The CIPHER telescope for hard X and soft γ-ray polarimetry

CIPHER (Coded Imager and Polarimeter for High Energy Radiation) is a hard X and soft γ-ray spectroscopic and polarimetric coded mask telescope based on an array of Cadmium Telluride microspectrometers. The position sensitive detector (PSD) is arranged in 4 modules of 32x32 crystals, each of 2x2 mm2 cross section and 10 mm thickness giving a total active area of about 160 cm2. The PSD is actively shielded by CsI crystals on the bottom and sides in order to reduce background and operates over a wide energy range (∼10 keV to 1 MeV). The mask, based on a modified uniformly redundant array (MURA) pattern, is about four times the area of the PSD and, being situated at about 100 cm from the CdTe array top surface, provides a wide field of view. 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 (or an external ISS-alpha payload) survey mission and to perform an innovative measurement of the Crab polarization level. CIPHER's wide field of view allows it 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 on CdTe detectors. Furthermore we present the expected performance in terms of image and spectral quality (angular and energy resolution) and polarimetric capabilities for an observation of the Crab pulsar from balloon altitudes