W. Dusi

Radiation effects on II-VI compound-based detectors

The performance of room temperature CdTe and CdZnTe detectors exposed to a radiation source can be strongly altered by the interaction of the ionizing particles and the material. Up to now, few experimental data are available on the response of II–VI compound detectors to different types of radiation sources. We have carried out a thorough investigation on the effects of g-rays, neutrons and electron irradiation both on CdTe : Cl and Cd0.9Zn0.1Te detectors. We have studied the detector response after radiation exposure by means of dark current measurements and of quantitative spectroscopic analyses at low and medium energies. The deep traps present in the material have been characterized by means of PICTS (photo-induced current transient spectroscopy) analyses, which allow to determine the trap apparent activation energy and capture cross-section. The evolution of the trap parameters with increasing irradiation doses has been monitored for all the different types of radiation sources. A comparison of the results obtained for CdTe : Cl and Cd0.9Zn0.1Te detectors allows to deepen our understanding of the detectors’ properties and performance. r 2002 Elsevier Science B.V. All rights reserved.

Radiation damage induced by 2 MeV protons in CdTe and CdZnTe semiconductor detectors

Abstract An experimental investigation of the radiation damage induced on CdTe and CdZnTe semiconductor detectors has been performed by exposing a set of samples to increasing doses of 2 MeV protons produced by a 1.7 MV Tandetron accelerator. The modifications in the detector performances have been studied through the dark current measurements and spectroscopic response analyses at low and medium energies. The deep levels of the materials have been investigated by means of Photo Induced Current Transient Spectroscopy analyses. The evolution of some important parameters (energy resolution, charge collection efficiency, leakage current, activation energies and capture cross-section of deep level defects) have been monitored with respect to increasing proton exposures and the results obtained give us some important indications on the modifications of the material properties as well as on the performances degradation of the detectors.

An evaluation of the possible use of CdTe microdetectors for astrophysical, biomedical and industrial imaging

Abstract Various imaging techniques in fields as diverse as astrophysical research, biomedical diagnostics and industrial tomography are closely related to the development of γ-ray detectors with improved spectral and imaging performances. From this point of view, cadmium telluride solid state detectors are very promising due to their capability to operate at room temperature, their high stopping power and their possibility of miniaturization. The necessity to have contemporaneously good efficiency and high spatial resolution demands that these microdetectors are produced in a novel manner, where the polarization of the applied electric field is perpendicular to the direction of the incoming radiation. In this way, it is possible to achieve a good charge collection efficiency and hence a high energy resolution, together with a high absorption thickness while at the same time having a spatial resolution commensurate with the detector size of about 2×2 mm 2 . The results of measurements regarding the energy resolution of various sizes of μdetector, ranging from 2×2×2 mm 3 to 2.5×2.5×20 mm 3 , are presented and discussed.

Investigation of response behavior in CdTe detectors versus inter-electrode charge formation position

Some important features of semiconductor detectors (pulse height, energy resolution, photopeak efficiency) are strongly affected by charge collection efficiency; therefore low charge mobility and trapping/detrapping phenomena can more or less degrade the CdTe based detectors performance, depending on the distance between the charge formation location and the collecting electrodes. Using a narrow photon beam, obtained by a 20 mm thick W collimator having a 0.2/spl times/2 mm/sup 2/ collimating channel, we have studied the response of a 3/spl times/5/spl times/2 mm/sup 3/ CdTe(Cl) planar detector, having the electrodes deposited on the 3/spl times/5 mm/sup 2/ sides. In order to investigate the behavior of the above parameters vs. the charge formation position induced by the incoming radiation we have performed a fine scanning of the interelectrodic region irradiating laterally the detector, that is in the configuration that we indicate as a planar transverse field (PTF). For comparison the same detector was irradiated also with a non collimated beam both in lateral (PTF) and in the classical configuration, that is with the beam entering through the cathode, a planar parallel field (PPF). When irradiated in PTF way, a region of the detector having a very good spectroscopic performance can be identified close to the cathode; this allows one to select region sizes with definite energy resolution values, suitable for different applications.

Charge transients locally induced by laser pulses in CdTe planar and multi-strip detectors

In detectors based on semiconductor compounds, trapping effects, material nonhomogeneities, and anomalous distribution of the internal electric field are known to affect the charge collection of the photo-generated carriers, and then the spectroscopic performance. Hence, it becomes important to use techniques able to access the local charge collection properties of the detector. To this scope an experimental set-up, which uses a collimated laser beam impinging on the detector at different positions to induce charge signal transients, has been developed. Linear scans and mapping have been performed at different voltages and the charge transients of CdTe planar and multi-strip detectors have been recorded by a proper front-end electronics. Electron and hole contributions have been identified, allowing us to extract relevant charge transport parameters.

