High-Z semiconductors are excellent materials for the development of efficient X-ray and Gamma-ray pixel detectors. The state of the art of readout electronics ROIC allows pixel sizes down to 50 μm. The Medipix 2 is such a photon counting chip with two energy thresholds.The Medipix chip was developed by the Medipix collaboration of CERN. The Medipix can be used with silicon, CdTe / CdZnTe or GaAs detectors. These small pixels make a strongdemand for the material quality of the semiconductor crystals concerning homogeneity and performance.At the Freiburger Materialforschungszentrum we have developed the technology of processing CdTe pixel detectors adjusted to the Medipix requirements with small pixels downto 55 μm and high connection density with more than 65,000 pixels. Several detector assemblies were assembled using CdTe with pixel sizes of 55 and 110 μm. The detectorperformance of these assemblies was tested with different radiation sources and x-ray tubes.The signal of the pixels was quite homogeneous and the number of working pixels was more than 92 percent. In this paper will discuss the very good performance of the pixel detectors regarding the concentration of material quality. The properties of the used crystals will be presented by the concentration of defects and the active deep levels as well as the structure and crystallinity of the detector material.

Bismuth tri-iodide (BiI3) is a promising material for room temperature gamma-ray detection owing to its wide bandgap, high density and high effective atomic number. Monte Carlo simulations show that BiI3 has a gamma-ray detection efficiency approximately 2 to 3 times higher than CdZnTe over the energy range of 0 to 3 MeV, with energy resolution (FWHM) less than 2% at 662 keV. In this work, we present the growth of high purity BiI3 single crystals by a modified vertical Bridgman method. Single crystal {0003} oriented platelets, 8 mm x 9 mm x 2 mm (the largest ever reported), were obtained using a 0.5 mm/hour growth velocity and 10 °C/cm temperature gradient. The crystallinity, stoichiometry and overall quality of the crystals characterized by X-ray diffraction, X-ray rocking curve, electron dispersive spectroscopy, and inductively coupled plasma atomic emission spectroscopy will be reported. In addition, the bandgap of the crystal calculated by measuring UV-Vis absorption spectra and the electrical properties investigated through I-V curves will be presented. The measured bandgap is also compared with density-functional theory (DFT) simulations and the result will be discussed. The performance of a prototype radiation detector fabricated from the grown crystals will be reported.

A single material capable of distinguishing between fast and thermal neutrons while discriminating them cleanly from gammas would be of tremendous value in large area radiation monitors and for cargo interrogation applications. We report on the solution growth of single crystal Lithium Salicylate and present initial measurements of the time profiles of light emission for gammas via the Compton process, fast neutrons via proton scattering, and thermal neutrons via the 6Li capture process. The spatial density of excitations varies dramatically for these three processes resulting in very different pulse shapes. We demonstrate the discriminating potential of this material using full waveform digitization.This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and funded by DOE NA-22 and LLNL LDRD.

