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Neutron, photon and proton energy spectra at high altitude measured using a phoswich-type neutron detector
Radiation Measurements 45 (2010) 1297e1300
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Neutron, photon and proton energy spectra at high altitude measured using a phoswich-type neutron detector M. Takada a, *, K. Yajima a, H. Yasuda a, T. Sato b, T. Nakamura a, c a
National Institute of Radiological Sciences, Inage-ku Chiba, 263-8555, Japan Japan Atomic Energy Agency, Tokai 319-1195, Japan c Cyclotron and Radioisotope Center, Tohoku University, Sendai 980-8578, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 November 2009 Received in revised form 31 May 2010 Accepted 9 June 2010
Neutron energy spectrum from 7 to 180 MeV, photon energy spectrum from 4 to 50 MeV and proton energy spectrum from 94 to 145 MeV were measured simultaneously using a phoswich-type neutron detector with particle discrimination methods at atmospheric depth of 249 g/cm2, a vertical cut-off rigidity of 10.2 GV and at a heliocentric potential of 312 MV. We compared our results with other measured and calculated particle energy spectra. Our measured results give a large, sharp neutron peak around 70 MeV, although Bonner balls show a broad peak around 100 MeV due to low energy resolution. The measured photon and proton spectra are between the calculated energy spectra. This onboard study provides the first experimental neutron energy spectrum over 10 MeV with a highenergy resolution. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Cosmic ray Neutron Photon Proton Energy spectrum Phoswich-type Liquid scintillator Plastic scintillator Particle discrimination Aviation altitude
1. Introduction Radiation environments at high altitudes are composed of a variety of particles such as neutron, proton, photon, electron and muon. Aircrews are exposed to elevated levels from 2 to 7 mSv/hr of cosmic radiation (Lewis et al., 2004) and larger than on the ground, 0.1 mSv/h. Ionizing components of cosmic particles have been measured using several detectors with good accuracy. The neutron component contributes about half of dose equivalent and its assessment is important for radiation protection of flight crews, however, neutron energy spectra were measured using multimoderator spectrometers (Goldhagen et al., 2002), so called Bonner balls, despite of low energy resolution and strong dependence on initial-guess spectra postulated a priori. High energy neutrons over 10 MeV contribute about 80% in neutron effective dose rates at high altitudes using the fluence-to-equivalent dose coefficients for isotropic exposure given by ICRP
* Corresponding author. Tel.: þ81 43 205 3464; fax: þ81 43 206 3514. E-mail address: [email protected] (M. Takada). 1350-4487/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2010.06.012
publication 92 (ICRP92, 2003). This study focuses on the highenergy neutron, photon and proton spectra over 10 MeV. We attempted to measure energy spectra of neutrons, photons and protons with high energy resolution using a newly developed phoswich-type detector simultaneously with particle discrimination methods. We compared our results with several experimental and calculated results.
2. Flight experiment 2.1. Flight route Cosmic neutron, photon and proton energy spectra were measured near the central part of Japan at an altitude of 10.8 0.03 km (249 g/cm2 atmospheric depth) and geographical latitude 39 N and longitude from 134.6 to 136.9 E on 13th February 2009 (Takada et al., 2010a) onboard an MU300 business jet operated by Diamond Air Service Inc. The vertical cut-off rigidity in the observation area was 10.2 GV. The solar modulation was reported to be at a heliocentric potential of 312 MV.
1298
M. Takada et al. / Radiation Measurements 45 (2010) 1297e1300
2.2. Phoswich-type detector
Neutrons
The phoswich-type neutron detector consists of an EJ309 organic liquid scintillator, 121.7 mm in diameter and a 121.7 mm long, covered with a 15-mm-thick EJ299-13 plastic scintillator (Takada and Nakamura, 2007) and coupled with a photomultiplier, 120 mm in diameter, R1250, via an acrylic light guide. The EJ309 liquid scintillator exhibits high flash-point (140 C) and auto-ignition (440 C) temperatures. Using a harmless and biodegrable EJ309 liquid scintillator improves the safety of the neutron detector for onboard experiments. Signals from the phoswich detector were acquired using a fast analog-to-digital converter and then converted to digital data with a 12 bit dynamic range for each 2 ns period from 10 ns before to 400 ns after the rise of the signal. 150e160 signals per second were measured throughout the flight and with 7.3% dead time. The phoswich detector can measure neutron, photon and proton separately in mixed radiation fields. Non charged particles of neutrons and photons deposit their energies mainly in the inner liquid scintillator, and on the other hand, charged particles deposit their energies both in the inner liquid and outer plastic scintillators. The inner liquid and outer plastic scintillators produce signals with very different decay times of 3.5 ns and 285 ns, respectively. Using particle-dependent pulse shapes, neutron, photon and proton events were discriminated each other (Takada et al., 2004, 2002, 2001).
