Proton irradiation of PACS stressed Ge:Ga detector arrays to simulate L2-orbit conditions
2006
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 25×16 pixel arrays of Ge:Ga crystals: a low stressed configuration is used in the wavelength range from 55
to 105 μm, and a high stressed device covers the range 105 to 210 μm. 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 ps -1 cm -2 and to 400 ps -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.
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