The use of semiconducting polymers in radiation detectors offers many advantages over their conventional inorganic counterparts, such as the capability of coating over large areas and on curved surfaces, mechanical flexibility, lower costs, and ease of processing. In addition, polymers act as “tissue-equivalent” materials, as the low atomic numbers of their constituent atoms closely match those of biological tissue. In this work, we have established the feasibility of using a thick layer of a conjugated polymer as a charge-sensitive, directly-detecting radiation detector. A typical diode-based structure was fabricated by sandwiching a polymeric active layer of poly(tri-aryl amine) (PTAA) [1] between a transparent indium tin oxide (ITO) bottom contact and a top contact of gold. The thicknesses of the active polymer layers, which were measured by profilometry, were up to 30µm. The electrical characteristics of the devices were investigated using the current-voltage (I-V) technique. A distinct rectifying behaviour with a low reverse bias current density of 72 nA/cm2 at a high electric field strength (100kV/cm) was observed. The charge transport properties of the device were assessed using the spectral photocurrent method. Increased photocurrent sensitivity was observed for polymer films annealed at a temperature of 150°C in comparison to 110°C. This result is explained by the completion of solvent loss, which was determined by thermal gravimetric analysis, and by structural relaxation. The device’s sensitivity to ionizing radiation was assessed using X-ray irradiation with 50 kV photons over a range of dose rates from 1.5 to 6.5 mGy/s. A linear relationship between dose rate and X-ray photocurrent was established. The observed sensitivity, >6000 nC/mGy/cm3, is comparable to what has been reported elsewhere for Si devices [2], and it is greater than what we reported previously for other conjugated polymers (up to 500 nC/mGy/cm3)[3]. The effects of storage time, ambient atmosphere, and visible light on the stability of devices have also been investigated. Good reproducibility of X-ray sensitivity was found. Our results indicate that PTAA is a highly-promising material for the direct detection of X-rays and potentially other types of radiation. [1] J. Veres, S. D. Ogier, A. W. Leeming, D. C. Cupertino and S. M. Khaffaf, Adv. Func. Mat. 13, 199(2003)[2] Bruzzi, F. Nava, S. Pini, and S. Russo, Appl. Surf. Sci. 184, 425 (2001)[3] F. A. Boroumand, M. Zhu, A. B. Dalton, J. L. Keddie, P. J. Sellin and J. J. Gutierrez, Appl. Phys. Lett. 91, 033509 (2007)

The development of the next generation high-flux x-ray imaging technology requires very high speed energy-sensitive room temperature detector arrays. Pulse-mode monolithic pixilated CdZnTe detector arrays offer a range of advantages for this family of applications such as room-temperature operation, high detection efficiency, excellent energy and spatial resolution and relatively mature device fabrication technology. Here we discuss the challenges of CdZnTe detector technology to meet the requirements of x-ray Computed Tomography (CT) applications in four critical areas: high-flux capability, high-flux temporal response, response uniformity, and response speed. We show that significant progress has been made both in the understanding of the physical processes controlling detector response and in device fabrication technology to achieve certain performance requirements. We demonstrate the performance of ultra-fast response CdZnTe detectors and a photon flux capability exceeding 175x10⁶ photons/s/mm².

Cadmium Zinc Telluride (CZT) ingots have been grown at WSU for use as room temperature radiation detectors. Growth, compensation, fabrication and characterization are performed at WSU. However, others have fabricated the Center’s CZT material with positive results. The growth process in carried out in a multi-zone electro-dynamic gradient (EDG) freeze Bridgman Furnace. A research item from CMR results is clearly what dopants are needed to grow fully active CdZnTe. Another critical issue in the growth (without a flux) is Te inclusions. How and can a new approach reduce their impact on electron trapping? Radiation detectors have been grown and fabricated by the Center show photo-peak resolution for radioactive sources, such as 57Co approaching ~3.5% FWHM. The growths have a reasonable yield including twins and grain boundaries. Crystal growths encompassing various dopants and types of crucibles provide a possible roadmap to reduce these effects. This paper addresses how to compensate intrinsic defects and produce a reliable detector. A study of intrinsic crucible and ingot impurities and detector performance will be presented. To strive for higher resolution and larger detectors, the highest purity materials are desired while using controlled amounts of dopants and reducing Te inclusions to achieve reasonable controlled compensation for CZT. Understanding, controlling and modifying the role of these effects are a key to the success for the production of cost-effective spectroscopic grade detectors. This research was funded by DOE NA-22, Office of Non-Proliferation Research and Engineering and by donations to the Center for Materials Research.