0.4
2.3. Response function Response functions of neutron, photon and proton are necessary to obtain these energy spectra using the unfolding method (Shin et al., 1981). The neutron and photon response matrices were created by Takada et al. (Takada et al., 2010b) on the basis of simulation using the Monte Carlo N-Particle eXtended (MCNPX) code (MCNPX, 2005). Maximum energies of the response matrices were 300 MeV for neutrons and 50 MeV for photons. The created response functions were verified by comparing the calculated and experimental results using several quasi-monoenergetic neutron beams produced by 40e80 MeV protons at three cyclotron facilities in the National Institute of Radiological Sciences, Cyclotron and Radioisotope Center, Tohoku University and Takasaki ion accelerators for advanced radiation application of the Japan Atomic Energy Agency (JAEA) in Japan; and mono-energy photon beams produced by the p-F reactions in JAEA. Absolute responses were agreed each other within 15% difference. Proton response matrices were also obtained with the MCNPX calculation in the proton energy range from 78 to 190 MeV, which protons deposit their energies in the inner liquid scintillator. Proton pulse heights are selected using the boundaries of proton events in calculated particle discrimination plots. The response matrices were made with source geometry of the upper hemisphere covering the phoswich detector, which simulates cosmic radiation field at the high altitude. 3. Results and discussion 3.1. Neutron energy spectrum The measured neutron energy spectrum from 7 to 180 MeV using the phoswich detector is plotted with error bars in Fig. 1. The measured spectrum shows a large, sharp neutron peak around 70 MeV and a lower energy peak of 12 MeV. We compared our result with other experimental and calculated results using three codes: LUIN2000 (Code for the Calculation of Cosmic Ray Propagation in the Atmosphere) (O’Brien, 1978), EXPACS (Excel-based Program for calculating Atmospheric Cosmic-ray Spectrum)
This work Yajima et al. (Bonner Ball) Goldhagen (Extended Bonner Ball) Nakamura (Liq. Scint.) LUIN2000 (FLUKA) EXPACS (PHITS) RMC Model (MCNPX)
0.2
Energy
Flux (n cm-2 s-1)
0.3
0.1
0 1
10
102 Neutron Energy (MeV)
Fig. 1. Measured neutron energy spectrum at 249 g/cm2 atmospheric depth, 10.2 GV vertical cut-off rigidity and 312 MV heliocentric potential, compared with experimental results using Bonner balls by Goldhagen (Goldhagen et al., 2004) and Yajima (Yajima et al., 2010) and using an organic liquid scintillator on the ground by Nakamura (Nakamura et al., 2005), and calculated results using EXPACS (Sato et al., 2008), LUIN2000 (O’Brien, 1978) and RMC (Takada et al., 2007) codes. The measured results were corrected with cut-off rigidity and atmospheric depth.
