Proton irradiation of PACS stressed Ge:Ga detector arrays to simulate L2-orbit conditions

The ESA Herschel space observatory will be launched in 2008 into the Earth-Sun L2 orbit and the three instruments onboard will be exposed to cosmic radiation during the 4 years lifetime of the satellite. To study the impact of ionizing radiation on the Ge:Ga photoconductors of the PACS instrument (Photodetector Array Camera and Spectrometer), we performed a series of irradiation measurements at the cyclotron of the University of Louvain la Neuve, Belgium simulating the in-flight predicted proton fluxes including solar flare events. The PACS integral field spectrometer contains two 25x16 pixel arrays of Ge:Ga crystals: a low stressed configuration is used in the wavelength range from 55 to 105 mu m, and a high stressed device covers the range 105 to 210 Imi. Calibration of the detector modules under realistic IR background fluxes is done at MPE Garching and MPIA Heidelberg. 70 MeV protons were generated at the cyclotron test site. They were attenuated on their way to the detectors by beam conditioning elements and the metal shields of the cryostat before they reached the Ge:Ga crystals with a mean energy of 17 MeV and a standard deviation of 1.5 MeV. According to predictions the expected proton fluxes were set to nominally 10 P S(-1)cm(-2) and to 400 p s(-1)cm(-2) simulating solar flares. We observed radiation-induced glitches in the detector signal, changes in responsivity, increase in noise and transient behavior. The ongoing data evaluation indicates optimal operating parameters, the best curing method and frequency, calibration procedures and data processing algorithms aiming for a high photometric accuracy.