N. Auricchio

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.

Polarimetric performance of a Laue lens gamma-ray CdZnTe focal plane prototype

A gamma-ray telescope mission concept [gamma ray imager (GRI)] based on Laue focusing techniques has been proposed in reply to the European Space Agency call for mission ideas within the framework of the next decade planning (Cosmic Vision 2015-2025). In order to optimize the design of a focal plane for this satellite mission, a CdZnTe detector prototype has been tested at the European Synchrotron Radiation Facility under an similar to 100% polarized gamma-ray beam. The spectroscopic, imaging, and timing performances were studied and in particular its potential as a polarimeter was evaluated. Polarization has been recognized as being a very important observational parameter in high energy astrophysics (> 100 keV) and therefore this capability has been specifically included as part of the GRI mission proposal. The prototype detector tested was a 5 mm thick CdZnTe array with an 11 x 11 active pixel matrix (pixel area of 2.5 x 2.5 mm(2)). The detector was irradiated by a monochromatic linearly polarized beam with a spot diameter of about 0.5 mm over the energy range between 150 and 750 keV. Polarimetric Q factors of 0.35 and double event relative detection efficiency of 20% were obtained. Further measurements were performed with a copper Laue monochromator crystal placed between the beam and the detector prototype. In this configuration we have demonstrated that a polarized beam does not change its polarization level and direction after undergoing a small angle (< 1 degrees) Laue diffraction inside a crystal

Characterization of a CZT focal plane small prototype for hard X-ray telescope

The promise of good energy and spatial resolution coupled with high efficiency and room temperature operation has fuelled a large international effort to develop cadmium zinc telluride (CZT) for hard X-ray applications. We are involved on the development of a hard X-ray telescope based on multilayer optics and focal plane detector operative in the 10-80 keV energy range. This telescope requires a high efficiency focal plane providing both fine spatial resolution and spectroscopy with a compact and robust design. This paper reports preliminary results on the characterization both in spectroscopic and spatial response of two small pixellated CZT detectors (10times10times1 mm3 and 10times10times2 mm3 single crystals) with 0.45 mm pixel size. We present the results obtained using both standard commercial read-out electronics Readout Electronics for Nuclear Applications (RENA) and innovative low noise and low power dissipation ASICs developed within the collaboration

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.

CZT X-ray detectors obtained by the boron encapsulated vertical Bridgman method

Recently, some of the authors showed that it is possible to grow CZT crystals by the boron oxide encapsulated vertical Bridgman method. The most important feature of the technique is that the crystal, during the growth, is fully encapsulated by a thin layer of liquid boron oxide, so that the crystal-crucible contact is prevented. The stress of the crucible to the crystal is strongly reduced also during the cooling, because the boron oxide layer is molten down to about 500°C. A number of detectors have been prepared out of these crystals. The transport properties (μτ product) have been studied by photoconductivity measurements as well as by determining the response to hard X-ray irradiation. The transport properties have been studied as a function of the indium content and of the position of the wafer which the detector was cut out.

SiliPET: design of an ultra high resolution small animal PET scanner based on stacks of semiconductor detectors

We have studied with Monte Carlo simulations, using the EGSnrc code, a new scanner for small-animal positron emission tomography (PET) based on stacks of double sided semiconductor detectors. In small animal PET imaging (mice) Compton scattering within the animal itself is not an issue and therefore registration of the energy information is not necessary. Low Z materials can therefore be studied for designing a high performance scanner. Each stack is composed of planar detectors with dimension 60times60times1 mm3 and 128 orthogonal strips on both sides to read the two coordinates of the interaction, the third coordinate being the detector number in the stack. Coincidence events are recorded only if two plane detectors in two different stacks registered an energy deposition over threshold. In this way we achieve a precise determination of the interaction point of the two 511 keV photons, overcoming three of the crucial scanner limits: depth of interaction measurement thus eliminating parallax error; inter-detector multiple interactions by requiring a single hit; reduced distance between the detectors reduces significantly effects due to the non colinearity of the emitted gamma rays. The reduced dimensions of the scanner also improve the sold angle coverage resulting in a high sensitivity. This suggests that this is a promising new approach for small animal PET imaging. The sinogram profile for a pointlike source of 18F in a 3 cm diameter water phantom is 0.52 mm FWHM. Due to the large solid angle coverage and relatively high Compton efficiency, the simulated sensitivity at the center of the FOV is 5.1% with 4 stacks each 4 cm thick placed in a box-like configuration. Preliminary results of a proof of principle measurement done with the MEGA advanced Compton imager using a sime1 mm diameter 22Na source showed a focal plane ray tracing FWHM of 1 mm

