CACTμS: A small CdTe array for a prototype balloon experiment

The Compact Array of Cadmium Telluride Micro Spectrometers (CACTμS) instrument was constructed as a prototype detection plane for the Coded Imager and Polarimeter for High Energy Radiation (CIPHER) telescope. The instrument, flown as a ‘piggy-back’ experiment on a stratospheric balloon from the Milo Italian balloon base in Sicily in July 2002, was constructed to verify the feasibility of using this kind of position sensitive detector for hard X- and soft γ-ray polarimetry and imaging. The main objective was to study the instrumental background at stratospheric balloon altitudes over the 20–1000 keV energy range. Of particular interest is the spectrum and distribution of Compton scattered events that trigger two pixel, the recognition of which is essential for providing a high sensitivity to linear polarization. The CACTμS data will be also used to evaluate the efficiency and the reliability of an off-line numerical algorithm for CdTe signal compensation, which uses the amplitudes obtained by a double shaping filter stage readout electronics. Herein we present both a description of the experiment and preliminary results from the flight data.