Caliste 64: detection unit of a spectro imager array for a hard x-ray space telescope

Abstract

In the frame of the hard X-ray Simbol-X observatory, a joint CNES-ASI space mission to be flown in 2014, a prototype of miniature Cd(Zn)Te camera equipped with 64 pixels has been designed. The device, called Caliste 64, is a spectroimager with high resolution event time-tagging capability. Caliste 64 integrates a Cd(Zn)Te semiconductor detector with segmented electrode and its front-end electronics made of 64 independent analog readout channels. This 1 × 1 × 2 cm 3 camera, able to detect photons in the range from 2 keV up to 250 keV, is an elementary detection unit juxtaposable on its four sides. Consequently, large detector array can be made assembling a mosaic of Caliste 64 units. Electronics readout module is achieved by stacking four IDeF-X V1.1 ASICs, perpendicular to the detection plane. We achieved good noise performances, with a mean Equivalent Noise Charge of ~65 electrons rms over the 64 channels. Time resolution is better than 70 ns rms for energy deposits greater than 50 keV, taking into account electronic noise and technological dispersal, which enables to reject background by anticoincidence with very low probability of error. For the first prototypes, we chose CdTe detectors equipped with Al-Ti-Au Schottky barrier contacts because of their very low dark current and excellent spectroscopic performances. So far, three Caliste 64 cameras have been realized and tested. When the crystal is cooled down to -10°C, the sum spectrum built with the 64 pixels of a Caliste 64 sample results in a spectral resolution of 664 eV FWHM at 13.94 keV and 841 eV FWHM at 59.54 keV. SCIENTIFIC AND TECHNOLOGICAL CONTEXT Simbol-X missionThe Simbol-X mission is a hard X-ray space telescope. It is currently undergoing a phase B with the French and the Italian space agencies. This telescope will fill the gap between X-ray telescopes with grazing mirrors, efficient until ~10 keV (XMM-Newton, Chandra missions) and hard X-ray and gamma-ray instruments using indirect imaging technique with coded masks to detect higher energies (INTEGRAL, SWIFT missions). Simbol-X will be a 20 meter focal length observatory, achieved by two satellites flying in formation. One carries the grazing incidence mirror to focus X-rays until 80 keV. The other satellite carries the detector payload. This focusing technique improves sensitivity and angular resolution by two orders of magnitude compared to the current instruments above 10 keV. As a consequence, a wide range of sources can be studied in detail, such as galactic and extra-galactic compact sources, supernovae remnants, cluster of galaxies, or young stellar objects. The core scientific objectives of Simbol-X are black holes physics and particle acceleration mechanisms [1,2].The detection set consists of three detector units [3]. The Low Energy Detector is a silicon drift detector with DEPFET readout, also called "Macro Pixel Detector" [4]. It covers the energy range from 0.5 to 20 keV. The High Energy Detector (HED) installed below the first detector is efficient from 8 to 100 keV. The third unit...

Micro hard-X ray camera: From Caliste 64 to Caliste 256

“…As the shaping time is several JlS whereas the required timing resolution in Silnbol-X is only few hundreds of ns, time-walk correction is necessary to derive the accurate arrival time of a photon from the raw data. Measurements and calibrations conclude that front-end electronics contributes to an error on time-tagging inferior to 70 ns rms for energy deposits greater than 50 keV [10].…”