Development of Thallium Bromide Detectors for Compton Imaging (#2645)
K. Hitomi1, M. Nogami1, N. Nagano1, T. Onodera2, K. Watanabe3, M. Matsumura3, S. - Y. Kim1, T. Ito1, K. Ishii1
1 Tohoku University, Sendai, Japan
Thallium bromide is a promising material for fabrication of gamma-ray detectors because of its high atomic numbers (81, 35) and high density (7.56 g/cm3). In this study, pixelated TlBr detectors were fabricated for constructing Compton imagers. Commercially available TlBr material was purified by the zone melting method. A TlBr crystal was grown by the traveling molten zone method using the purified material. Electrodes were formed on the crystal with the dimensions of approximately 6.5 mm × 6.5 mm × 5.5 mm by vacuum evaporation of Tl. The TlBr detector had a planar cathode and nine pixelated anodes (pixel size: 1 mm × 1 mm) surrounded by a guard ring. The output waveforms from all electrodes connected to preamplifiers were recoded with a digitizer. The detector was irradiated with 137Cs and 60Co gamma-ray sources at room temperature. Two pixel events, in which full-energy of the incident gamma-ray is deposited in a pixelated detector by Compton scattering under the first pixel followed by photoelectric absorption under the second one, was extracted from the measured waveforms to construct Compton images of the gamma-ray sources. Compton images of the two gamma-ray sources were successfully obtained from the pixelated TlBr detector.
Keywords: Thallium bromide, Compton Imaging, Semiconductor detector, Gamma-ray detector
Fabrication and characterization of n-type 4H-SiC epitaxial Schottky detectors: Deep level transient spectroscopy, Cathodoluminescence, Electron beam induced current, and Pulse height studies (#4146)
C. Oner1, T. A. Chowdhury1, J. W. Kleppinger1, K. C. Mandal1
1 University of South Carolina, Department of Electrical Engineering, Columbia, South Carolina, United States of America
High resolution Schottky barrier alpha detectors have been fabricated on 50, 20, and 12 micron thick n-type 4H-SiC epitaxial layers. The junction properties of the detectors were evaluated using current-voltage and capacitance-voltage measurements. The forward I-V characteristic revealed an effective barrier height between 1.75 – 1.45 eV and a diode ideality factor between 1.12 – 1.65. The C-V measurements revealed doping concentration ~ 1014 cm-3 which ensured a fully depleted detector at bias voltages as low as ~50 V. Alpha spectroscopic measurements revealed energy resolution between of 0.5 to 0.3% for 5.48 MeV alpha particles. The performance of these detectors is limited by the presence of microscopic and macroscopic defects. Deep level transient spectroscopy (DLTS) studies revealed the presence of Z1/2, Ci1 and EH6/7 defect centers. Cathodoluminescence (CL) studies revealed three photon peaks with energies centered on 3.2 eV, 2.33 eV, and 1.6 eV respectively. The presence of green luminescence (GL) at 2.33 eV suggests the presence of boron impurity centers, and it is suspected that the peak centered at 1.6 eV is related to carbon vacancies. Correlation of EBIC contrast with extended defects in 50 micron n-type 4H-SiC epitaxial layers has been studied using an oxidizing chemical etching salt in molten KOH. EBIC results indicate that under bias stress of the Schottky devices, the screw dislocations produce a dark EBIC contrast indicating high current flow in the defective region. The mechanism of EBIC contrast has been investigated in details. The fabricated detectors have been tested with thermalized neutrons from an isotopic 252Cf source and have shown to provide robust pulse-height spectra, which are easily separable from the anticipated gamma-ray response. The fabricated detectors are highly useful for neutron and gamma-ray fluence rate measurements in high gamma-ray background for extended period of time and at high temperatures up to 700 °C.
Keywords: 4H-SiC, Schottky barrier, Neutron detector, DLTS, EBIC, Defect levels, Etching, Alpha spectroscopy, High temperature measurements
Flexible, multi-layer oxide field effect transistor for direct X-ray dosimetry (#4048)
1 Bologna, Physics and Astronomy, Bologna, Italy, Italy
Distributed X-ray radiation dosimetry is crucial in diverse areas such as nuclear waste management, radiotherapy or personal protection devices. Most current sensitive wearable dosimeters employ an optical readout after exposure and do not provide real-time information.
Here we present novel thin film flexible transistors (TFTs) fabricated on plastic substrates, that behave as real time direct X-ray detectors. They are based on high mobility oxides, that show a quantitative shift in threshold voltage of up to 5.5 V/Gy upon exposure to ionizing radiation. The transistors employ Gallium-Indium-Tin-Oxide as semiconductor and a multilayer dielectric containing layers of Silicon Oxide and Tantalium Oxide.
