M.P. Morigi

Experimental micro-CT system for x-ray NDT

This work describes the setup of an experimental system for microtomography developed in the framework of a collaboration between the Physics Department of the University of Bologna (Italy) and the Geosphaera Research Center of Moscow (Russia). The main goal of this inspection system is to carry out high-resolution analysis in vitro of biomedical samples as well as nondestructive testing (NDT) of industrial components. The detection system consists of a 30x15 mm2 rectangular fiberoptic taper (ratio 2:1) optically coupled to a cooled 12-bit CCD camera (1024x512 pixels). On the entrance window of the taper is deposited a thin layer of Gd2O2S:Tb phosphor which provides the X-light conversion. The image readout is carried out by means of a commercial frame grabber installed on a personal computer and specific software is used for data acquisition and control of the tomographic process. The object under investigation is arranged on a 3-degree micro-positioning system (x-y translation and rotation) and irradiated by an X-ray microfocus beam (up to 200 kVp). The sample can be positioned easily along the source-detector axis in order to obtain a large magnification of details of interest. The X-ray detector has been intensively tested in order to determine its performance in terms of MTF, NPS, and DQE. Moreover, preliminary tests have been carried out on several samples in order to evaluate the performance of the micro-CT system.

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.

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.

Numerical Determination of Personal Aerosol Sampler Aspiration Efficiency

In this work the determination of the aspiration efficiency of personal aerosol samplers, commonly used in occupational exposure assessment, is investigated by means of CFD techniques. Specifically, it will be described a code to calculate the particle trajectories in a given flow field. At the present state the code considers only the effects of the mean flow field on the particle motion, whereas the turbulent diffusion effects are neglected. Comparisons with experimental measurements are also given in the framework of a research contract, supported by the European Community, with several experimental contributions from the participants. The main objective of the European research is to develop a new approach to experimentation with airborne particle flows, working on a reduced scale. This methodology has the advantage of allowing real-time aerosol determination and use of small wind tunnels, with a better experimental control. In this article we describe how the methodology has been verified using computational fluid dynamics. Experimental and numerical aspiration efficiencies have been compared and the influence of gravity and turbulence intensity in full and reduced scale has been investigated. The numerical techniques described here are in agreement with previous similar research and allow at least qualitative predictions of aspiration efficiency for real samplers, taking care of orientation from the incoming air flow. The major discrepancies among predicted and experimental results may be a consequence of bounce effects, which are very difficult to eliminate also by greasing the sampler surface.

Contrast evaluations in a digital mammographic system

It is worldwide accepted that the enhancement of the dynamic range is probably the most important improvement of digital mammography based on solid state detectors with respect to its screen-film version. In turn, this means a better contrast resolution of the image and then an improved detectability of low-contrast features. Moreover the acquisition of intrinsically digital mammographic images allows to perform specific numerical processing for further a posteriori enhancement of contrast in regions where suspect pathologic details appear. On these bases, a new system for digital mammography has been set up at the Department of Physics of the University of Bologna. Some conventional mammographic screens are alternatively coupled to a fiber optic taper which drives the light onto a CCD camera through a MCP image intensifier. This paper describes the detailed characterization of the system in terms of the achievable spatial resolution and image contrast.

Comparison of the radiation damage induced by thermal neutrons in CdTe and CdZnTe detectors

In recent years, room temperature semiconductor detectors like CdTe and CdZnTe have been proposed for several scientific, industrial and medical applications. In some cases, these applications require the capability to operate for a long time in intense, sometimes mixed, radiation fields while retaining full spectroscopic performances. In spite of its importance, a detailed characterization of the effects of the radiation damage has not been competed yet for these materials. However, preliminary results carried out by irradiating CdTe detectors with 60Co gamma rays demonstrated that the fatal photon dose is on the order of some tens of thousands Gy, while the detectors are rather insensitive to doses up to few thousands Gy. A wider activity is now being performed by the authors studying both the CdTe and the CdZnTe when involving several types of irradiating sources and different techniques to quantify the induced damaging. This includes the detailed quantitative analysis of retained spectroscopic performances at low and medium energies, the measurement of the dark current and the characterization of the defects and modifications induced in the structure of the crystal by the irradiation process.

A proposed multi-detector method for prompt 90Sr detection in post-accident environmental matrices.

Strontium-90 is of relevant biological importance among fission products released during a fission reactor accident. Rapid information on its concentration in environmental matrices is extremely valuable, yet this implies chemical separations and handling with a shift in daughter equilibrium and therefore the need to wait a sufficient time for the equilibrium to be reestablished. The work concerns a feasibility study of a multi-detector system for a prompt evaluation of the 90Sr activity or, at least, a prompt determination of its order of magnitude in the presence of other pure beta and beta-gamma emitters; their interferences are examined on the basis of the Chernobyl releases and their decay properties. The technique is based on a plastic scintillator beta detector and a guard ring of BGO counters which can be logically connected in coincidence and anticoincidence. The evaluations show that a few hours are sufficient to determine a specific activity comparable with the Maximum Permissible Concentration in air by sampling 10 m3.