Performance characteristics of doped-Ge photoconductors for the Infrared Space Observatory Long Wavelength Spectrometer

The Long Wavelength Spectrometer (LWS), part of the Infrared Space Observatory (ISO), uses an array of ten doped-germanium photoconductors to cover the wavelength range 45 - 200 micrometers . The array comprises a Ge:Be detector for the 45 - 50 micrometers range, five unstressed Ge:Ga detectors for wavelengths between 50 - 110 micrometers and four stressed Ge:Ga for the long wavelength range 110 - 200 micrometers . We have calibrated the performance of the detectors, both individually and in the LWS. Optimum values of detector bias and operating temperature that maximize the sensitivity of the LWS have been established, resulting in detector NEPs of 10-17 WHz-1/2 for Ge:Be, 5 X 10-18 WHz-1/2 for unstressed Ge:Ga and 5 X 10-19 WHz-1/2 for stressed Ge:Ga. Under the low flux conditions expected in the LWS (< 10-14 W at the detector), doped-Ge photoconductors exhibit a number of other non-linearities and memory effects. Additionally, the performance of the detectors will be strongly influenced by ionizing radiation encountered during orbital perigee. The characteristics of these effects are described and the implications for operation and calibration of the LWS considered