R15  Materials2

Friday, Nov. 4  08:00-10:00  Schuman

Session Chair:  Shariar Motakef, , United States

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(08:30) R15-1, X-Ray Transparent Multi-Channel Dosimeter Based on Organic Photodiodes and Plastic Scintillators for Real-Time IVR Monitoring

F. Nishikido1, E. Takada2, M. Nogami2, T. Maeda2, T. Moritake3, T. Yamaya1

1National Institute of Radiological Sciences, Chiba, Japan
2National Institute of Technology, Toyama College, Toyama, Japan
3University of Occupational and Environmental Health, Fukuoka, Japan

Interventional radiology (IVR) is a medical subspecialty of radiology to realize image-guided surgical procedures using imaging modalities, such as x-ray fluoroscopy. However, skin injuries by prolonged x-ray exposure in duration of procedure have been reported. Therefore, monitoring of skin dose is desired in clinical sites for reduction of excessive x-ray exposure. We are developing a real-time dose distribution monitoring system for the IVR in which only radiolucent materials were in the field of view not to interfere with the IVR procedure. The organic photodiode (OPD) is mainly composed of organic materials, which has good transparency for x-ray. As a result, the detector using the OPDs combined with a plastic scintillator connected to thin flexible cable is expected to have transparency to x-rays. In this paper, therefore, we developed a new detector based on the OPD connected to the thin flexible cable for the real-time monitoring of skin dose distribution in IVR. The device structure of the OPD was plastic scintillator (BC-408, 10 mm 10 mm 1 mm)/ IZO (100nm)/ PEDOT: PSS (30 nm) / PCBM: P3HT (200 nm) / Al (70 nm). The size of a sensing area is 6 mm 4 mm on the plastic scintillator. The readout flexible cable was made of 25 m polyimide board and 12 m copper line. The length of the flexible cable is 30 cm and there are three readout electrodes to connect to the OPD electrode. We evaluated performance of the x-ray detectors for with a micro CT (R_mCT2, RIGAKU). As a result, the proposed x-ray detector had a enough transparency to be used in the IVR dosimeter. In addition, we obtained the sufficient detector responses for the various tube current, tube voltage and irradiation angle for x-ray irradiation in real-time with 1 s interval.

(08:45) R15-2, invited, Flexible, Ultra-Low Voltage, Fully Printed Radiation Detectors Based on Organic Semiconductors

B. Fraboni

Department of Physics and Astronomy, University of Bologna, Bologna, Italy

A new generation of ionizing radiation sensors based on organic materials is attracting a large attention exploiting appealing features of such as ease of processing, low power supply and mechanical flexibility. Moreover, the equivalence of the typical density of organic molecules to that of human tissue makes them very suitable for medical X-ray direct dosimetry. Our approach is based on the use of organic semiconductors as the active material for the direct detection of ionizing radiation, implementing real-time and room temperature operating sensors. We fabricated fully printed detectors based on micro-crystalline thin films of TIPS-pentacene deposited by drop casting onto flexible substrates and we assessed their high X-ray sensitivity (Figure 1). We investigated the direct X-ray photo-conversion process and we developed a kinetic model that gives an important insight into the physical process that leads low-Z organic materials to sucha highly sensitive response. This investigation allowed us to realize, by printing processes only, a flexible, 2x2 pixelated X-ray detector based on organic thin films, operating at very low voltage (below 1V) (Figure 2). Moreover, we assessed how detectors based on solution-grown Organic Semiconducting Single Crystals (OSSCs) are capable to detect charged particles with very good peak discrimination and an Hecht-like behavior. It has been possible to estimate the mobility-lifetime product obtaining values comparable to polycrystalline inorganic detectors [3]. The good detection efficiency and peak discrimination observed for charged particle detection in organic materials (hydrogen-rich molecules) are encouraging for their further exploitation in the detection of thermal and high-energy neutrons. These results open the way for novel flexible, large area and low voltage ionizing radiation detection systems, capable of providing quantitative and real time information on dose rate and spatial distribution of impinging radiation.

