M. Zanarini

Deep levels and compensation in γ-irradiated CdZnTe

The behavior of detector-grade Cd0.9Zn0.1Te in a radiation-hostile environment has been investigated by studying the effects on the material defective states induced by γ irradiation. The detector performance is strongly affected by the presence of charge-trapping centers which may also intervene in the material compensation properties. We have investigated by photoinduced current transient spectroscopy analyses the evolution with increasing irradiation dose of the deep levels both present in the as-grown material and induced by the ionizing radiation. A significant correlation between the material resistivity and some deep levels behavior has been observed. We have compared this trend to the results obtained from γ-irradiated CdTe:Cl to better understand the role deep traps play in the compensation process of II–VI materials.

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.

Defective states induced in CdTe and CdZnTe detectors by high and low energy neutron irradiation

We present a study of the effects of high and low energy neutron irradiation on CdTe and CdZnTe high resistivity detector grade material. The evolution of the defect states under increasing irradiation fluence has been monitored by means of photoinduced current transient spectroscopy analyses. Particular attention is placed on the comparison and correlation between the results from the two materials under the two different neutron irradiation energies. The behavior of the observed deep traps associated with defect states allowed us to attribute an origin to most of them and to understand the role they play in the charge carrier collection process and in the degradation of material spectroscopic capabilities.

Defects introduced in cadmium telluride by γ irradiation

The properties peculiar to high resistivity CdTe:Cl are of great interest because of its application as a radiation detector. The compensation process responsible for the materials semi-insulating character implies the presence in the lattice of impurities and defects which have not yet been thoroughly characterized. The use of CdTe:Cl as a detector exposes the material to high fluxes of ionizing radiation which alter the crystal stoichiometry and affect the resulting electrical and optical properties, but few and scattered experimental data are available about radiation effects on this compound. In this work we have carried out an extensive investigation of the effects of γ irradiation on CdTe:Cl by photoinduced current transient spectroscopy analyses. We have identified the deep levels with activation energies up to midgap and we have followed their evolution with increasing irradiation doses up to 50 kGy, the dose which totally degrades the material detecting properties.

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.

Behavior of CdTe and CdZnTe detectors following electron irradiation

When exposed to an intense radiation field, the spectroscopic capabilities of room temperature CdTe and CdZnTe detectors can be strongly altered by the remarkable injection of energy in the material structure due to the absorption of ionizing, particles. Up to date few experimental data are available about the behavior of II-VI compound detectors to various types of radiation. For this reason we are carrying out an extended study using a large variety of particles for irradiation. In this work we report on the effects of electron irradiation both on CdTe and CdZnTe detectors. We have studied the detector response to electron irradiation through dark current measurements and by 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 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 and compared with the results already obtained by using other radiation sources.

Time and thermal recovery of irradiated CdZnTe detectors

The exposure of Cd0.9Zn0.1Te detectors to increasing doses/fluences of ionizing radiation seriously affects their spectroscopic performance. We have investigated the recovery process of irradiated detectors by means of photon spectroscopy (241Am and 57Co) and PICTS (photo-induced current transient spectroscopy) analyses, to study the evolution with time of their spectroscopic performance and to correlate it with the presence of defective states in the material. The temperatures of the annealing stages in our recovery process varied from room temperature to 380 K, while the time scale varied from 1 h to 4 years. We have observed an improvement of the material detecting properties with time at room temperature only after a long time (years) and for detectors that had not been severely degraded by the irradiation. Preliminary results on thermal treatments are also reported as they proved necessary to recover heavier damage. The recovery effect can be associated with a decrease in concentration of a few specific defective states, thus assessing the crucial role they play in determining the charge collection processes in the material.

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.