Characterization of high- and low-stressed Ge:Ga array cameras for Herschel's PACS instrument

Abstract

The Herschel Space Observatory is a passively cooled 3.5 m telescope (T_mirror < 90 K) scheduled for launch in 2007. One of its three scientific instruments is PACS (Photoconductor Array Camera and Spectrometer) which will carry out astronomical observations in the wavelength range of 57 &mu;m to 210 &mu;m with unprecedented sensitivity and spatial resolution. PACS has two cameras for imaging spectroscopy in two wavelength bands from 57 &mu;m to 130 &mu;m and 130 &mu;m to 210 &mu;m. Both cameras are built up from 25 linear arrays, each with 16 detector pixels consisting of Gallium doped Germanium crystals. By stressing these crystals with a bolt­spring mechanism, the desired cut­-off wavelengths of ~ 127 &mu;m and ~ 205 &mu;m can be reached. The detectors are operated at temperatures of ~ 2 K and read out by cryogenic readout electronics (CRE), featuring pre­amplifiers and multiplexers. Test facilities have been designed and built up at MPIA, Heidelberg, and MPE, Garching, in order to characterize and calibrate the spectrometer cameras before integration into the instrument. Both test facilities have a cryostat cooled by superfluid liquid helium. While the MPE facility uses an internal cold black body to illuminate the camera, the facility at MPIA makes use of an external black body and cold attenuation filters. Tests of the qualification models of the spectrometer cameras show that the detector responsivity is ~ 8 A/W and ~ 40 A/W for the low and high stressed detectors respectively, surpassing the requirements. The NEP is currently limited by CRE readout noise and will be improved with the new generation of FM CREs. Ionizing irradiation significantly increases the detector responsivity, which might make it necessary to operate them with a lower bias voltage. On the other hand, radiation effects can be reliably cured by a combination of bias boosts and infrared flashes.