E. Caroli

Progress in the Development of CdTe and CdZnTe Semiconductor Radiation Detectors for Astrophysical and Medical Applications

Over the last decade, cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) wide band gap semiconductors have attracted increasing interest as X-ray and gamma ray detectors. Among the traditional high performance spectrometers based on silicon (Si) and germanium (Ge), CdTe and CdZnTe detectors show high detection efficiency and good room temperature performance and are well suited for the development of compact and reliable detection systems. In this paper, we review the current status of research in the development of CdTe and CdZnTe detectors by a comprehensive survey on the material properties, the device characteristics, the different techniques for improving the overall detector performance and some major applications. Astrophysical and medical applications are discussed, pointing out the ongoing Italian research activities on the development of these detectors.

Coded aperture imaging in X- and gamma-ray astronomy

Coded aperture imaging in high energy astronomy represents an important technical advance in instrumentation over the full energy range from X- to γ-rays and is playing a unique role in those spectral ranges where other techniques become ineffective or impracticable due to limitations connected to the physics of interactions of photons with matter. The theory underlying this method of indirect imaging is of strong relevance both in design optimization of new instruments and in the data analysis process. The coded aperture imaging method is herein reviewed with emphasis on topics of mainly practical interest along with a description of already developed and forthcoming implementations.

IBIS/PICsIT in-flight performances

PICsIT (Pixellated Imaging CaeSium Iodide Telescope) is the high energy detector of the IBIS telescope on-board the INTEGRAL satellite. PICsIT operates in the gamma-ray energy range between 175 keV and 10 MeV, with a typical energy resolution of 10% at 1 MeV, and an angular resolution of 12 arcmin within a100 square degree field of view, with the possibility to locate intense point sources in the MeV region at the few arcmin level. PICsIT is based upon a modular array of 4096 independent CsI(Tl) pixels,0.70 cm 2 in cross-section and 3 cm thick. In this work, the PICsIT on-board data handling and science operative modes are described. This work presents the in-flight performances in terms of background count spectra, sensitivity limit, and imaging capabilities.

Growth and Characterization of CZT Crystals by the Vertical Bridgman Method for X-Ray Detector Applications

CdZnTe crystals were grown by the vertical Bridgman method in closed quartz ampoules. The crystalline quality and the impurity content of these crystals were studied. Several X-ray detectors were cut out of these crystals. The resistivity, emission spectra, μτ product, and spectroscopic characteristics of these detectors were extensively measured and compared with the characteristics of detectors obtained from CdZnTe crystals grown by the boron oxide encapsulated vertical Bridgman technique. The detectors prepared from crystals grown without boron oxide show good μτ value, spectroscopic resolution, and higher reproducibility. The influence of growth method on impurity content and on detector response was discussed.

Hard x-ray response of pixellated CdZnTe detectors

In recent years, the development of cadmium zinc telluride (CdZnTe) detectors for x-ray and gamma ray spectrometry has grown rapidly. The good room temperature performance and the high spatial resolution of pixellated CdZnTe detectors make them very attractive in space-borne x-ray astronomy, mainly as focal plane detectors for the new generation of hard x-ray focusing telescopes. In this work, we investigated on the spectroscopic performance of two pixellated CdZnTe detectors coupled with a custom low noise and low power readout application specific integrated circuit (ASIC). The detectors (10×10×1 and 10×10×2 mm3 single crystals) have an anode layout based on an array of 256 pixels with a geometric pitch of 0.5 mm. The ASIC, fabricated in 0.8 μm BiCMOS technology, is equipped with eight independent channels (preamplifier and shaper) and characterized by low power consumption (0.5 mW/channel) and low noise (150–500 electrons rms). The spectroscopic results point out the good energy resolution of both dete...

