Characterization of a CdTe microstrip detector as a hard X ray focal plane prototype

Our collaboration is developing a hard X-ray telescope operating in the 10-100 keV energy range and based on focusing multi-layers coupled to a CdTe/CZT focal plane detector. Two classes of detectors can comply with the focal plane requirements: pixel detectors, where an electrode is segmented as a bi-dimensional array and micro-strip detectors. In the last ones each of the opposite crystal surfaces contains a set of orthogonal micro-strips. This geometry reduces noticeably the number of readout independent channels even if it requires the coincidence among at least two opposite strips to determine the position. In the current work we present the characterization of the spectroscopic and spatial response of CdTe prototype (1 cm2) equipped with two orthogonal micro-strip electrodes with 0.5 mm pitch and mounted on an alumina substrate using both collimated and un-collimated radioactive sources. The 16 anode strips use, as front-end analogical electronics, an innovative low noise and low power dissipation (<1 mW/ch) ASIC, whereas the cathode strips pulses are readout by 16 charge sensitive preamplifiers. The handling and acquisition electronics implements a gamma camera logic, where the equivalent of 256 channels are acquired and converted via 4 ADC and afterwards sent to a 32 bit parallel interface. This system calculates a weighted average of the signals from the irradiated strips to determine the mean position and the energy of the interaction. The comparison between these results and those obtained with an equivalent pixel detector should allow to decide the definitive design of the hard X ray focal plane detector.