FIRST-PACS:: Design and performance of the sensor engineering models

The Photoconductor Array Camera and Spectrometer (PACS) will be equipped with two sensor arrays consisting of 16x25 pixels each. Arranged in linear arrays of 16 detectors the sensitivity of the sensors is tuned to the wavelength ranges 60 mu m to 130 mu m and 130 mu m to 210 mu m, by applying different levels of stress to the Ge:Ga crystals utilizing a special leaf spring which is part of each of the 25 modules. The electronics of the sensors are mounted on the same module but thermally isolated from the sensor level which is at a lower temperature of about 2 K. The sensors are read out by a specially developed integrating and multiplexing cryogenic read-out electronics (CRE). With a fore optics made of light cones in front of the detector cavities a 100% filling factor is achieved and a high quantum efficiency close to 0.5 is expected. in order to achieve extremly good stress uniformity in all detectors and therefore equal cutoff wavelengths, a high degree of the quality of the Ce:Ga detectors and of the assembling components used for this dedicated stress mechanism is required. The first 6 engineering modules have been successfully manufactured and tested afterwards. The relative responsivity of a set of pixels has been determined and a good performance has been demonstrated for the sensors, which are very close to fulfilling the requirements for PACS aboard the infrared space telescope FIRST.