Calibration and performance of doped-Ge photoconductors for the ISO Long Wavelength Spectrometer

The Long Wavelength Spectrometer (LWS), part of the Infrared Space Observatory (ISO), uses an array of 10 doped-germanium photoconductors to cover the wavelength range 45–180 μm. The array comprises one Ge:Be detector for the 45–50 μm range, five unstressed Ge:Ga detectors for wavelengths between 50 and 110 μm and four stressed Ge:Ga detectors for the long wavelength range 110–180 μm. 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 ranging from 10⁻¹⁷ W Hz^{−$\text{1}\text{2}$} for Ge:Be to 5 × 10⁻¹⁹ W Hz^{−$\text{1}\text{2}$} 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. The characteristics of these effects are described and the implications for operation and calibration of the LWS considered.