Our measurements demonstrate that the threshold voltage shift is caused by the accumulation of positive ionization charge in the dielectric layer as a consequence of high energy photon absorption in the high-Z dielectric. The high mobility combined with a steep sub-threshold slope of the transistors allows for a fast and ultra-low power readout of the deposited radiation dose.
It is noteworthy that the preliminary studies we have carried out on the radiation hardness of the here described thin oxide transistors, indicate that transport properties of the oxide semiconductor do not show radiation damage 
We employ the radiation sensitive multi-layer TFTs in two examples of novel real-time dosimeter applications: i) a flexible dosimeter matrix able to map in real-time deposited doses with micrometer resolution; ii) we demonstrate the feasibility of low-cost passive RFID sensor tags that report in real time the exposure to critical fluxes of ionizing radiation.
 T. Cramer, A. Sacchetti, M. T. Lobato, P. Barquinha, V. Fischer, E.Fortunato, R.Martins and B.Fraboni., “Radiation-Tolerant Flexible Large-Area Electronics Based on Oxide Semiconductors,” Adv. Electron. Mater., vol. 2, no. 7, pp. 1–8, 2016.
Keywords: high mobility semiconducting oxide, field effect transistors, flexible radiation detectors
Hybrid organic-inorganic perovskites for medical X-ray imaging (#2045)
S. F. Tedde1, R. Fischer1, 2, J. Hürdler1, O. Schmidt1
1 Siemens Healthineers, Technology Center, Erlagen, Bavaria, Germany
Fabrication of medical X-ray imaging detectors which combines minimum manufacturing cost and high performance has been the major challenge for the last decades. Recently, hybrid organic-inorganic perovskites revealed promising performance as direct X-ray converters both as polycrystalline and single crystal structures. The high X-ray absorption, high electron and hole diffusion lengths with high charge carrier mobility and long lifetimes together with relatively low costs of the manufacturing precursors makes them really attractive as potentially novel X-ray converting material.
Here, we present an effortless powder route which enables cost effective fabrication of microcrystalline methylammonium lead iodide layers with a thickness of more than 1000 µm on large areas. First, microcrystalline powder of methylammonium lead iodide with particle size is in the range of few microns is synthesized by a precipitation process. After removal of all solvents, the dried powder is compacted with moderate heat and pressure in a soft-sintering process to form either free standing pellets or soft-sintering directly on top of a desired substrate. Charge extraction proved to be efficient, resulting in an X-ray-to-charge conversion of 200 µCGy-1cm-2 and a fast response in the ms regime. A dedicated interlayer engineering helped to reduce dark currents densities for several orders of magnitude down to 1E-3 mA/cm² at 0,2 V/µm applied electrical field and improve the adhesion of the perovskite layer on the substrate. The direct integration of a soft-sintered microcrystalline methylammonium lead iodide layer on top of an a-Si TFT backplane with 256x256 pixels and 98 µm pixel pitch, resulted in the first perovskite based active-matrix flat panel digital X-Ray detector prototype. X-ray images show a promising resolution of 4-5 lp/mm.
Keywords: X-Ray detectors, Perovskites, flat panel detectors, imaging, pixelated detectors
Thallium Bromide Detectors for PRD and SPRD Applications (#3387)
A. Datta1, J. Fiala1, P. Becla1, S. Motakef1
1 CapeSym, Inc., Natick, Massachusetts, United States of America
Thallium bromide (TlBr) is a wide bandgap compound semiconductor which has a number of advantages relative to the incumbent CdZnTe. Until recently the suitability of this material for gamma-ray detection was challenged by its short room temperature lifetime. Recently we have shown that periodic switching of bias on TlBr detectors can result in room temperature device lifetimes exceeding 10,000 hours of operation. In this presentation, we will give an overview of the status of radiation detection technologies based on scintillators and semiconductors, and describe the relative advantages of TlBr within this context. We will report on the performance of a prototype Personal Radiation Detector (PRD) based on bias-switching of planar TlBr devices, and will show that TlBr detectors can now meet the required sensitivity and other performance criteria of PRD applications. We will also report on performance of a number of TlBr single charge detectors for spectroscopy, where their uncorrected and corrected energy resolution competes with those of CZT detectors.
This work was supported in part by the U.S. Department of Homeland Security, Domestic Nuclear Detection office contract HSHQDN-16 -C-00025. This support does not constitute an express or implied endorsement on the part of the Government.
Keywords: personal radiation detector, Spectral PRD, Room temperature semiconductor detector