Showing papers by "Michael Fiederle published in 2008"

Energy calibration measurements of MediPix2

Abstract: Real-time measurement of the composition and spectroscopic characteristics of the radiation field inside the ATLAS detector and cavern will be carried out with MediPix2 devices. In view of this application, a calibration of the low energy threshold was performed by illuminating the devices with low energy photons, using 241Am sources and X-rays. A procedure was thereafter established for the calibration of all MediPix2 devices to be used in ATLAS. Using X-ray tubes with filters at the Czech Metrology Institute, photons of energies between 8 and 250 keV were obtained and the detection efficiency of the MediPix devices to these radiations was determined. An exponential decrease of the photon intensity as a function of detector depth was observed, illuminating the detector with the beam almost parallel to the surface. The properties of the decrease of intensity were used to determine the exact incident angle of the beam.

Distribution of zinc, resistivity, and photosensitivity in a vertical Bridgman grown Cd1−xZnxTe ingot

Abstract: We present the results of a comprehensive study of distribution of zinc, resistivity, and photosensitivity in a Cd{sub 1-x}Zn{sub x}Te ingot grown by the Vertical Bridgman method. We used several complementary methods, viz., glow discharge mass spectroscopy, photoluminescence-, resistivity-, and photosensitivity-mapping, along with photo-induced current transient spectroscopy to characterize the material. We identified electronic levels in the band-gap responsible for compensation, recombination, and photosensitivity.

New approaches in order to enlarge the grain size of bulk CdZnTe (CZT) crystals

Abstract: For the few decades, II-VI compound semiconductors are gaining attention because of its numerous applications in the field of detector technology, photovoltaic, nuclear medicine, astronomy etc. In the recent past, materials scientists focused their attention for the growth of CdTe/CdZnTe single crystals because it doesn’t require any specialized cooling and detects higher energy photos as in comparison with the existing Ge, Si and HgI2 detectors. In the present study, we are going to discuss five main approaches in order to get good quality CZT crystal and we have successfully grown the CZT crystal by adopting these approaches. They are: i) oscillatory Bridgman technique previous to the growth process, ii) modifying the thermal environments in a Bridgman geometry using a Pt tube as a cold finger in order to reduce the growth velocity iii) growth from the vapour phase using Bridgman geometry with a pyrolitic boron nitride (PBN) crucible to locate the feed material, and with a special temperature profile, iv) microgravity experiments in the FOTON M3 mission using magnetic field prior to the growth process and v) growth by a boron oxide encapsulation. The detailed discussions are given in the following sections.

Evaluation of the ATLAS-MPX devices for neutron field spectral composition measurement in the ATLAS experiment

Abstract: A network of 15 Medipix2-based devices (ATLAS-MPX devices) has been installed at various positions in the ATLAS detector within the framework of the ATLAS-MPX collaboration. The aim of the network is to perform real-time measurement of spectral characteristics and composition of the main radiation types in the experiment including slow and fast neutrons, especially during the initial low luminosity LHC operation. This contribution describes the network structure and focuses on the neutron efficiency calibration process of the ATLAS-MPX devices and its simulation in order to predict the behavior of the device in complex neutron fields.

Doping, compensation, and photosensitivity of detector grade CdTe

Abstract: We studied the resistivity, photosensitivity, photoluminescence, and surface photovoltage of CdTe crystals doped with Ge or Sn to extend our knowledge of the influence of the deep-donor level on compensation and afterglow effects. We demonstrated a strong correlation between photosensitivity caused by photoelectrons with Fermi-level variations near the GeCd0/2+ or SnCd0/2+ energy levels. Surface photovoltage measurements confirmed that when the concentration of residual acceptors varied along the direction of growth, then trapping conditions dramatically changed as a defect was converted from a neutral state to doubly charged positive one.

Characterization of Medipix2 assemblies with CdTe sensor using synchrotron radiation

Abstract: Medipix2 assemblies with CdTe sensors have been characterized using synchrotron radiation. Processing of the assemblies has been done at the Freiburg material research center (FMF), the characterization has been performed at the ANKA at the Forschungszentrum Karlsruhe. The following paper reports the first results of the research of efficient pixelarray detectors for application in a synchrotron providing high energetic photons. Examinations on the homogeneity of the sensor, charge sharing and an energy scan will be shown.