Development status of a Laue lens project for gamma-ray astronomy

We report the status of the HAXTEL project, devoted to perform a design study and the development of a Laue lens prototype. After a summary of the major results of the design study, the approach adopted to develop a Demonstration Model of a Laue lens is discussed, the set up described, and some results presented.

Spectroscopic response of CZT detectors obtained by the boron encapsulated vertical Bridgman method

A great effort is being presently devoted in growing and studying CdTe and CdZnTe detectors which can be used in a large variety of applications, such as the basic, medical, industrial, and space research.

Polarimetric performance study of a CZT gamma-ray burst monitor concept

In astrophysics, hard X- and soft gamma-ray polarimetry has had limited development. To date, no dedicated polarimeters have ever been launched into space. However, previous Monte Carlo simulations and prototype experimental studies have been carried out by the authors in order to evaluate the polarimetric performance of pixelised CZT matrices. We have applied this Monte Carlo code to the case of a CZT pixel instrument proposed as a Gamma-ray burst monitor (GRBM) for an X-ray payload on board the International Space Station (ISS): the LOBSTER experiment, which has already successfully passed the ESA Phase A study for a future flight in 2009. Herein we present the results of the study of the 4 detection units that compose the GRBM detector. Each of these units is a 24times12 matrix of CZT elementary crystals and the pixels have a cross section of 8times8 mm2, giving an active area of about 184 cm2 for each unit. The detector thickness is nominally 3 mm, with a maximum of 5 mm should this thickness be crucial to exploit the GRBM as a polarimeter. The 4 detection units have a rectangular FOV of 55degtimes 35deg FWHM and their axes are misaligned with each other by 45deg in the direction perpendicular to the ISS motion and 10deg along the ISS direction of motion. The GRBM will operate in the energy range between 3 keV and 300 keV. The energy dependent polarimetric Q factor and detection efficiencies are presented and the expected minimum detectable polarization for gamma ray bursts is discussed

Measurements for the SiliPET project: A small animal PET scanner based on stacks of silicon detectors

In this paper we propose a new scanner for small-animal positron emission tomography (PET) based on stacks of double sided silicon detectors. Each stack is composed of 40 planar detectors with dimension 60 × 60 × 1 mm3 and 128 orthogonal strips on both sides to read the two coordinates of interaction, the third being the detector number in the stack. Multiple interactions in a stack are discarded by an exclusive OR applied between each plane detector of a stack. In this way we achieve a precise determination of the interaction point of the two 511 keV photons. The reduced dimensions of the scanner also improve the solid angle coverage resulting in a high sensitivity. Preliminary results were obtained with MEGA prototype tracker (11 double sided Si detector layers, each with a thickness of 0.5 mm and a strip pitch of 470 μm), divided into two stacks 2 cm apart made of respectively 5 and 6 prototype layers, placing a small spherical 22Na source in different positions. We report on the results, spatial resolution, imaging, spectral and timing performances obtained with double sided silicon detectors, manufactured by ITC-FBK, having an active area of 3 × 3 cm2 and a strip pitch of 500 μm. Two different strip widths of 300 μm and 200 μm, and two thicknesses of 1 mm and 1.5 mm, equipped with 64 orthogonal p and n strips on opposite sides were read out with the VATAGP2.5 ASIC, a 128-channel “general purpose” charge sensitive amplifier.