P. Chirco

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.

X-ray 3D computed tomography of bronze archaeological samples

X-ray cone-beam computed tomography (CT) is one of the most powerful non-destructive testing (NDT) technique for the whole 3D inspection of a sample. The development of suitable 2D detectors has permitted the set-up of very fast and efficient CT systems able to collect and reconstruct hundreds of tomographic slices in short time and with a more efficient use of the X-ray flux. At present, this technique is widely applied in industrial field but it can be employed successfully even in the analysis and restoration of archaeological samples. An experimental 3D CT system has been set up at the Physics Department of University of Bologna (Italy) for the investigation of small bronze objects. Preliminary tests have been carried out on an Etruscan fibula and results concerning radiographic inspection, slice tomographic analysis and 3D reconstruction of the object will be presented.

Evaluation of hydrogen content in metallic samples by neutron computed tomography

Neutron radiography is currently a well-known technique, which is employed for non-destructive testing in a number of industrial and environmental applications. Originally developed for reactor fuel examinations, it is now effective in detecting small amounts of corrosion and infiltrations of hydrogen or light materials within thick metallic structures due to the particular behaviour of total neutron cross sections. Nevertheless, improvements related to the development of tomographic systems, which allow far better imaging performances, have been achieved only in the last few years, as a consequence, primarily, of the production of large, charge coupled device (CCD) arrays. Nowadays, neutron computed tomography is the technique most suited for the study of the distribution of hydrogen within metallic matrices. In this field, a series of experimental tests were carried out employing a set of nickel samples containing a H/sub 2/O-D/sub 2/O solution in known percentages. It was possible, therefore, to obtain a calibration curve for the total neutron cross section vs. Gray level in the reconstructed image. >

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.

Computer simulation of charge trapping and ballistic deficit effects on gamma-ray spectra from CdTe semiconductor detectors

Abstract In recent years, a variety of special-purpose software has been developed to simulate γ-ray semiconductor detector and associated electronics performances. Unfortunately, these software systems often lack flexibility and cannot be applied outside rather limited ranges. General radiation transport Monte Carlo codes such as ITS and EGS4 can achieve high levels of physical accuracy, but the simulated pulse-height spectra are completely noise free, and therefore differ significantly from experimental results. In this work, the ITS output files have been modified, so as to add to Monte Carlo simulated spectra the subsequent degradation effects inherent in the detection process. Ballistic deficit losses, electronic noise, charge trapping and detrapping are taken into account: the corresponding simulation algorithms are independently combined into the final spectra. As the charge collection efficiency depends upon the position of γ-ray interactions within the bulk, the detector has been virtually sliced into a large number of sections so as to statistically evaluate the spectrum distortion on a local spectra basis. Although the algorithms developed were applied to γ-ray spectra obtained from CdTe detectors only, the generality of the method makes it suitable for any semiconductor detector.

CdTe detectors' response to irradiation with high-energy gamma-rays

In recent years the performance of room-temperature semiconductor detectors such as CdTe are improved and they are now suitable candidates for several applications. However, some key parameters that can severely affect such performances 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 /sup 60/Co 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.

Comparative evaluation of the temperature dependence of different noise sources in CdTe detectors

Abstract The number of applications of CdTe detectors for imaging purposes is currently growing at a fast rate. Nevertheless, noise is still a limiting factor in several circumstances, particularly when a high dynamic range is needed. Thus, knowledge of the comparative importance of various noise sources (mainly referring to the dark current in the detector, the electronic noise in the preamplifier and the statistical noise) is valuable for the development of efficient noise reduction techniques. The dependence on temperature of these noise sources has been evaluated over the range from −30°C up to 40°C. Although some components of the noise show a marked temperature dependence, the overall detector spectroscopic performances are constant up to 10°C, while above this level an appreciable degradation occurs.