Recent results on the factors governing the energy resolution in compound semiconductors to be used as future spectroscopic detectors for hard X-ray astronomy S. Kraft, T. Peacock, M. Bavdaz, A. Owens Space Science Department of the European Space Agency, P.O.Box 299, NL-2200AG Noordwijk M.-A. Gagliardi, S. Nenonen Metorex International Oy, P.O. Box 85, FIN-02201 Espoo F. Schoize, G. Ulm Physikalisch-Technische Bundesanstalt, Abbestr. 2—12, D-10587 Berlin T. Tuomi, M. Juvonen, R. Rantamãki Helsinki University of Technology, P.O. Box 3000, FIN-O215OHUT Espoo ABSTRACT The next generation of X-ray astrophysics missions may well extend the energy range beyond the current limit of 10 keV studied by existing X-ray Astrophysics space missions such as ASCA or future missions such as AXAF and XMM to be launched in the next few years. To address with a high degree of sensitivity the astrophysical problems associated with X-ray emission in the X-ray band from 10 keY to 100 keY a significant extension of the capabilities of focusing X-ray optics and imaging broad band hard X-ray detectors will be required. In this paper we present experimental results from the study of the X-ray energy response from two compound semiconductors: GaAs and CdZnTe. The limitations on the energy resolution due to leakage currents, incomplete charge collection and spatial non-uniformities are presented based on the detailed mapping of the energy response of each type of detector to highly monochromatised synchrotron radiation and radioactive sources in the photon energy range from 1 keY to 60 keV. Correlation of the observed response variations with crystal morphology and imperfections are described using collimated synchrotron radiation in the soft x-ray energy range as a diagnostic tool. Energy resolutions around and well below 1 keY have been measured at photon energies between 1 keV and 60 keV. It is clear that based on these results imaging arrays could be fabricated which will provide an important advance in the capabilities of detectors in this energy range. Keywords Compound semiconductors, CdZnTe, GaAs, TlBr, high resolution spectroscopy, photon detectors 1. INTRODUCTION Astrophysics space missions to be launched in the next few years will mainly address the photon energy range between 100 eV and 10 keV using silicon based detectors such as CCDs which are effectively restricted to x-ray energies below 10 keY by the low absorption efficiency. Recent results obtained by the high energy experiment on BeppoSax"2 together with supporting missions such as the Compton Gamma Ray Observatory (CGRO) , the Rossi X-ray Timing Explorer (RXTE) and GRANAT suggest a need to develop imaging medium energy resolution spectrometers covering the energy range between 10 keV and 100 keY. Future x-ray astronomy missions will therefore address the x-ray production mechanism in such objects as Active Galactic Nuclei (AGN), neutron stars, black holes etc., but now extending the energy range up to 100 keV. Such missions will require imaging detectors either with a wide bandwidth (1 keY to 100 keV) or high energy detectors with a significant response below 10 keY. The latter requirement allows for the overlap with the response of conventional silicon detectors. Missions undertaken at the beginning of the next century and operating in the photon energy range above 15 keV like INTEGRAL will use detectors made of high Z material such as Ge and CdTe. Due to the technological limits of array fabrication, CdTe or GaAs is to be favoured as an array for coded mask imaging spectroscopy. Note such imaging does not rely on the x-rays being brought to a focus and as such the array format has to be very large to provide sufficient collecting area. These types of CdTe arrays coupled to a coded mask can provide only a crude spatial resolution of about 10 arcmin and can only be used for rather moderate energy resolution of about 7% at 100 keY.3 Clearly the future exploration of the x-ray band Part of the SPIE Conference on EUV, X-Ray, and Gamma-Ray 236 Instrumentation for Astronomy IX • San Diego. California • July 1998 SPIE Vol. 3445 • 0277-786X/98/$1O.OO Downloaded From: http://proceedings.spiedigitallibrary.org/ on 12/03/2012 Terms of Use: http://spiedl.org/terms