2078 zyxwvutsrqponm IEEE TRANSACfIONS zyxwvuts ON NUCLEAR SCIENCE, VOL. 47, NO. 6, DECEMBER 2000 CdTe Detectors’ Response to Irradiation with High-Energy Gamma-Rays P.Chirco2.* zyxwvuts , E. Caroli’, A. Cavallini3, W. Dwi”*, B.Fraboni3, M. Hage-Ali4, M.P.Morigi2, P. Siffert4, M. Zanarini2 ‘Istituto TESRE/CNR, Via Gobetti 10 1,40 126 Bologna, Italy *Physics Department, University of Bologna and zyxwvu I” Bologna, v.le Berti Pichat 6/2, Bologna, Italy Physics Department, University of Bologna and INFM Bologna, v.le Berti Pichat 6/2, Bologna, Italy 4Laboratoire PHASE, Strasbourg, France ’ zyxw IEEE zyxwvut Member 3 Abstract In recent years the performance of room-temperature semiconductor detectors such as CdTe has improved and they are now suitable candidates for several applications. However, some key parameters that can severely affect such perfomances have not been measured in detail yet. We have extended previous studies on the radiation damage of a set of CdTe detectors irradiated in a 6oCo gamma-cell in a wide range of doses. A full characterization of the performance of irradiated detectors has been obtained by means of spectroscopic, electrostatic, photo-induced current transient spectroscopy and photo-deep level transient spectroscopy measurements to quote the energy resolution, the leakage current, the activation energy and capture cross-section of deep level defects, respectively. I. INTRODUCTION‘ The interest towards the use of cadmium telluride detectors was greatly increased in the recent years because they offer a good trade-off between key performance, such as the energy resolution and the absorption efficiency, and the complexity of the experimental equipment. Indeed, a wide range of application is possible; extensive studies were devoted to biomedical and industrial imaging and non destructive testing. This is due to their well-known capability to operate at room temperature with minor degradation of spectroscopic performance. Furthermore, the lack of bulk cooling devices and the miniaturized dimensions (down to few cubic millimetres) are a particular premium for those applications where size and weight are critical parameters, like the space applications [l]. It is worth noting that many of these applications require that the detectors are operative in a hostile environment where they are exposed for long periods to a high flux of ionising radiation. Some problems and uncertainties are still to be addressed for an extensive deployment of CdTe detectors. In particular, the low value of the hole mobility inhibits the production of spectrometric grade gamma-ray detectors with an interelectrode distance greater than few mm . This limits the maximum absorption length or the active entrance area, depending on the geometrical position of the electrodes with respect to the impinging radiation [2]. Furthermore, the detectors often present different physical properties even if they were produced from the same crystal. 0018-9499/00$10.00 For this reason, a quantitative and extensive characterization of all the trapping sites is not yet completed. The exploitation of CdTe detectors for applications involving their operativity under a hostile environment requires an extensive characterisation of the degradation of spectroscopic performances, that is far from complete. However, a preliminary study under irradiation with 6oCo gamma rays [3] revealed that CdTe detectors are quite insensitive to the radiation damage up to some thousands Gy. When the delivered dose exceeded this value, the performances began to degrade, although they were still working in a spectroscopy mode when irradiated at some tens of kGys. A dose of 100 kGy was fatal for all the tested samples. The above results were significant but should be refined to characterise the detector performance with better accuracy. For this reason, a new set of CdTe detectors has been irradiated to study their behaviour in detail for doses greater than 10 kGy. 11. EXPERIMENTAL SETUP AND RESULTS A set of CdTe:Cl detectors of 1 x 2 ~ 5 mm3 was roduced from a single ingot. They were contacted on 2x5 mm surfaces by using electroless gold deposition to be used in the PTF (Planar Transverse Field) configuration. Each detector was then coated with a thin plastic layer (100 pm) of a special polymer - named GALXILTM - that was developed specifically to give these devices a good insulation against humidity, moisture and scratches. They were mounted on a plastic base to provide them support and ruggidized contacts were made for the bonding wires to the read-out electronics. The characterization of the detectors involved the measurement of some key parameters: the leakage current, the energy resolution, the photopeak efficiency, the stability of gain and the energy distribution of trapping levels in the band gap. All these parameters was measured prior to and again after each irradiation. Radiations were performed by using a 6oCo gamma-cell. The dose-rate of the gamma-cell was measured at a value of 1940 Gy/h by means of a standard Fricke solution. P The leakage current The importance of the leakage current for optimal spectroscopic performance is well-known. This is particularly true for room-temperature semiconductor detectors. Indeed, it has been previously demonstrated [4] that the fluctuations of zy 0 2000IEEE