Low Energy Characterization of Caliste HD, a Fine Pitch CdTe-Based Imaging Spectrometer

Abstract : Caliste HD is a recently developed micro-camera designed for X and gamma-ray astronomy, based on a 1×1 cm$^2$ CdTe Schottky pixelated detector. Its entire surface is composed of 256 pixels, disposed on a 16 × 16 pixel array. This spectrometer is buttable on its 4 sides and can be used to create a large focal plane. It is also designed for space environment. Its IDeF-X front-end electronics has low power consumption, excellent noise performance and a wide dynamic range, from 2 keV to 1 MeV. Moreover, electronic noise performances of this device were optimized to set the low level energy threshold lower than 2 keV. This paper focuses on the Caliste HD performance near the low energy limit. For this purpose, we have exposed the Caliste HD module to a mono-energetic X-ray beam, and set energies between 2 and 12 keV. We measured accurately the detection efficiency in this energy range and found it to be ranging from 39% to 75% for energies from 2.2 keV to 11.6 keV, considering only particles detected in the single-event photopeak and ignoring events impinging between two adjacent pixels. This efficiency detection profile thereby highlights crucial effects of the Pt electrode opacity on Caliste HD low energy response, and suggests the presence of absorption zones at the interface between CdTe crystal and platinum. Respective thickness of each layer were estimated by simulation and confirmed by RBS ($Rutherford\ Backscattering\ Spectroscopy$). Besides, using a mono-energetic beam allows fine energy resolution measurement, which was found to be ranging from 560 to 760 eV FWHM between 2 and 12 keV. In addition, the linearity of this spectrometer and the issue of charge sharing between adjacent pixels were studied. This study revealed that spectroscopic performances remain excellent for such boundary operating conditions.
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Sebastien Dubos, Olivier Limousin, Claire Blondel, Remi Chipaux, Youri Dolgorouky, et al.. Low Energy Characterization of Caliste HD, a Fine Pitch CdTe-Based Imaging Spectrometer. IEEE Transactions on Nuclear Science, Institute of Electrical and Electronics Engineers, 2013, 60 (5), pp.3824 - 3832. ⟨10.1109/TNS.2013.2281572⟩. ⟨cea-01791971⟩

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