Investigation of the electric field distribution in x-ray detectors by Pockels effect

In x- and γ-ray semiconductor detectors, the distribution of the internal electric field strongly affects their charge-collection properties. In planar detectors, the electric field distribution depends not only on the semiconductor characteristics, but mainly on the nature of the electrical contacts. This is an issue in CdTe and CdZnTe (CZT) detectors, where many efforts have been devoted to the study and the improvement of contacts, with the aim of suppressing dark current to low values and enhancing the collection of the photo-generated carriers. This work explores the effect of recently developed electrodes for CdTe detectors on the internal electric field distribution by exploiting the voltage-induced birefringence, which is shown by zincblende-structured materials such as CdTe. We used the transverse Pockels effect, in which near-infrared radiation impinges on the side of the CdTe detector parallel to the contact planes, and therefore perpendicular to the electric field. The implemented set-up and the procedure to reconstruct the electric field distribution is presented. The system allows us to extract maps of the electric field with a good spatial resolution, and to study its dependence on the applied voltage. The use of the Pockels effect appears to be a powerful tool, not only to gain information on the electric field, but also to analyse the time evolution observed, at room temperature, in the performance of these detectors.

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.

Characterization of Bulk and Surface Transport Mechanisms by Means of the Photocurrent Technique

The transport properties of ternary alloys, as CdZnTe, affect heavily the features required in X and Gamma spectroscopic detectors. The density and the nature of bulk defects and the lacking quality of contacts, characterized by high recombination center densities near the surface, damage strongly the quality of these materials. The photocurrent technique is a powerful tool to study the bulk properties and to investigate contact and surface quality. From steady-state photocurrent spectra information about the bulk trap defects and about the kinds of surface states far and near the contacts, has been obtained. By varying the bias at fixed wavelength, instead, it can be calculated the transport parameters like the product mobility-lifetime ¿¿ and the ratio s/¿, this last one related to the surface underneath the metal contacts. This paper gives some contribution in the interpretation of high energy region of the spectra. The authors correlate the response of samples illuminated with energies above the band gap to the surface states and show the different effects of this states on the electron and the hole transport case. Moreover in this work the authors compare the photocurrent measurement with the X ray spectroscopy and also discuss several phenomena like the shape difference between the spectra taken at different photon flux and bias polarities. Also the band edge shift and the modification of S/¿ parameter observed on varying the bias polarity have been discussed giving new a possible interpretations.

CIPHER, A POLARIMETER TELESCOPE CONCEPT FOR HARD X-RAY ASTRONOMY

The polarisation of astrophysical source emission in the energy range from a few tens of keV up to the MeV region is an almost unexplored field of high-energy astrophysics. Till date, polarimetry in astrophysics–in the energy domain from hard X-rays up to soft γ-rays–has not been pursued due to the difficulties involved in obtaining sufficient sensitivity. Indeed for those few instruments that are capable of performing this type of measurement (e.g. the COMPTEL instrument on the Compton Gamma-ray Observatory and the IBIS instrument on INTEGRAL), polarimetry itself plays a secondary role in the mission objectives, as the efficiencies (0.5% and 10% maximum, respectively) and polarimetric Q factors (0.1 and 0.3, respectively) are relatively limited. In order to perform efficient polarimetric measurements for hard X-ray and soft gamma-ray sources, with an instrument of relatively robust and simple design, a CdTe based telescope (CIPHER: Coded Imager and Polarimeter for High Energy Radiation) is under study. This instrument is based on a thick (10 mm) CdTe position-sensitive spectrometer comprising four modules of 32 × 32 individual pixels, each with a surface area of 2 × 2 mm2 (about 160 cm2 total detection area). The polarimetric performance and design optimisation of the CIPHER detection surface have been studied by use of a Monte Carlo code. This detector, due to its intrinsic geometry, can allow efficient polarimetric measurements to be made between 100 keV and 1 MeV. In order to predict the polarimetric performance and to optimise the design and concept of the CIPHER detection plane, a Monte Carlo code based on GEANT4 library modules was developed to simulate the detector behaviour under a polarised photon flux. The Compton double event efficiency, as well bi-dimensional double event distribution maps and the corresponding polarimetric modulation factor will be presented and discussed. Modulation Q factors better than 0.50 and double event total efficiencies greater than 10% were calculated in the energy range between 100 keV and 1 MeV. Herein we will present and discuss the general problems that affect polarimetric measurements in space, such as the inclination of the source with respect to the telescope optical axis and background radiation. Q factor calculations for several beam inclinations as well as for background together with simulated astronomical sources will be presented and discussed.

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