(Sato et al., 2008) and RMC (Royal Military College of Canada) (Takada et al., 2007) codes. Yajima et al. (Yajima et al., 2010) and Goldhagen et al. (Goldhagen et al., 2004) used Bonner balls and Bonner balls with extended energy responses above 1 GeV. Nakmaura et al. measured the neutron energy spectra above 6 MeV with high energy resolution using an organic liquid scintillator but their measurements were done on the ground. The other measured results were normalized at our measured locations, 249 g/cm2 in atmospheric depth and geographical latitude 93 N, by correcting for neutron attenuation in air (166 g/cm2, measured by Nakamura (Nakamura et al., 2005)), considering the cut-off rigidity, evaluated by Gordon (Gordon et al., 2004). The LUIN2000 calculates atmospheric cosmic-ray propagation based on an analytical solution of Boltzmann transport theory, the EXPACS is also based on the analytical solution induced from the PHITS (Particle and Heavy Ion Transport code System) simulation (Iwase et al., 2002) and the RMC code uses a Microsoft Excel lookup table based on the MCNPX simulation (MCNPX, 2005). Particle energy spectra at a top of atmosphere are transported in the air and then at high altitude (10 km) the particle energy spectra are much affected with the incident particle spectrum at the top of atmosphere. Local interstellar spectrum (LIS) modulated with the solar wind and shielded by the geomagnetic field is used as the particle spectra at the top of atmosphere. LIS in the RMC code, referred from the LUIN code, is calculated with energies below 10 GeV/nucleon using the equation,
h
f ¼ 9:9 104 E þ 780exp 2:5 104
i2:65
;
where E is MeV/nucleon, and above 10 GeV is represented by Peters’ model; however, LIS in the EXPACS is calculated using the Nymmik model. These calculated LISs agree well with measured
Table 1 Measured neutron fluxes compared with other experimental and calculation results. Numbers in parentheses indicate the flux ratios of each result to our measured result. The last line shows the flux ratios considering the extrapolated neutron spectrum in our result. Flux in n cm2 s1
This study
Extended Bonner ball
RMC
7e180 MeV 30e100 MeV (peak) 7e2 GeV
0.27 (1.0) 0.15 0.0028 (1.0) 0.32 (1.0)
0.25 (0.91) 0.11 (0.73) 0.31 (0.96)
0.25 (0.91) 0.10 (0.67) 0.35 (1.08)
proton and helium spectra. Different solar modulation models are used in the LUIN2000 and RMC codes (heliocentric potential), and the EXPACS (force field formalism). Incident particle spectra close to each other were used in the calculations. Our neutron energy spectrum shows a large sharp peak around 70 MeV different from the other experimental and calculated results with a broad peak around 100 MeV. Different widths of the peaks in our result and the Bonner ball result are observed because of the energy resolutions of the detectors. The liquid scintillator with the high energy resolution shows a sharp peak around 70 MeV. Our sharp peak can be demonstrated to result from the good energy resolution of this detector. In the neutron energy range below 30 MeV our result shows larger than the measurement on the ground using the liquid scintillator. It could be caused by the different neutron attenuation in air. The sharp neutron peak around 70 MeV and a valley around 30 MeV may be explained by a combination of neutron production and attenuation through in air. Our measured neutron fluxes are compared with the extended Bonner ball and RMC code results integrated in the measured energy region, 7e180 MeV, our peak energy region, 30e120 MeV and 7e2 GeV, including the extrapolated neutron energy spectrum above 180 MeV in our result (Table 1). Although our results are 9 and 27% larger than the extended Bonner ball fluxes and 9 and 33% larger than the RMC code calculations in the neutron energy range from 7 to 180 MeV (measured energy range) and from 30 to 100 MeV (peak energy range), our result agree well with the extended Bonner ball flux within 4% and the RMC code calculation
Photon 1 LUIN2000 (SemiEmpirical Model) EXPACS (PHITS Base)
Flux (n cm-2 MeV-1 s-1)
This work
10-1
10-2
-3
10
1
10
102 Photon Energy (MeV)
Fig. 2. Measured photon energy spectrum compared with spectra calculated using the LUIN2000 and EXPACS codes.
Proton Flux (n cm-2 s-1 MeV-1)
M. Takada et al. / Radiation Measurements 45 (2010) 1297e1300
1299
10-3
10-4
-5
10
This Work LUIN2000 EXPACS RMC
10-6
3
102
10 Proton Energy (MeV)
Fig. 3. Measured proton energy spectrum compared with energy spectra calculated using the LUIN2000, EXPACS and RMC codes.
