Irfan Kuvvetli

Characterization of large cadmium zinc telluride crystals grown by traveling heater method

The focus of this paper is to evaluate thick, 20×20×10 and 10×10×10mm3, cadmium zinc telluride (CZT), Cd0.9Zn0.1Te, crystals grown using the traveling heater method (THM). The phenomenal spectral performance and small size and low concentration of Te inclusions/precipitates of these crystals indicate that the THM is suitable for the mass production of CZT radiation detectors that can be used in a variety of applications. Our result also proves that with careful material selection using IR and high-quality fabrication processes, the theoretical energy resolution limit can be achieved.

The e-ASTROGAM mission

Abstracte-ASTROGAM (‘enhanced ASTROGAM’) is a breakthrough Observatory space mission, with a detector composed by a Silicon tracker, a calorimeter, and an anticoincidence system, dedicated to the study of the non-thermal Universe in the photon energy range from 0.3 MeV to 3 GeV – the lower energy limit can be pushed to energies as low as 150 keV, albeit with rapidly degrading angular resolution, for the tracker, and to 30 keV for calorimetric detection. The mission is based on an advanced space-proven detector technology, with unprecedented sensitivity, angular and energy resolution, combined with polarimetric capability. Thanks to its performance in the MeV-GeV domain, substantially improving its predecessors, e-ASTROGAM will open a new window on the non-thermal Universe, making pioneering observations of the most powerful Galactic and extragalactic sources, elucidating the nature of their relativistic outflows and their effects on the surroundings. With a line sensitivity in the MeV energy range one to two orders of magnitude better than previous generation instruments, e-ASTROGAM will determine the origin of key isotopes fundamental for the understanding of supernova explosion and the chemical evolution of our Galaxy. The mission will provide unique data of significant interest to a broad astronomical community, complementary to powerful observatories such as LIGO-Virgo-GEO600-KAGRA, SKA, ALMA, E-ELT, TMT, LSST, JWST, Athena, CTA, IceCube, KM3NeT, and the promise of eLISA.

Spectral Response of THM Grown CdZnTe Crystals

The spectral response of several crystals grown by the Traveling Heater Method (THM) were investigated. An energy resolution of 0.98% for a Pseudo Frisch-Grid of 4 times 4 times 9 mm3 and 2.1% FWHM for a coplanar-grid of size 11 times 11 times 5 mm3 were measured using 137Cs-662 keV. In addition a 4% FWHM at 122 keV has also been measured on 20 times 20 X 5 mm3 monolithic pixellated devices. The material shows great potential toward producing large-volume detectors with spectral performance that meets the requirement for high-resolution gamma-ray spectroscopy.

Modeling charge-sharing effects in pixellated CZT detectors

While charge-sharing in pixellated CdZnTe (CZT) room temperature radiation detectors has been studied numerically and experimentally in the last 5-10 years, there are no analytical models that will guide the optimization of CZT detector operations and enable efficient use of the correction algorithms. In this paper, we propose a new analytical model to provide an effective framework to study these effects over a wide range of detector geometries, biasing conditions, and material properties. The proposed model correctly predicts the amount of charge-sharing events versus photon energy and detector pixel size which are confirmed experimentally and numerically at the Danish National Space Center.

The Effect of Cathode Bias (Field Effect) on the Surface Leakage Current of CdZnTe Detectors

Surface resistivity is an important parameter of multi-electrode CZT detectors such as coplanar-grid, strip, or pixel detectors. Low surface resistivity results in a high leakage current and affects the charge collection efficiency in the areas near contacts. Thus, it is always desirable to have the surface resistivity of the detector as high as possible. In the past the most significant efforts were concentrated to develop passivation techniques for CZT detectors. However, as we found, the field-effect caused by a bias applied on the cathode can significantly reduce the surface resistivity even though the detector surface was carefully passivated. In this paper we illustrate that the field-effect is a common feature of the CZT multi-electrode detectors, and discuss how to take advantage of this effect to improve the surface resistivity of CZT detectors.

