Gamma-ray spectrometer of high purity germanium detector cooled by the Stirling cycle cryostat in the lunar mission SELENE

The gamma ray spectrometer (GRS) will be onboard the Japanese lunar polar orbiter SELENE, to be launched in 2004, to obtain the information of element abundance on the entire lunar surface. GRS consists of a high-purity Ge detector as a main detector, which is an n-type Ge crystal with a high detection efficiency of 60% encapsulated in an aluminum canister. In addition, BGO and plastic scintillators are employed for an anti-coincidence shield and for suppression of Compton photons and energetic ion backgrounds. GRS will be the first gamma ray spectrometer employing a Ge detector for the lunar mission. GRS adopted a Stirling cycle refrigerator, whose cooling capacity is 2.0 W at 80 K. The mechanical vibration from the cryosystem to the Ge detector is effectively reduced by the dual opposed pistons in the compressor and the flexible thermo-coupling link between the two components. An excellent energy resolution of <3 keV at 1.33 MeV has been achieved in the prototype-model of GRS.