within 8% by considering the extrapolated energy region, 180e2 GeV. This confirms that our measured neutron energy fluxes are quite reasonable. 3.2. Photon energy spectrum Fig. 2 shows the photon energy spectrum, 4e50 MeV, with error bars from unfolding errors and the calculated results using the LUIN and EXPACS codes. Our results fall between the two calculations. Our results agree well with the LUIN calculation within 3%, 4e10 MeV and with the EXPACS calculation within 11% above 25 MeV. 3.3. Proton energy spectrum The proton energy spectrum was measured in the energy range of 94e145 MeV in Fig. 3. We compared our experimental results with energy spectra calculated using the LUIN2000, EXPACS and RMC codes. The proton energy spectrum was obtained from the proton events, where protons deposited their energies in the inner liquid scintillator and stopped in the liquid scintillator. The proton energy spectrum below 100 MeV is increased. It could be considered by protons scattered materials around the detector. Our result shows different spectrum shape than the calculated results. We compared proton fluxes (Table 2). The calculations using EXPACS and RMC codes show larger fluxes than our results, however, the LUIN2000 code shows smaller fluxes. This comparison shows opposite results of neutron fluxes. Smaller neutrons could be created with smaller cross sections of neutron productions by protons and larger protons are transmitted through the atmosphere in the Monte Carlo calculations.
Table 2 Measured proton fluxes compared with other calculated results in n cm2 s1. Numbers in parentheses indicate the flux ratios of each result to our measured result. This work
LUIN2000
EXPACS
RMC
5.28 103 (1)
3.26 103 (0.62)
10.30 103 (1.95)
8.55 103 (1.62)
1300
M. Takada et al. / Radiation Measurements 45 (2010) 1297e1300
4. Conclusion
References
The neutron energy spectrum from 7 to 180 MeV, the photon energy spectrum from 4 to 50 MeV and the proton energy spectrum from 94 to 145 MeV are measured simultaneously using the phoswich neutron detector at the 10.8 km altitude (atmospheric depth of 249 g/cm2), the vertical cut-off rigidity of 10.2 GV and the heliocentric potential of 312 MV. This onboard study provides the first experimental results of neutron energy spectrum over 10 MeV, and the photon and proton energy spectra. Our experimental result shows a large, sharp neutron peak around 70 MeV where the previous research had a broad neutron peak around 100 MeV. The measured neutron spectrum is reasonably good by comparing our results with other experimental and calculation results. The measured photon and proton energy spectra are between the results calculated with LUIN2000, EXPACS and RMC codes. The measured neutron, photon and proton energy spectra are very useful for benchmark simulation of computer codes.
Goldhagen, P., Reginatto, M., Kniss, T., Wilson, J.W., Singleterry, R.C., Jones, I.W., Van Steveninck, W., 2002. Nucl. Instrum. Methods Phys. Res. A. 476, 42e51. Goldhagen, P., Clem, J.M., Wilson, J.W., 2004. Radiat. Prot. Dosimetry 110, 387e392. Gordon, M.S., Goldhagen, P., Rodbell, K.P., Zabel, T.H., Tang, H.H.K., Clem, J.M., Bailey, P., 2004. IEEE Trans. Nucl. Sci. 51 (6), 3427e3434. International Commission on Radiation Protection, 2003. ICRP Publ. 92. Pergamon Press, Oxford. 92. Iwase, H., Niita, K., Nakamura, T., 2002. J. Nucl. Sci. Technol. 39 (11), 1142e1151. Lewis, B.J., Desormeaux, M., Green, A.R., Bennett, L.G.I., Butler, A., McCall, M., Saez Vergara, J.C., 2004. Radiat. Prot. Dosimetry 111 (2), 151e171. MCNPX User’s Manual. 2005. Los Alamos National Laboratory report, LA-CP-05-0369. Nakamura, T., Nunomiya, T., Abe, S., Terunuma, K., Suzuki, H., 2005. J. Nucl. Sci. Technol. 42 (10), 843e853. O’Brien K., 1978. EML-338 (update of HASL-275). Sato, T., Yasuda, H., Niita, K., Endo, A., Sihver, L., 2008. Radiat. Res. 170, 244e259. Shin, K., Uwamino, Y., Hydo, T., 1981. Nucl. Technol. 