Laboratory simulation of cosmic radiation effects on stressed Ge:Ga detectors at L2 and curing

The Photodetector Array Camera and Spectrometer (PACS) is one of three science instruments on board the Herschel space observatory to be launched in 2008. It will perform imaging photometry and spectroscopy in the wavelength range from 57 mu m to 210 mu m. The integral field spectrometer contains two 25 x 16 pixel cameras of Gallium doped Germanium crystals (Ge:Ga). By stressing these crystals, cutoff wavelengths of 127 mu m (low-stressed, 200 N) and 205 mu m (high-stressed, 800 N) are reached. The characterization of these detectors (responsivity, noise equivalent power (NEP), dark current....) is carried out at the Max-Planck-Institutes for Astronomy (MPIA, Heidelberg) and Extraterrestrial Physics (MPE, Garching). Both test facilities allow simulation of the in-flight operational conditions of the arrays and provide accurate IR fluxes by means of external/internal black bodies and calibrated cold attenuation filters. A radioactive Cs-137 source is used at MPIA to simulate the steady cosmic radiation impact on the photoconductor arrays in order to study the radiation induced changes in responsivity, noise, and the transient behavior. The goal is to determine the optimal operating parameters (temperature, bias, integration time...) for the operation at the L2 orbit, the best curing method, curing frequency and calibration procedure for high photometric accuracy. The "lessons learned" on operating, curing, deglitching and calibrating stressed Ge:Ga detectors during the ISO mission are applied as well as the relevant reports from IRAS and Spitzer.