Radon alpha-ray detector on-board lunar mission SELENE

Alpha-ray detector (ARD) will be on-board SELENE, a Japanese lunar orbiter to be launched around 2006. Primary target is the alpha particles emitted by ²²² Rn and ²¹⁰ Po. ²²² Rn is produced by the decay of ²³⁸ U and emanates from the lunar surface. It is trapped by the lunar gravity and decays with the half-life of 3.8 days emitting 5.490 MeV alpha particle. In the decay sequence of ²²² Rn, ²¹⁰ Po emits alpha particle with the energy of 5.305 MeV. Time scale of the activity is dominated by the 21-year half-life of ²¹⁰ Pb. Thus, alpha particle intensity from ²¹⁰ Po is an indicator of the change of radon emanation rate and change of crust condition due to seismic activity or impact events for the time scale of ∼50 years, while that of ²²² Rn reflects the current emanation rate. Results from Apollo 15, 16, and recent Lunar Prospector mission indicate that the average amount of radon on the lunar surface is much smaller than expected, and the radon-alpha distribution suggests that radon comes out through gas emanation from fissures of the lunar surface. We developed a large-area detector of 326 cm ² for the ARD, which is 15-20 times larger than the detectors of Apollo and Lunar Prospector. Reduction of the background was achieved with the anti-coincidence by rejecting cosmic-ray tracks. It will enable: (1) precise global mapping of the radioactive material on the lunar surface; (2) identification of gas emanation location; (3) study of the radon gas emanation mechanism on the lunar surface and the origin of the lunar atmosphere; (4) obtaining information on the crustal movement during the last ∼50 years.