Irradiation-induced defects in CdTe and CdZnTe detectors

Abstract The performance of room-temperature CdTe and CdZnTe detectors exposed to a radiation source can be strongly altered by the interaction of the ionizing particles and the material. Up to now few experimental data are available on the response of II–VI compound detectors to different types of radiation sources. We have focussed our attention on the effects of γ-rays and neutron irradiation and we have investigated the exposed detectors by means of dark current measurements and of quantitative spectroscopic analyses at low and medium energies. The deep traps present in the material have been characterized by means of photo-induced current transient spectroscopy analyses, which allow for the determination of the trap apparent activation energy and capture cross-section. The evolution of the trap parameters with increasing irradiation doses has been monitored for both types of radiation sources. The comparison of the results obtained for CdTe and CdZnTe detectors allows us to deepen our understanding of the detectors’ properties and performance.

Timing response of CdTe detectors

Abstract Semiconductor CdTe detectors are gaining wide acceptance in many applications where X- and γ-ray measurements are necessary, such as in astrophysical research, medical imaging and industrial radiography. Good timing response is critical both in applications like positron emission tomography, where fast coincidence capabilities are required, and in single photon counting when a high counting rate is needed. The typical configuration employed, where the direction of the impinging radiation beam is parallel to the collecting electric field, has one well known drawback: an increase in active layer, necessary in order to reach a satisfactory absorption efficiency for the detection of high energy photons, leads to a longer transport path for the charge carriers generated. As a consequence, there is a degradation in energy resolution and a broadening in time response. In the present paper, measurements of the timing response for an unusual configuration of CdTe detectors are presented. In this configuration, which we call a PTF (planar transverse field) detector, the collecting electric field and hence the transport direction of carriers is transverse to the direction of the incoming photons and so detection thickness and transport length are independent. In this way the absorption layer can be increased without impairing the timing performance. The measurements described herein have been performed using a PTF detector having dimensions of 2.5 × 2.5 × 20 mm 3 , in order to have a good efficiency for annihilation γ-ray photons.

An experimental method to evaluate the dead Layer thickness of X- and gamma-ray semiconductor detectors

In the classic irradiation configuration of solid-state X- and Gamma-ray detectors, where the photons impinge normally to the cathode, the electrode and detector dead layer thickness affect the detection efficiency and the spectroscopic performance of the device, mainly at the lowest incident photon energies. The concentration of defects in the near-electrode regions greatly depends on the mechanical and chemical treatments used for the surface preparation (polishing and etching), before electrode deposition, as well as on metallic diffusion in the crystal, which result in differently thick dead layers. In this paper, we evaluate, in an easy, experimental way, the dead layer thickness, irradiating a detector by a narrow photon beam, at various incident angles. The areas relevant to the 14-keV (/sup 57/Co) and to 22-keV (/sup 109/Cd) photopeaks, at different angles of incidence, are independently used to solve a linear equation depending on the photons absorption in the electrodes (Pt) and the dead layer (CdTe) material. As an example, data obtained with a CdTe detector 2-mm-thick and 3/spl times/10 mm/sup 2/ electrode area are presented and discussed.

Characterization of thin back-to-back CdTe detectors

Thin CdTe detectors (3/spl times/5 mm/sup 2/ electrode area, 0.5 and 0.8 mm thick), mounted in back-to-back configuration with common anode have been characterized. This configuration allows one to double the useful absorbing thickness in the classical planar parallel field (PPF) irradiation geometry and to double the sensitive area in the planar transverse field (PTF) geometry, while maintaining the same interelectrode distance (0.5 or 0.8 mm) and one electronic chain as for single detectors. The tests performed aim at understanding the effects on the spectroscopic performance of various interelectrode distances and in particular of the chemical and mechanical treatments used to make thin detectors. A narrow photon beam, 10-150 keV in energy, obtained using a 20-mm-thick tungsten collimator, was employed. The results obtained, compared with previous measurements on various thicknesses devices, indicate that the optimum single detector thickness is 1 mm.

A CdTe gamma-ray spectrometer with imaging capabilities for astrophysics: Monte Carlo simulation results

One of the critical design constraints for satellite-borne gamma-ray astronomy telescopes incorporating solid-state (mainly Ge) detectors has been the problem of keeping the detection plane at a very low temperature. This problem should be alleviated by the considerable progress being made in the technology of room temperature devices such as CdTe detectors. Furthermore, a new geometrical arrangement in the design of these particular detectors allows the use of small devices (μ-detectors) with high detection efficiency and good spectroscopic performance (E/δE ∼ 100). These μ-detectors can be mosaiced in order to form a large area position sensitive detector (PSD) with fine spatial resolution (about a few millimeters). Such a PSD could be coupled with a coded aperture to realise a telescope for use in high energy astronomy (0.05–5 MeV). Herein we give preliminary results from a Monte Carlo simulation of such a PSD concerning the detection efficiency and spatial resolution as a function of photon energy and incident angle. The results suggest that CdTe is a promising new detection medium for use in high energy astronomy.