agreement between experiment and calculation is rather good, being within 50%, except for that of the thin moderator of 1.5 cm thickness.
4. Particle discrimination Because of the sensitivities of the lithium scintillators to gamma rays, detector performance in discriminating neutron events from gamma-ray events was first tested by using point radioisotope sources of 252Cf and 60Co. These measurements were performed with a pair of lithium glass scintillators without using a moderator. Neutron and gamma-ray dose rates were monitored by a neutron rem counter and an NaI scintillation survey meter, respectively. It was confirmed that neutron events were buried within gamma-ray events for gamma-ray fluxes ten times higher than the neutron flux under the rough approximation of a neutron quality factor of 10.
Table 1 C/E values of response functions Neutron energy (MeV)
0.25 0.55 1.0 5.0 15.0 22.0
Moderator thickness (cm) 1.5
3.0
5.0
9.0
0.784 0.581 0.346 0.369 0.327 0.231
0.989 0.715 0.581 0.471 0.456 0.383
1.23 0.923 0.749 0.558 0.509 0.499
1.68 1.37 1.18 0.705 0.660 0.851
Conversely, for a ratio of neutron flux to gamma-ray flux higher than 0.3, the neutron events could be clearly separated from the gamma-ray events. Neutron discrimination from charged particles for the detector without moderator was then investigated in a neutron and proton mixed field that was realized by bombarding a 35 MeV proton beam with a 2 mm thick lithium target at the CYRIC. The neutron events were clearly distinguished from proton events in this field in which the neutron flux was approximately equal to the proton flux.
5. Neutron spectrum measurement The neutron spectrum measurement was performed in the proton and neutron mixed field using the AVF
S. Taniguchi et al. / Radiation Physics and Chemistry 61 (2001) 523–526
525
Fig. 3. Experimental geometry in the neutron-proton mixed field at the AVF cyclotron of NIRS.
cyclotron at the National Institute of Radiological Sciences (NIRS), Japan. The protons which were accelerated up to 70 MeV irradiated a 2 mm thick beryllium target. The experimental geometry is shown in Fig. 3. The detector was placed at 451 to the proton beam axis 350 cm behind the target. The protons passing through the target were stopped in an aluminum beam dump and the polyethylene blocks were put between the beam dump and the detector for neutron shielding from the dump. Counts were measured by using five different moderators and were then unfolded with the SAND-II code to get the neutron spectrum in this field. In this
unfolding the 1=E spectrum was used as an initial guess spectrum. The unfolded spectrum is shown in Fig. 4 with the spectrum calculated by the MCNPX code for comparison. The spectrum measured with the detector gives rather good agreement with the calculation both in spectral shape and in absolute values.
6. Conclusion A new type multi-moderator neutron spectrometer using a pair of 6Li and 7Li glass scintillators has
526
S. Taniguchi et al. / Radiation Physics and Chemistry 61 (2001) 523–526
been developed. Neutron discrimination performance of this detector, from gamma rays and protons, was investigated and it was verified that this spectrometer can be used to measure the neutron spectrum in a mixed field of neutrons and protons.
References
Fig. 4. Comparison of the unfolded neutron spectrum with the SAND-II code and the calculated one.
Baba, M., et al., 1996. Development of monoenergetic neutron calibration fields between 8 keV and 15 MeV. Nucl. Instrum. Methods A 376, 115–123. Rose, P.F., 1991. ENDF-201, ENDF/B-VI summary documentation. BNL-NCS-17541, 4th edition. Takada, M., et al., 1996. Characterization of 22 and 33 MeV quasi-monoenergetic neutron fields for detector calibration at CYRIC. Nucl. Instrum. Methods A 372, 253–261. Waters, L.S. (Ed.), 1999. MCNPXt USER’S MANUAL. TPO-E83-G-UG-X-00001.