Over the last decade, cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) wide band gap semiconductors have attracted increasing interest as X-ray and gamma ray detectors. Among the traditional high performance spectrometers based on silicon (Si) and germanium (Ge), CdTe and CdZnTe detectors show high detection efficiency and good room temperature performance and are well suited for the development of compact and reliable detection systems. In this paper, we review the current status of research in the development of CdTe and CdZnTe detectors by a comprehensive survey on the material properties, the device characteristics, the different techniques for improving the overall detector performance and some major applications. Astrophysical and medical applications are discussed, pointing out the ongoing Italian research activities on the development of these detectors.

Abstract. The scientific instrumentation on board the X-ray Astronomy Satellite BeppoSAX includes a Medium Energy Concentrator Spectrometer (MECS), operating in the energy range 1.3 − 10 keV, which consists of three units, each composed of a grazing incidence Mirror Unit and of a position sensitive Gas Scintillation Proportional Counter. The design and performance of the MECS instrument are here described, together with its on-ground calibration.

We report the discovery of a transient equivalent hydrogen column density with an absorption edge at ∼3.8 kiloelectron volts in the spectrum of the prompt x-ray emission of gamma-ray burst (GRB) 990705. This feature can be satisfactorily modeled with a photoelectric absorption by a medium located at a redshift of ∼0.86 and with an iron abundance of ∼75 times the solar one. The transient behavior is attributed to the strong ionization produced in the circumburst medium by the GRB photons. The high iron abundance points to the existence of a burst environment enriched by a supernova along the line of sight. The supernova explosion is estimated to have occurred about 10 years before the burst. Our results agree with models in which GRBs originate from the collapse of very massive stars and are preceded by a supernova event.

We report on the 0.5È200 keV spectral properties of Cyg X-1 observed at di †erent epochs with the Narrow Field Instruments of the BeppoSAX satellite. The source was in its soft state during the Ðrst observation of 1996 June. In the second observation of 1996 September, the source had parameters characteristic to its hard state. A soft X-ray excess, a broad Fe Ka line and Compton reÑection are clearly detected in both states. The soft-state broadband continuum is well modeled by a disk blackbody (accounting for the soft excess) and Compton upscattering of the disk photons by a hybrid, thermal/ nonthermal plasma, probably forming a corona above the disk (also giving rise to the ComptonreÑection component). In the hard state, the primary hard X-ray spectrum can be well modeled by Compton upscattering of a weak blackbody emission by a thermal plasma at a temperature of D60 keV. The soft excess is then explained by thermal Comptonization of the same blackbody emission by another hot plasma cloud characterized by a low value of its Compton parameter. Finally, we Ðnd the characteristic ratio of the bolometric Ñux in the soft state to that in the hard state to be about 3. This value is much more compatible with theories of state transitions than the previously reported (and likely underestimated) value of 1.5.

We present BeppoSAX follow-up observations of GRB980425 obtained with the Narrow Field Instruments (NFI) in April, May, and November 1998. The first NFI observation has detected within the 8 ′ radius error box of the GRB an X-ray source positionally consistent with the supernova 1998bw, which exploded within a day of GRB980425, and a fainter X-ray source, not consistent with the position of the supernova. The former source is detected in the following NFI pointings and exhibits a decline of a factor of two in six months. If it is associated with SN 1998bw, this is the first detection of X-ray emission from a Type I supernova above 2 keV. The latter source exhibits only marginally significant variability. The X-ray spectra and variability of the supernova are compared with thermal and non-thermal models of supernova high energy emission. Based on the BeppoSAX data, it is not possible to firmly establish which of the two detected sources is the GRB X-ray counterpart, although probability considerations favor the supernova.