53, 78e85. Takada, M., Taniguchi, S., Nakamura, T., Fujitaka, K., 2001. Nucl. Instrum. Methods Phys. Res. A. 465, 512e524. Takada, M., Taniguchi, S., Nakamura, T., Nakao, N., Uwamino, Y., Shibata, T., Fujitaka, K., 2002. Nucl. Instrum. Methods Phys. Res. A. 476, 332e336. Takada, M., Awaya, I., Iwai, S., Iwaoka, M., Masuda, M., Kimura, T., Takagi, S., Sato, O., Nakamura, T., Fujitaka, K., 2004. J. Nucl. Sci. Technol. (Suppl. 4), 399e402. Takada, M., Lewis, B.J., Boudreau, M., Anid, H. Al, Bennett, L.G.I., 2007. Radiat. Prot. Dosimetry 124 (4), 289e318. Takada, M., Yajima, K., Yasuda, H., Sato, T., Nakamura, T., 2010a. J. Nucl. Sci. Technol. 47 (10). Takada, M., Yajima, K., Yasuda, H., Nakamura, T., Baba, M., Honma, T., Endo, A., Tanimura, Y., 2010b. J. Nucl. Sci. Technol. 47 (10). Takada, M., Nakamura, T., 2007. Radiat. Prot. Dosimetry 126, 178e184. Yajima, K., Yasuda, H., Takada, M., Sato, T., Goka, T., Matsumoto, H., Nakamura, T., 2010. J. Nucl. Sci. Technol. 47 (1), 31e39.
Acknowledgements The experimental flight was financially supported by the Ground-based Research Program for Space Utilization promoted by the Japan Space Forum (JSF). We offer our heartfelt appreciation to the Diamond Air Services, Inc. for their flight operation.
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Neutron, photon and proton energy spectra at high altitude measured using a phoswich-type neutron detector M. Takada a, *, K. Yajima a, H. Yasuda a, T. Sato b, T. Nakamura a, c a
National Institute of Radiological Sciences, Inage-ku Chiba, 263-8555, Japan Japan Atomic Energy Agency, Tokai 319-1195, Japan c Cyclotron and Radioisotope Center, Tohoku University, Sendai 980-8578, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 November 2009 Received in revised form 31 May 2010 Accepted 9 June 2010
Neutron energy spectrum from 7 to 180 MeV, photon energy spectrum from 4 to 50 MeV and proton energy spectrum from 94 to 145 MeV were measured simultaneously using a phoswich-type neutron detector with particle discrimination methods at atmospheric depth of 249 g/cm2, a vertical cut-off rigidity of 10.2 GV and at a heliocentric potential of 312 MV. We compared our results with other measured and calculated particle energy spectra. Our measured results give a large, sharp neutron peak around 70 MeV, although Bonner balls show a broad peak around 100 MeV due to low energy resolution. The measured photon and proton spectra are between the calculated energy spectra. This onboard study provides the first experimental neutron energy spectrum over 10 MeV with a highenergy resolution. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Cosmic ray Neutron Photon Proton Energy spectrum Phoswich-type Liquid scintillator Plastic scintillator Particle discrimination Aviation altitude
1. Introduction Radiation environments at high altitudes are composed of a variety of particles such as neutron, proton, photon, electron and muon. Aircrews are exposed to elevated levels from 2 to 7 mSv/hr of cosmic radiation (Lewis et al., 2004) and larger than on the ground, 0.1 mSv/h. Ionizing components of cosmic particles have been measured using several detectors with good accuracy. The neutron component contributes about half of dose equivalent and its assessment is important for radiation protection of flight crews, however, neutron energy spectra were measured using multimoderator spectrometers (Goldhagen et al., 2002), so called Bonner balls, despite of low energy resolution and strong dependence on initial-guess spectra postulated a priori. High energy neutrons over 10 MeV contribute about 80% in neutron effective dose rates at high altitudes using the fluence-to-equivalent dose coefficients for isotropic exposure given by ICRP
* Corresponding author. Tel.: þ81 43 205 3464; fax: þ81 43 206 3514. E-mail address: [email protected] (M. Takada). 1350-4487/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2010.06.012
publication 92 (ICRP92, 2003). This study focuses on the highenergy neutron, photon and proton spectra over 10 MeV. We attempted to measure energy spectra of neutrons, photons and protons with high energy resolution using a newly developed phoswich-type detector simultaneously with particle discrimination methods. We compared our results with several experimental and calculated results.
