Twin shaping filter technique for signals compensation in CZT detectors grown by the vertical bridgman method

CdTe/CdZnTe is a consolidated material to realize detectors for a large variety of applications, such as medical, industrial, and space research. An Italian collaboration, involving the CNR/IMEM and INAF/IASF institutes, was born some years ago with the aim to develop a national capability to produce CZT detectors starting from the material growth to the final detection device. Some important features of these detectors (pulse height, energy resolution, photopeak efficiency) are affected by the charge collection efficiency: the low mobility of the charge carriers (particularly the holes) and trapping/detrapping phenomena can degrade the CdTe/CZT detectors response, depending on the distance between the charge formation position and the collecting electrodes. Several efforts have been made to improve the detection efficiency as well as the energy resolution, using both the optimization of the electrode geometry (drift strip technique, coplanar-grid, small pixel effect) and pulse height compensation methods to overcome the hole trapping problem. We have studied a bi-parametric method that uses a twin pulse shaping active filter to analyze the same signal: one slow, which is proportional to the energy of the photon, and one fast, which depends on the position of the interaction with respect to the collecting electrode. The experimental results obtained with the application of this bi-parametric technique on planar CZT detectors of good quality grown by the Vertical Bridgman method at CNR/IMEM are presented as a function of the bias voltage, photon energy and shaping time pairs

Interface shape control and tellurium inclusion concentration distribution in CdZnTe crystals grown by vertical Bridgman for X-ray detector applications

In spite of the efforts devoted to the task, many problems connected with the growth of CdZnTe (Zn>0) crystals are still unresolved, in particular tellurium inclusion density control, large single crystalline yield, seeding, and interface shape control. Moreover, also the electrical properties of the crystals (high resistivity and mobility-lifetime product) must be taken into account if detector performances have to be improved. In this work, the authors report on the growth and characterization of several CdZnTe crystals (Zn=10%) by vertical Bridgman, with and without the use of boron oxide as encapsulant. Different techniques were used to characterize the crystals: i) PL mapping for determining interface shape and to study the nucleation ii) a novel IR mapping apparatus to obtain fully 3D reconstruction of the inclusion distribution iii) X-ray detector characterization by means of nuclear sources to study the transport properties of the material (with mobility-lifetime product for electrons up to 6x10-3 cm2/V)

Charge Transport Properties in CZT Detectors Grown by the Vertical Bridgman Technique

Great efforts are being presently devoted to the development of CdTe and CdZnTe detectors for a large variety of applications, such as medical, industrial, and space research. We present the spectroscopic properties of some CZT crystals grown by the standard vertical Bridgman method and by the boron oxide encapsulated vertical Bridgman method, which has been recently implemented at IMEM-CNR. By this technique the crystal is grown in an open quartz crucible fully encapsulated by a thin layer of liquid boron oxide. This technique prevent the crystal-crucible contact allowing larger single grains with lower dislocation density to be obtained. Several mono-electrode detectors were realized with two planar gold contacts. The samples are characterized by an active area of ≈4x4 mm2 or ≈7x7 mm2 and with thickness ranging from 1 to 2 mm. The charge transport properties of the detectors have been studied by mobility-lifetime (μτ) product measurements, carried out at the European Synchrotron Radiation Facility (Grenoble) in PTF configuration, where the impinging beam direction is orthogonal to the collecting electric field. We have performed several fine scans between the electrodes with a beam spot of 10x10 μm2 at different energies from 60 keV to 400 keV. In this work we present the test results in terms of μτ product of both charge carriers and an evaluation of the spectroscopic response uniformity across the sensitive volume of tested samples

The TRILL project: increasing the technological readiness of Laue lenses

Hard X-/soft Gamma-ray astronomy (> 100 keV) is a crucial field for the study of important astrophysical phenomena such as the 511 keV positron annihilation line in the Galactic center region and its origin, gamma-ray bursts, soft gamma-ray repeaters, nuclear lines from SN explosions and more. However, several key questions in this field require sensitivity and angular resolution that are hardly achievable with present technology. A new generation of instruments suitable to focus hard X-/soft Gamma-rays is necessary to overcome the technological limitations of current direct-viewing telescopes. One solution is using Laue lenses based on Bragg's diffraction in a transmission configuration. To date, this technology is in an advanced stage of development and further efforts are being made in order to significantly increase its technology readiness level (TRL). To this end, massive production of suitable crystals is required, as well as an improvement of the capability of their alignment. Such a technological improvement could be exploited in stratospheric balloon experiments and, ultimately, in space missions with a telescope of about 20 m focal length, capable of focusing over a broad energy pass-band. We present the latest technological developments of the TRILL (Technological Readiness Increase for Laue Lenses) project, supported by ASI, devoted to the advancement of the technological readiness of Laue lenses. We show the method we developed for preparing suitable bent Germanium and Silicon crystals and the latest advancements in crystals alignment technology.Comment: arXiv admin note: text overlap with arXiv:2211.1688

Twin-Shaping Filter Technique Applied to CZT Detectors

CdTe/CdZnTe is an attractive and consolidated material with which to realize detectors with good efficiency and energy resolution, operating at room temperature for a large variety of applications such as astrophysics, medical imaging and security. However, this type of material suffers from the low mobility of the charge carriers (particularly the holes), which are trapped and so degrade the detector response in terms of charge collection efficiency, energy resolution and photopeak efficiency. The response of a planar CdTe/CdZnTe detector, which depends on the distance between the charge formation position and the collecting electrodes, can be improved by using two kinds of techniques, based on the optimization of the electrode geometry and/or signal compensation methods. We are studying the feasibility and the reliability of a biparametric method that uses a twin pulse shaping active filter to analyze each signal from the detector twice: one “Slow”, which is proportional to the energy of the incident photon, and one “Fast”, which depends on the position of the interaction with respect to the collecting electrode. In this paper we describe the bi-parametric technique applied to planar CdZnTe detectors grown by CNR/IMEM and to Spectrometer Grade detectors. We report the experimental results in terms of energy resolution, peak-to valley ratio and photopeak efficiency, as well as the compensated spectra obtained as a function of the bias voltage, photon energy and shaping time pairs. We also report the results obtained by using a CdZnTe drift strip detector. Furthermore, this technique could be implemented in an array of detectors, whose front-end electronics is composed of ASICs, where the shaping time can be selected for each channel, like the RENA-3 IC (NOVA R&D)