Gianni Landini

Twin shaping filter techniques to compensate the signals from CZT/CdTe detectors

A Gamma Ray Burst Monitor (GRBM) has been proposed to form part of the LOBSTER experiment, approved by ESA for a Phase A study for a future flight (2009) aboard the International Space Station (ISS). The GRBM detector would be based on CdZnTe array modules, maintained, together with the front-end electronics, at a temperature of about 250 K by using Peltier effect or passive cooling systems. To improve the detector performance several hardware (HW) and software (SW) techniques are being tested. In addition to the strip readout technique for rejecting charged particles interacting with the detector, we have investigated a method which employs a pair of active filters (one slow and one fast) to analyze differently shaped signals from the same charge sensitive preamplifier. This technique could be particularly useful for application with multi-element detection systems requiring dedicated front end and readout integrated circuits (ASICs) especially designed for the implementation of the HW correction procedure. Some experimental results are presented from the application of the biparametric technique on CdZnTe/CdTe detectors with planar and segmented electrodes in order to study the influence of the correction parameters and to verify the efficiency of the correction algorithm on groups of neighboring pixels.

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.

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.

A Polarimetric Experiment With a Laue Lens and CZT Pixel Detector

A new generation of high sensitivity telescopes based on the use of Laue lenses coupled with high efficiency solid state focal plane detectors has been identified as a possibility for hard X and soft gamma ray astronomy. For this kind of space mission, polarimetry is recognized as a very important observational parameter and therefore this capability should be included as one of the primary scientific requirements. In this framework our group has realized an experiment to demonstrate that the combination of a Laue Lens, built using Cu mosaic crystals, with a CZT pixel detector is capable of measuring the polarization of hard X ray sources contemporaneously with spectroscopy and imaging. This experiment was performed at the beginning of March 2008 using the ID15B beam line at ESRF (Grenoble). The instrument was based on a pixel CZT detector (5 mm thick array with 11times11 pixels of 2.5times2.5 mm2) with a sensitive area of 3times3 cm2 in conjunction with a mosaic Cu crystal (15times15 mm2, 4 mm thick) used in the Laue diffraction configuration. During the tests the Cu crystal was rotated so as to simulate a Laue lens ring, and the detector was moved in order to have the diffracted beam always impinging on the same pixel. The ID15B beamline allowed us to test the response of this system to almost 100% linearly polarized photons at ~90, 270, and 350 keV. In this paper, we describe the experimental setup and we report on the first results, with particular emphasis on the evaluation of possible systematic effects introduced in the detected polarization of the impinging photons by the Laue diffraction process.

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.

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.

Development status of a Laue lens for high-energy x rays (>60 keV)

A Laue lens for focusing X-ray photons with energies above 60 keV for astrophysical applications is being developed. The lens is based on mosaic crystals of Cu (111) produced at the Institute Laue-Langevin. A feasibility study has allowed to establish lens geometry and crystal properties required. The test of the crystals has provided very satisfactory results. We are now developing a Demonstration Model (DM) of the lens in order to establish the best assembling technique of the crystals. We will discuss the status of the project and its prospects.

A study of temperature dependence of some relevant parameters performed on a set of CdTe detectors

Cadmium telluride is a semiconductor material which is now widely used in both spectroscopic and imaging applications. Among its properties, the most advantageous is its ability to operate at room-temperature, so as to avoid the need of bulk cooling devices. Nevertheless, a quantitative evaluation of the temperature dependence of the detectors response is needed in order to establish a temperature range for safe operations. In this paper, some parameters are evaluated at temperatures ranging from -35/spl deg/C up to 40/spl deg/C with an energy source of 511 keV for a set of three spectroscopy grade CdTe detectors. The data obtained seem to indicate a significant temperature independence below 0/spl deg/C, while at temperatures over 10/spl deg/C spectra degradations occur. >

Prototype of a mercuric iodide X-ray camera: Preliminary results

Abstract We present a prototype of an X- and γ-ray camera based on a semiconductor crystal of mercuric iodide (HgI 2 ) having a cross-sectional area of 10 × 10 mm 2 and a thickness of 0.5 mm. Two orthogonal sets of 20 charge-collecting μstrips are deposited on the two opposite surfaces of the detector giving a pixel size of 0.5 × 0.5 mm 2 . Using commercial hybrid preamplifiers several experimental tests have been performed in order to assess the performance of this device. Preliminary results on the spectroscopic capabilities together with an evaluation of the attainable spatial resolution are presented.

Hard x-ray imaging via crystal diffraction: first results of reflectivity measurements

Hard x-ray reflectivity measurements of mosaic crystals are being performed at the x-ray facility of the physics department of the University of Ferrara. This paper reports on preliminary results obtained by using flat samples of pyrolytic graphite (002) with a thickness of 2 mm and a mosaic spread of 0.3 deg. A short description is given of the experimental apparatus and calibration procedures followed.