2. Flight experiment 2.1. Flight route Cosmic neutron, photon and proton energy spectra were measured near the central part of Japan at an altitude of 10.8 0.03 km (249 g/cm2 atmospheric depth) and geographical latitude 39 N and longitude from 134.6 to 136.9 E on 13th February 2009 (Takada et al., 2010a) onboard an MU300 business jet operated by Diamond Air Service Inc. The vertical cut-off rigidity in the observation area was 10.2 GV. The solar modulation was reported to be at a heliocentric potential of 312 MV.
1298
M. Takada et al. / Radiation Measurements 45 (2010) 1297e1300
2.2. Phoswich-type detector
Neutrons
The phoswich-type neutron detector consists of an EJ309 organic liquid scintillator, 121.7 mm in diameter and a 121.7 mm long, covered with a 15-mm-thick EJ299-13 plastic scintillator (Takada and Nakamura, 2007) and coupled with a photomultiplier, 120 mm in diameter, R1250, via an acrylic light guide. The EJ309 liquid scintillator exhibits high flash-point (140 C) and auto-ignition (440 C) temperatures. Using a harmless and biodegrable EJ309 liquid scintillator improves the safety of the neutron detector for onboard experiments. Signals from the phoswich detector were acquired using a fast analog-to-digital converter and then converted to digital data with a 12 bit dynamic range for each 2 ns period from 10 ns before to 400 ns after the rise of the signal. 150e160 signals per second were measured throughout the flight and with 7.3% dead time. The phoswich detector can measure neutron, photon and proton separately in mixed radiation fields. Non charged particles of neutrons and photons deposit their energies mainly in the inner liquid scintillator, and on the other hand, charged particles deposit their energies both in the inner liquid and outer plastic scintillators. The inner liquid and outer plastic scintillators produce signals with very different decay times of 3.5 ns and 285 ns, respectively. Using particle-dependent pulse shapes, neutron, photon and proton events were discriminated each other (Takada et al., 2004, 2002, 2001).
0.4
2.3. Response function Response functions of neutron, photon and proton are necessary to obtain these energy spectra using the unfolding method (Shin et al., 1981). The neutron and photon response matrices were created by Takada et al. (Takada et al., 2010b) on the basis of simulation using the Monte Carlo N-Particle eXtended (MCNPX) code (MCNPX, 2005). Maximum energies of the response matrices were 300 MeV for neutrons and 50 MeV for photons. The created response functions were verified by comparing the calculated and experimental results using several quasi-monoenergetic neutron beams produced by 40e80 MeV protons at three cyclotron facilities in the National Institute of Radiological Sciences, Cyclotron and Radioisotope Center, Tohoku University and Takasaki ion accelerators for advanced radiation application of the Japan Atomic Energy Agency (JAEA) in Japan; and mono-energy photon beams produced by the p-F reactions in JAEA. Absolute responses were agreed each other within 15% difference. Proton response matrices were also obtained with the MCNPX calculation in the proton energy range from 78 to 190 MeV, which protons deposit their energies in the inner liquid scintillator. Proton pulse heights are selected using the boundaries of proton events in calculated particle discrimination plots. The response matrices were made with source geometry of the upper hemisphere covering the phoswich detector, which simulates cosmic radiation field at the high altitude. 3. Results and discussion 3.1. Neutron energy spectrum The measured neutron energy spectrum from 7 to 180 MeV using the phoswich detector is plotted with error bars in Fig. 1. The measured spectrum shows a large, sharp neutron peak around 70 MeV and a lower energy peak of 12 MeV. We compared our result with other experimental and calculated results using three codes: LUIN2000 (Code for the Calculation of Cosmic Ray Propagation in the Atmosphere) (O’Brien, 1978), EXPACS (Excel-based Program for calculating Atmospheric Cosmic-ray Spectrum)
This work Yajima et al. (Bonner Ball) Goldhagen (Extended Bonner Ball) Nakamura (Liq. Scint.) LUIN2000 (FLUKA) EXPACS (PHITS) RMC Model (MCNPX)
0.2
Energy
Flux (n cm-2 s-1)
0.3
0.1
0 1
10
102 Neutron Energy (MeV)
Fig. 1. Measured neutron energy spectrum at 249 g/cm2 atmospheric depth, 10.2 GV vertical cut-off rigidity and 312 MV heliocentric potential, compared with experimental results using Bonner balls by Goldhagen (Goldhagen et al., 2004) and Yajima (Yajima et al., 2010) and using an organic liquid scintillator on the ground by Nakamura (Nakamura et al., 2005), and calculated results using EXPACS (Sato et al., 2008), LUIN2000 (O’Brien, 1978) and RMC (Takada et al., 2007) codes. The measured results were corrected with cut-off rigidity and atmospheric depth.
