CIPHER: coded imager and polarimeter for high-energy radiation

The CIPHER instrument is a hard X- and soft gamma-ray spectroscopic and pol arimetric coded mask imager based on an array of cadmium telluride micro-sp ectrometers. The position-sensitive detector (PSD) will be arranged in 4 mo dules of 32 x 32 crystals, each of'x 2 mm(2) cross section and 10 mm thickn ess giving a total active area of about 160 cm(2). The micro-spectrometer c haracteristics allow a wide operating range from similar to 10keV to 1 MeV, while the PSD is actively shielded by CsI crystals on the bottom in order to reduce background. The mask, based on a modified uniformly redundant arr ay (MURA) pattern, is four times the area of the PSD and is situated at abo ut 100 cm from the CdTe array top surface. The CIPHER instrument is propose d for a balloon experiment, both in order to assess the performance of such an instrumental concept for a small/medium-size satellite survey mission a nd to perform an innovative measurement of the Crab polarisation level. The CIPHER's field of view allows the instrument to keep a single source withi n the field of view for a long observation period without requiring a preci se pointing system. Herein we describe the instrument design: together with results obtained in our development studies, in particular on CdTe micro-s pectrometers and the integrated front-end electronics. Furthermore, we pres ent the expected operational performance in terms of image and spectral qua lity (angular and energy resolution) and polarimetric capabilities for an o bservation of the Crab nebula from balloon altitudes. (C) 2000 Elsevier Sci ence B.V. All rights reserved.