A high degree of control of the material charge transport and trapping properties is one of the crucial issues for a successful application of Cd-based compounds as room-temperature X- and γ-ray detectors. The behaviour of detectors when exposed to prolonged or intense radiation fields may be significantly altered due to the interaction between the impinging beam and the electronic structure of the atoms. The formation of electrically active defects following such irradiation alters the charge compensation processes that are responsible for the high resistivity of the material and ultimately affects the macroscopic performance of the detector.This contribution deals with the irradiation of Cd_(1-x)Zn_xTe based detectors with X-rays of 40keV energy using a Mo and a W anode to simulate the damage resulting from a prolonged exposure to diagnostic apparatuses for mammography and general purpose medical imaging.We investigated the effects induced by high X-ray doses by correlating γ-spectroscopy, current-voltage analyses and photo-induced current spectroscopy (PICTS). The evolution of the detector performance with the increasing radiation dose and the effects of the two different X-ray energies have been analysed and compared to previous results obtained on γ-ray (⁶⁰Co) irradiated detectors to shine more light on the microscopic mechanism underlying the defect formation processes in Cd_(1-x)Zn_xTe . The formation of intrinsic defects is likely to be the major outcome of X-ray irradiation and we investigated the origin and nature of the dominant electrically active defects induced.

The introduction of large single crystal and high performance CdZnTe (CZT) grown by the traveling heater method (THM) in 2006 has defined conventional myths about the capability of this crystal growth method with respect to the production of spectroscopic grade CZT and its commercialization prospect in medical imaging application. Since then, a lot of progresses have been made, both in the crystal growth and the devices sides. This paper focuses on the development of THM CZT in recent years. Crystalline defects which challenge the thickness scalability of large volume CZT detectors along with efforts and achievements in overcoming these challenges are discussed. Advances in THM CZT crystal growth include 100mm diameter ingot and state-of-the-art device fabrication will also be presented.

Radiation interactions with materials cause changes in the electronic and physical properties of the material, which affect the performance of the radiation detectors. In our research defects are induced in the crystals by exposure to Co⁶⁰ standard sources with a radiation dose rate of 6x10⁴ rad/hr. The dose used to irradiate the CdZnTe detectors is increased in steps of 1-2 Mrad to a maximum dose of 7 Mrad. The defects-related parameters were determined by Current Deep Level Transient Spectroscopy (I-DLTS). Results for the defects levels within the band gap, the carrier capture cross-sections and defect densities will be reported. The energy traps are mostly shallow and medium electron donors below the conduction band. The traps have energies between 15 - 665 meV, while the charge capture cross sections of these defects and their trap densities vary from 10^-19 - 10^-16 cm² and 10 ⁷ – 10¹⁰ cm^-3 respectively. The detector’s electrical characteristics and its performance are explored before and after the gamma exposure, using I-V, C-V and gamma-ray spectroscopy.

In the past we demonstrated the role of micro-defects such as Te inclusions on charge trapping and then proved their deleterious effect on CdZnTe detector performance. More thorough investigations revealed a more complex situation. For example, some CZT crystals with low concentration of Te inclusions demonstrated poor performance as detectors. By using destructive etching techniques, we observed that there are several types of extended defects in CdZnTe crystals (such as dislocations, microtwins and sub-grain boundaries), which are not readily visible with IR imaging techniques. These extended defects could be the cause of the deterioration of performance of the CZT detectors with low concentration of Te inclusions. These extended defects might also affect the internal E-field and local charge collection efficiency and, thus, the device performances. They could be more deleterious than Te inclusions as the primary cause of non-uniform charge transport. With nondestructive techniques employing synchrotron radiation, we investigated their role on charge trapping as well as the possible distortion of the internal electric field. These techniques and the latest experimental results will be presented.

Large-volume CdZnTe (CZT) radiation detectors would greatly improve radiation detection capabilities and, therefore, attract extensive scientific and commercial interests. CZT crystals with volumes as large as hundreds of centimeters can be achieved today due to improvements in the crystal growth technology. However, the availability of such large-volume CZT crystals with adequate transport properties is still a challenging problem, which substantially obstructs the commercialization of CZT detectors and imaging arrays. We have employed several different material characterization methods, such as differential interference contrast (DIC) infrared microscope, current-voltage (I-V) measurements, alpha-particle response, Pockels effect measurements, and synchrotron X-ray scanning techniques, to investigate the performance-limiting factors for large-volume CZT detectors. Experimental results with the above characterization methods will be presented and discussed. These results help us to better understand the effects of internal crystal defects on the performance of large-volume CZT detectors.