(Sato et al., 2008) and RMC (Royal Military College of Canada) (Takada et al., 2007) codes. Yajima et al. (Yajima et al., 2010) and Goldhagen et al. (Goldhagen et al., 2004) used Bonner balls and Bonner balls with extended energy responses above 1 GeV. Nakmaura et al. measured the neutron energy spectra above 6 MeV with high energy resolution using an organic liquid scintillator but their measurements were done on the ground. The other measured results were normalized at our measured locations, 249 g/cm2 in atmospheric depth and geographical latitude 93 N, by correcting for neutron attenuation in air (166 g/cm2, measured by Nakamura (Nakamura et al., 2005)), considering the cut-off rigidity, evaluated by Gordon (Gordon et al., 2004). The LUIN2000 calculates atmospheric cosmic-ray propagation based on an analytical solution of Boltzmann transport theory, the EXPACS is also based on the analytical solution induced from the PHITS (Particle and Heavy Ion Transport code System) simulation (Iwase et al., 2002) and the RMC code uses a Microsoft Excel lookup table based on the MCNPX simulation (MCNPX, 2005). Particle energy spectra at a top of atmosphere are transported in the air and then at high altitude (10 km) the particle energy spectra are much affected with the incident particle spectrum at the top of atmosphere. Local interstellar spectrum (LIS) modulated with the solar wind and shielded by the geomagnetic field is used as the particle spectra at the top of atmosphere. LIS in the RMC code, referred from the LUIN code, is calculated with energies below 10 GeV/nucleon using the equation,
h
f ¼ 9:9 104 E þ 780exp 2:5 104
i2:65
;
where E is MeV/nucleon, and above 10 GeV is represented by Peters’ model; however, LIS in the EXPACS is calculated using the Nymmik model. These calculated LISs agree well with measured
Table 1 Measured neutron fluxes compared with other experimental and calculation results. Numbers in parentheses indicate the flux ratios of each result to our measured result. The last line shows the flux ratios considering the extrapolated neutron spectrum in our result. Flux in n cm2 s1
This study
Extended Bonner ball
RMC
7e180 MeV 30e100 MeV (peak) 7e2 GeV
0.27 (1.0) 0.15 0.0028 (1.0) 0.32 (1.0)
0.25 (0.91) 0.11 (0.73) 0.31 (0.96)
0.25 (0.91) 0.10 (0.67) 0.35 (1.08)
proton and helium spectra. Different solar modulation models are used in the LUIN2000 and RMC codes (heliocentric potential), and the EXPACS (force field formalism). Incident particle spectra close to each other were used in the calculations. Our neutron energy spectrum shows a large sharp peak around 70 MeV different from the other experimental and calculated results with a broad peak around 100 MeV. Different widths of the peaks in our result and the Bonner ball result are observed because of the energy resolutions of the detectors. The liquid scintillator with the high energy resolution shows a sharp peak around 70 MeV. Our sharp peak can be demonstrated to result from the good energy resolution of this detector. In the neutron energy range below 30 MeV our result shows larger than the measurement on the ground using the liquid scintillator. It could be caused by the different neutron attenuation in air. The sharp neutron peak around 70 MeV and a valley around 30 MeV may be explained by a combination of neutron production and attenuation through in air. Our measured neutron fluxes are compared with the extended Bonner ball and RMC code results integrated in the measured energy region, 7e180 MeV, our peak energy region, 30e120 MeV and 7e2 GeV, including the extrapolated neutron energy spectrum above 180 MeV in our result (Table 1). Although our results are 9 and 27% larger than the extended Bonner ball fluxes and 9 and 33% larger than the RMC code calculations in the neutron energy range from 7 to 180 MeV (measured energy range) and from 30 to 100 MeV (peak energy range), our result agree well with the extended Bonner ball flux within 4% and the RMC code calculation
Photon 1 LUIN2000 (SemiEmpirical Model) EXPACS (PHITS Base)
Flux (n cm-2 MeV-1 s-1)
This work
10-1
10-2
-3
10
1
10
102 Photon Energy (MeV)
Fig. 2. Measured photon energy spectrum compared with spectra calculated using the LUIN2000 and EXPACS codes.