Suitability of the Medipix2 readout-chip for radiological imaging

Abstract: Photon Counting Chips, like the Medipix2 open the possibility of a further improvement of image quality and dose reduction for the patient in X-ray diagnostics. The aim of this paper is to demonstrate the usability of the Medipix2 readout-chip in the medical environment, especially in radiological imaging. Therefore, first measurements with a 300 μm thick Si detector in terms of homogeneity and energy calibration should be presented. The medical imaging capabilities are shown by the imaging of a of a tube phantom (three different tubes represent typical densities of the human body) with the Philips HOMX 161 X-ray source. Additionally measurements in reference to variations of the room temperature show that the Medipix2 readout-system is suitable for medical applications.

The characterization of CdTe TimePix device and the study of its capabilities for the double beta decay measurements

Abstract: This work aims at the characterization of cadmium-telluride detector bump-bonded on the TimePix readout chip and the evaluation of its potential as a tool to study the double beta decay (ββ) processes (such as β−β− or EC/EC), typically performed in ultra low background conditions First tests with the CdTe pixelated sensor (256x256 pixel matrix, 55μ, m pitch, 1mm sensor thickness) have been performed The results of the device calibration and testing of the spectroscopic properties as well as the estimation of intrinsic background of the device are presented

Optimization of Medipix-2 threshold masks for spectroscopic X-ray imaging

Abstract: Spectroscopic X-ray imaging enhances image contrast and provides advanced object information due to energy resolution. The Medipix-2 is a photon counting semiconductor detector and features two energy thresholds for spectroscopic imaging. The aim of this study is to present the development of optimized threshold adjustment masks with small energy windows of about 3 keV and to demonstrate spectroscopic X-ray imaging using the example of an integrated circuit.

Development of a GMCA (Gamma analysis digital filter and Multi Channel Analyzer)

Abstract: The development of a real-time digital signal processing unit for gamma ray detection is shown. A schematic of the contributing electronic components is illustrated along with the digital and analog specifications. The used digital algorithms for signal processing are presented and compared as well as energy spectra of 137Cs with different detector types, namely CPG CdZnTe and LaBr 3 , derived with an experimental setup.

Investigation of growth conditions of CdTe thick films on properties and demands for X-ray detector applications

Abstract: CdTe thick films were prepared by vacuum deposition on amorphous substrates using MBE technique. The growth was performed at different temperatures to investigate the development of the growth rate, surface morphology, structure and optical properties. Properties of films deposited with a single CdTe source are compared with films grown with an additional Cd source. The growth experiments are discussed with regard to demands for X-ray detector applications.

Dewetting during the crystal growth of (Cd,Zn)Te:In under microgravity

Abstract: The phenomenon of “Dewetting” associated with the Vertical Bridgman (VB) crystal growth technique leads to the growth of a crystal without contact with the crucible. One dramatic consequence of this modified VB process is the reduction of structural defects within the crystal. It has been observed in several microgravity experiments for different semiconductor crystals. This work is concentrated on the growth of high resistivity (Cd,Zn)Te:In (CZT) crystals by achieving the phenomenon of dewetting under microgravity condition and its application in the processing of CZT detectors. Two Cd 0.9 Zn 0.1 Te:In crystals were grown in space on the Russian FOTON satellite in the POLIZON-M facility in September 2007 (mission M3). At the end of the preliminary melting phase of one crystal, a Rotating Magnetic Field (RMF) was applied in order to reduce the typical tellurium clusters within the melt before the pulling. The other crystal was superheated with 20 K above the melting point before the pulling. A third reference crystal has been grown on the ground in similar thermal conditions. Profiles measurements of the space grown crystals surface gave the evidence of a successful dewetting during the crystal growth. Characterization methods such as IR microscopy and CoReMa have been performed on the three crystals. CZT detectors have been processed from the grown part of the different crystals. The influence of the dewetting on the material quality and the detector properties completes the study.

Comparison of different gamma-ray detectors with respect to energy resolution and performance

Abstract: The German Federal Office for Radiation Protection (BfS) maintains a network of about two thousand radiation detectors spread all over Germany. The task is to survey the exposure of humans and animals to radiation and its local variation. Because of the wide range of involved energies (a few keV up to several MeV) and to keep up constantly good performance during changes in temperature and humidity e.g., special detector systems are needed. With respect to these demands different commercially available detector systems are compared in this work: A Lanthanum Bromide (LaBr 3 ) scintillator, a Cadmium-Zinc-Telluride Coplanar Grid (CZT-CPG) detector and a Cesium-Iodide (CsI) scintillator.