A laboratory test setup to study the stability of operation of the CdTe detectors within Astro-H HXI

Abstract

ABSTRACT Astro-H 1 is a JAXA/NASA X-ray satellite launched in 17th Feb. 2016. The hard X-ray imager (HXI) 2 is aSi/CdTe stacked detector system which is placed in the focus of a hard x-ray telescope. HXI constitute one ofthe four di erent instruments onboard Astro-H.We are presenting the current status of a stacked detector setup which consists of two mini-HXI double sidedCdTe strip detectors (CdTe DSDs)|similar to those used in HXI|that are read out with the low-noise readoutASIC IDeF-X BD. We describe the con guration of the setup, its spectroscopic performance, and a long-termoperation of the setup. The long-term test simulates the orbital operation of HXI using identical detectortemperatures, bias voltages, and switch-on/switch-o cycles with the goal to study the detector stability and theevolution of its performance during operation.Keywords: Polarization e ect, CdTe instability, Astro-H, HXI, long-term test 1. MOTIVATION The high atomic number of cadmium (Z=48) and tellurium (Z=52) makes CdTe a favorable semiconductormaterial for hard X-ray detectors. Unfortunately, even the purest available CdTe crystals have numerous defectsacting as electron donors and acceptors.Cadmium vacancies create deep acceptor levels (DALs) located within the band gap of CdTe which arebelieved as the cause of the so called e ect. Under biased condition the Fermi level goes abovethese DALs and the DALs become ionized with In this way charge accumulates inside the crystal i.e. thecrystal gets polarized.The accumulated charge diminishes the electric eld near the cathode and enhances the electric eld nearthe anode. This results nally in a decreasing charge collection eciency which can be observed as a spectraldrift towards lower pulse-height amplitudes. The time after crystal biasing when the electric eld at the cathodevanishes is called the time. After the polarization time the detection eciency and the chargecollection eciency decrease tremendously and the detector is no longer usable as a spectroscopic device. SeeMeuris et al. 2011