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Earth Planets Space, Vol. 63 (No. 1), pp. 25-35, 2011
doi:10.5047/eps.2010.01.006

Neutron production in the lunar subsurface from alpha particles in galactic cosmic rays

Shuya Ota1,2,3, Shingo Kobayashi4, Lembit Sihver3,5,6, Naoyuki Yamashita1, and Nobuyuki Hasebe1

1Research Institute for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo, Japan
2National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage, Chiba, Japan
3Chalmers University of Technology, Goteborg, SE-412 96 Gothenburg, Sweden
4Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Sagamihara, Kanagawa, Japan
5Texas A&M University, Texas 77843-3133, USA
6Roanoke College, Salem, Virginia 24153, USA

(Received July 3, 2009; Revised January 19, 2010; Accepted January 22, 2010; Online published February 21, 2011)

Abstract: The neutron production from alpha particles in galactic cosmic rays (GCR) in the lunar subsurface has not been estimated with reliable precision despite its importance for lunar nuclear spectroscopy and space dosimetry. Here, we report our estimation of neutron production from GCR nuclei (protons and alpha particles) with the Particle and Heavy Ion Transport code System (PHITS), which includes several heavy ion interaction models. PHITS simulations of the equilibrium neutron density profiles in the lunar subsurface are compared with experimental data obtained in the Apollo 17 Lunar Neutron Probe Experiment. Our calculations successfully reproduced the data within an experimental error of 15%. Our estimation of neutron production from GCR nuclei, estimated by scaling that from protons by a factor of 1.27, is in good agreement within an error of 1% with the calculations using two different alpha particle interaction models in PHITS during a period of average activity of the solar cycle. However, we show that the factor depends on the incident GCR spectrum model used in the simulation. Therefore, we conclude that the use of heavy ion interaction models is important for estimating neutron production in the lunar subsurface.
Key words: Neutron, alpha particle, lunar and planetary spectroscopy, Moon, gamma ray spectrometer, SELENE, PHITS, heavy ion transportation.


Corresponding author E-mail: shuyaota@aoni.waseda.jp


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