The Herschel Space Observatory is a passively cooled 3.5 m telescope (Tmirror < 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 μm to 210 μm with unprecedented sensitivity and spatial resolution. PACS has two cameras for imaging spectroscopy in two wavelength bands from 57 μm to 130 μm and 130 μm to 210 μm. 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 μm and ~ 205 μm 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.
We are presently developing large format photoconductor arrays for the Herschel Space Observatory and for the Stratospheric Observatory For Infrared Astronomy (SOFIA). These arrays are based on individual Ge:Ga detectors contained in integrating cavities which are fed by an array of light cones to provide for area-filling light collection in the focal plane of an instrument. In order to detect light at wavelengths > 120 μm, uniaxial stress has to be applied to each detector crystal. We have developed a method to efficiently stress an entire stack of detector elements which allows us to form two-dimensional arrays from an arbitrary number of linear detector modules. Each linear module is read out by a cryogenic readout electronics circuit which operates at 4 K and is mechanically integrated into the module. We have measured effective quantum efficiencies of the light cone / detector /read-out chain of > 30% under realistic background conditions.
GaAs photoconductive detectors could extend the spectral response cut-off up to > 300 μm. In the past, a continuous progress in material research has led to the production of pure, lightly and heavily doped n-type GaAs layers using the liquid phase epitaxy technique (LPE). Sample detectors demonstrated the expected infrared characteristics of bulk type devices. Modeling of BIB detector types predicts an improved IR sensitivity due to the attainable higher doping of the infrared sensitive layer. However, the modeling gives also an estimate of the severe material requirements for the n-type blocking layer. With a new centrifugal technique for the LPE material growth we intend to achieve this goal. Technical details of this unique equipment, first results of the achieved material quality in the initial growth runs and future steps to optimize operational parameters are reported. If successful, this detector technology will be first implemented in our spectrometer FIFI LS for SOFIA.
The photoconductor detector arrays for the PACS instrument (Photoconductor Array Camera and Spectrometer) aboard the future ESA telescope Herschel have been developed during the engineering phase in 1999. In early 2000 the construction of the qualification models began for both, the highly and low stressed Ge:Ga arrays, which consist of 12 linear modules each. These two types of photoconductor arrays are dedicated for different wavelengths bands in the spectrometer section of the instrument. While the performance of a few engineering arrays has been studied and presented earlier, additional data are meanwhile available on the absolute responsivity and quantum efficiency of the detectors. Furthermore, experience has been obtained during manufacture of a larger series of arrays giving better statistics on performance aspects, such as uniformity of the cutoff wavelengths and of the responsivity or the maximum stress obtainable within such arrays. Considerable progress has also been made in the development and manufacture of the 4 Kelvin Cold Read-out Electronics (CRE), which will integrate and multiplex the signals generated in each linear array with its 16 detector pixels. Manufacture of the detector arrays for the qualification model is scheduled to be completed by this summer, and manufacture of the flight model has already started. The qualification model will be delivered to the test facilities, where absolute spectral performance of the 24 linear modules will be determined. In this paper we give a summary of the related activities and results as obtained during manufacturing and testing.