Proton Flux (n cm-2 s-1 MeV-1)
M. Takada et al. / Radiation Measurements 45 (2010) 1297e1300
1299
10-3
10-4
-5
10
This Work LUIN2000 EXPACS RMC
10-6
3
102
10 Proton Energy (MeV)
Fig. 3. Measured proton energy spectrum compared with energy spectra calculated using the LUIN2000, EXPACS and RMC codes.
within 8% by considering the extrapolated energy region, 180e2 GeV. This confirms that our measured neutron energy fluxes are quite reasonable. 3.2. Photon energy spectrum Fig. 2 shows the photon energy spectrum, 4e50 MeV, with error bars from unfolding errors and the calculated results using the LUIN and EXPACS codes. Our results fall between the two calculations. Our results agree well with the LUIN calculation within 3%, 4e10 MeV and with the EXPACS calculation within 11% above 25 MeV. 3.3. Proton energy spectrum The proton energy spectrum was measured in the energy range of 94e145 MeV in Fig. 3. We compared our experimental results with energy spectra calculated using the LUIN2000, EXPACS and RMC codes. The proton energy spectrum was obtained from the proton events, where protons deposited their energies in the inner liquid scintillator and stopped in the liquid scintillator. The proton energy spectrum below 100 MeV is increased. It could be considered by protons scattered materials around the detector. Our result shows different spectrum shape than the calculated results. We compared proton fluxes (Table 2). The calculations using EXPACS and RMC codes show larger fluxes than our results, however, the LUIN2000 code shows smaller fluxes. This comparison shows opposite results of neutron fluxes. Smaller neutrons could be created with smaller cross sections of neutron productions by protons and larger protons are transmitted through the atmosphere in the Monte Carlo calculations.
Table 2 Measured proton fluxes compared with other calculated results in n cm2 s1. Numbers in parentheses indicate the flux ratios of each result to our measured result. This work
LUIN2000
EXPACS
RMC
5.28 103 (1)
3.26 103 (0.62)
10.30 103 (1.95)
8.55 103 (1.62)
1300
M. Takada et al. / Radiation Measurements 45 (2010) 1297e1300
4. Conclusion
References
The neutron energy spectrum from 7 to 180 MeV, the photon energy spectrum from 4 to 50 MeV and the proton energy spectrum from 94 to 145 MeV are measured simultaneously using the phoswich neutron detector at the 10.8 km altitude (atmospheric depth of 249 g/cm2), the vertical cut-off rigidity of 10.2 GV and the heliocentric potential of 312 MV. This onboard study provides the first experimental results of neutron energy spectrum over 10 MeV, and the photon and proton energy spectra. Our experimental result shows a large, sharp neutron peak around 70 MeV where the previous research had a broad neutron peak around 100 MeV. The measured neutron spectrum is reasonably good by comparing our results with other experimental and calculation results. The measured photon and proton energy spectra are between the results calculated with LUIN2000, EXPACS and RMC codes. The measured neutron, photon and proton energy spectra are very useful for benchmark simulation of computer codes.
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Acknowledgements The experimental flight was financially supported by the Ground-based Research Program for Space Utilization promoted by the Japan Space Forum (JSF). We offer our heartfelt appreciation to the Diamond Air Services, Inc. for their flight operation.