A 3D CZT high resolution detector for x- and gamma-ray astronomy

At DTU Space we have developed a high resolution three dimensional (3D) position sensitive CZT detector for high energy astronomy. The design of the 3D CZT detector is based on the CZT Drift Strip detector principle. The position determination perpendicular to the anode strips is performed using a novel interpolating technique based on the drift strip signals. The position determination in the detector depth direction, is made using the DOI technique based the detector cathode and anode signals. The position determination along the anode strips is made with the help of 10 cathode strips orthogonal to the anode strips. The position resolutions are at low energies dominated by the electronic noise and improve therefore with increased signal to noise ratio as the energy increases. The achievable position resolution at higher energies will however be dominated by the extended spatial distribution of the photon produced ionization charge. The main sources of noise contribution of the drift signals are the leakage current between the strips and the strip capacitance. For the leakage current, we used a metallization process that reduces the leakage current by means of a high resistive thin layer between the drift strip electrodes and CZT detector material. This method was applied to all the proto type detectors and was a very effective method to reduce the surface leakage current between the strips. The proto type detector was recently investigated at the European Synchrotron Radiation Facility, Grenoble which provided a fine 50 × 50 μm2 collimated X-ray beam covering an energy band up to 600 keV. The Beam positions are resolved very well with a ~ 0.2 mm position resolution (FWHM ) at 400 keV in all directions.

Characterization of CZT Detectors for the ASIM Mission

The National Space Institute, of the Technical University of Denmark is responsible for the selection and characterization of the CZT detector crystals for the X- and Gamma-ray instrument, MXGS, onboard ESAs Atmospheric Space Interaction Monitor (ASIM) mission. The first CZT pixel detector modules for MXGS have recently been delivered by Redlen. Measurements at the University of Bergen demonstrate that the detectors exhibit the expected spectral performance; however, it was also found that the detector modules showed unexplained pixel-to-pixel count rate variations. At The National Space Institute a series of measurements were initiated focusing on the pixel-to-pixel count rate variations. Collimated monochromatic X-ray beams were used to scan a number of Redlen pixel detectors. The main conclusion of these measurements is that the pixel-to-pixel count rate variations are correlated with variations in the effective pixel areas. These variations are likely related to non-uniformity of the internal electric detector field.

The development of drift-strip detectors based on CdZnTe

The development results of drift-strip detectors on the basis of CdZnTe crystals of dimensions 10/spl times/10/spl times/3 mm/sup 3/ are presented. The pitch of the structure was 200 /spl mu/m, the strips width -100 /spl mu/m, the inter-strip gaps -100 /spl mu/m, the length of strips is 9.5 mm. Design and technological aspects of the development of drift strip detectors are described. The distribution histograms for leakage currents of strips as well as distribution of interstrip resistance by the numbers of strips on various detectors are presented. Energy resolution at energies 59.5 and 661 keV was 1.9 and 12 keV correspondingly. Energy resolution at high energies was limited by the transport characteristics of the charge carriers.

A three-dimensional CZT detector as a focal plane prototype for a Laue Lens telescope

The importance of hard X-ray astronomy (>10 keV) is now widely recognized. Recently both ESA and NASA have indicated in their guidelines for the progress of X- and γ-ray astronomy in the next decade the development of new instrumentation working in the energy range from the keV to the MeV region, where important scientific issues are still open, exploiting high sensitivity for spectroscopic imaging and polarimetry observations. The development of new concentrating (e.g. multilayer mirror) telescopes for hard X-rays (10 -100 keV) and focusing instruments based on Laue lenses operating from ~60 keV up to a few MeV is particularly challenging. We describe the design of a threedimensional (3D) depth-sensing position sensitive device suitable for use as the basic unit of a high efficiency focal plane detector for a Laue lens telescope. The sensitive unit is a drift strip detector based on a CZT crystal, (10×10 mm2 area, 2.5 mm thick), irradiated transversally to the electric field direction. The anode is segmented into 4 detection cells, each comprising one collecting strip and 8 drift strips. The drift strips are biased by a voltage divider, whereas the anode strips are held at 0 V. The cathode is divided in 4 horizontal strips for the reconstruction of the Z interaction position. The 3D prototype will be made by packing 8 linear modules, each composed of 2 basic sensitive units, bonded onto a ceramic layer together with the readout electronics.

Large-volume high-resolution cadmium zinc telluride radiation detectors: recent developments

The excellent room temperature spectral performance of cadmium zinc telluride detectors grown via the Traveling Heater Method (THM) makes this approach suitable for the mass deployment of radiation detectors for applications in homeland security and medical imaging. This paper reports our progress in fabricating thicker and larger area detectors from THM grown CZT. We discuss the performance of such 20x20x10 mm3, and 10x10x10 mm3 monolithic pixellated detectors and virtual Frisch-Grid 4x4x12 mm3 devices, and describe the various physical properties of the materials.