[P-PS06] 月の科学と探査

[PPS06-09] 月水資源探査に向けた放射線検出器MoMoTarOの研究開発

*長岡 央¹、榎戸 輝揚^1,2、辻 直希¹、加藤 陽¹、谷口 絢太郎 ¹、晴山 慎³、大竹 淑恵¹、岩本 ちひろ¹、高梨 宇宙¹、若林 泰生¹、小林 泰三⁴、池永 太一⁵、中野 雄貴⁵、塚本 雄士⁵、草野 広樹⁶、玉川 徹¹、星野 健⁷、上野 宗孝⁷ (1.理化学研究所、2.京都大学、3.聖マリアンナ医科大学、4.立命館大学、5.ソイルアンドロックエンジニアリング株式会社、6.量子科学技術研究開発機構、7.日本航空宇宙研究開発機構)

キーワード：中性子、ガンマ線、水資源

The Moon has been thought to be depleted in volatile elements since the original volatile elements of the Moon had not been observed from the retuned sample from mid- and low-latitudes of the Moon for long time. However, recent global remote sensing observations and lunar sample analyses by improvement of in-situ analyses techniques changed the insight of water-free moon (e.g., Colaprete et al. 2010; Pieters et al. 2009; Saal et al. 2008). Especially, the external origin of lunar water points the transport of volatiles to the lunar surface by the collisions of meteoroids and comets, and implantation of hydrogen by solar wind, and the implanted hydrogen formed hydroxyl (-OH) by reaction with the oxide in the lunar surface material. On the low latitude of the moon where temperatures reach about 400 K, water evaporate easily and cannot exist stably. Part of evaporated water dissipates to the space, and other moves to low-temperature regions (called migration). On the lunar poles, there are permanent shadow regions (PSR) in the bottom of crater where sunlight does not reach. In PSR, extremely low temperature environment keeps water stable. Therefore, it is possible that water was kept in the lunar poles. Many countries including Japan, have paid attention to the water distributed in the lunar polar region and are proceeding with resource exploration plans targeting water resources.

Moon Moisture Targeting Observatory (MoMoTarO) is a small radiation detector consisting of neutron detector and gamma-ray detector. Our team aim to apply it for lunar exploration of water resources. Fast neutrons which were produced by nuclear reactions between galactic cosmic rays and lunar soils, are greatly slowed down to thermal neutrons by the presence of water within the soil on the surface of the Moon. MoMoTarO aims to investigate water resources, by measuring neutrons leaking from the lunar surface onboard lunar orbiter or rover. MoMoTarO is expected to be applied for an elemental analyzer for geological exploration by measuring lunar gamma-rays at the same time. Simultaneous measurement of gamma-ray and neutron is essential for gamma-ray spectroscopy because a correction using neutron fluxes is required (e.g., Naito et al. 2018; Yamashita et al. 2012).

Our team aims to measure neutrons and gamma rays on the lunar surface by using the following three methods: 1) Development of neutron and gamma ray detector MoMoTarO for space observation, 2) Monte Carlo Simulation of lunar radiation environment with Geant4, 3) Water content measurement test on the ground using lunar simulant soil and radiation isotope, and a comparison of the experimental results with the simulation results. In this presentation, current status of their developments and water measurement test on the ground will be reported.