CIPHER: coded imager and polarimeter for high-energy radiation

The CIPHER instrument is a hard X- and soft γ-ray spectroscopic and polarimetric coded mask imager based on an array of cadmium telluride micro-spectrometers. The position-sensitive detector (PSD) will be arranged in 4 modules of 32×32 crystals, each of 2×2 mm ² cross section and 10 mm thickness giving a total active area of about 160 cm ². The micro-spectrometer characteristics allow a wide operating range from ∼10 keV 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 array (MURA) pattern, is four times the area of the PSD and is situated at about 100 cm from the CdTe array top surface. The CIPHER instrument is proposed for a balloon experiment, both in order to assess the performance of such an instrumental concept for a small/medium-size satellite survey mission and to perform an innovative measurement of the Crab polarisation level. The CIPHER's field of view allows the instrument to keep a single source within the field of view for a long observation period without requiring a precise pointing system. Herein we describe the instrument design, together with results obtained in our development studies, in particular on CdTe micro-spectrometers and the integrated front-end electronics. Furthermore, we present the expected operational performance in terms of image and spectral quality (angular and energy resolution) and polarimetric capabilities for an observation of the Crab nebula from balloon altitudes.