Radiation effects on stressed Ge:Ga array detector of far-infrared surveyor on AKARI

AKARI, the Japanese infrared astronomical satellite, was launched on 2006 February 21 ( UT) and put into a sun-synchronous polar orbit at an altitude of 700 km. Cosmic radiations, particularly protons in the South Atlantic Anomaly (SAA), were expected to affect the performance of the stressed Ge:Ga array far-infrared detector on board AKARI. One of the influences is the radioactivation of the detector housing; gamma-rays from the radioactivated detector housing interact with Ge: Ga elements, producing spikes (so-called glitches) in the electric outputs of the detector. Prior to the launch, we performed a 100 MeV proton-beam irradiation test for an engineering model of the stressed Ge: Ga array, which simulated the SAA passage. In the test, we observed glitches in the detector output that were due to the radioactivation of the detector housing. By investigating the test data, we have computed the glitch rate of the flight array detector expected in the AKARI orbit, including its change with time from the launch to the end of the AKARI mission. After the launch of AKARI, we have compared the performance observed in the orbit to that predicted by the proton-beam test. The glitch rate really changed with time after the launch; we have found that the in-orbit behavior is consistent with the prediction.