(09:00) R15-3, invited, X-Ray Imaging with Scintillator-Sensitized Hybrid Organic Photodetectors

O. Schmidt, P. B�chele, S. F. Tedde, R. Fischer, J. H�rdler

Technology Center, Siemens Healthcare GmbH, Erlangen, Germany

Organic semiconductors provide exiting new opportunities for the realization of flat panel X-ray detectors as they can be processed from the solution phase on large areas at low cost. Here, we demonstrate a disruptive approach that improves the resolution of such X-ray imaging detectors by incorporating terbium-doped gadolinium oxysulfide (GOS:Tb) scintillator particles into an organic photodetector matrix. Light emission from the scintillating particles is detected by the surrounding organic photodetection matrix. Optical cross-talk is minimized as the particles are small (1-5 �m) and the penetration depth of the photons is short (100-500 nm) compared to the pixel size of the X-ray detector. We will discuss various options to process the hybrid material on a 256x256 pixel detector and will demonstrate a working device with a thickness of 170 �m. From this demonstrator we estimate an electron-hole pair generation energy of ~70 eV and achieve a resolution of 4.75 lp/mm at a MTF of 0.2.

(09:15) R15-4, Performance of Thallium Bromide devices for Room Temperature Field Applications

A. Datta, J. Fiala, P. Becla, K. Becla, S. Motakef

CapeSym, Inc., Natick, MA, US

Thallium Bromide (TlBr) is a wide bandgap, compound semiconductor with high gamma-ray stopping power and promising physical properties. However, performance degradation and the eventual irreversible failure of TlBr devices can occur due to polarization, caused by the electro-migration of Tl+ and Br- ions to the electrical contacts across the device. With recent developments, we fabricated TlBr devices which gives reliable detection for more than a year at room temperature under continuous bias and gamma irradiation. These results are reproducible, and shows highly reliable performance of TlBr devices. The evidence and statistics of these developments will be demonstrated in the paper. Effects of ambient atmosphere and applied detector bias on the long-term detector performance are also explored. PRD prototypes were prepared using these devices. The performance of these PRDs are comparable to the COTS scintillator-based units, but with much higher efficiency and lower cost.

(09:30) R15-5, Direct Gamma-Ray Detection with Strip TlBr Detectors for Nuclear Medicine Applications

G. Ario-Estrada1, L. Cirignano2, H. Kim2, J. Du1, K. Shah2, S. R. Cherry1, G. S. Mitchell1

1Department of Biomedical Engineering, University of California Davis, Davis, CA, USA
2Radiation monitoring Devices, Inc., Watertown, Watertown, MA, USA

Thallium bromide (TlBr) is a promising semiconductor material for use in positron emission tomography (PET) because of its very high detection efficiency, excellent spatial resolution, low noise, and high energy resolution. We report on the fabrication process and performance of two thin orthogonal strip TlBr detectors. TlBr ingots have been grown at Radiation Monitoring Devices (RMD) by the travelling molten zone method and show high material purity, high resistivity, and potential long term stability (up to one month continuously biased) at room temperature. The detectors have approximate dimensions of 10 mm x 10 mm x 0.75 mm and on each side their electrodes are divided into 9 strips with 1 mm pitch (0.9 mm width and 0.1 mm inter-strip space). Initial studies include the pulse height spectra of the detector strips for the radioactive sources Am-241 and Cs-137 at different bias voltages. A multi-channel readout board has each of the strips (18 per detector) coupled to a charge sensitive preamplifier. The preamp output is shaped with a 2-pole slow spectroscopy amplifier to measure the deposited energy. The same preamp output feeds a fast shaper and is used to obtain the trigger signal. Coincident events between the two sides of a detector are observed and used to localize the interaction region to the intersection of the orthogonal strips. The two detectors will be used to evaluate detector performance with 511 keV gamma rays, with the eventual design goal of using a stack of thin detectors to build a three-dimensional detector block of TlBr material.

(09:45) R15-6, Characterization of pin GaN diodes radiation detection for a-ray

T. Arikawa1, K. Mochizuki1, M. Sugiura1, H. Nakagawa2, S. Usami3, M. Kushimoto3, Y. Honda3, H. Amano3,4, S. Schtt5, A. Vogt5, M. Fiederle5, H. Mimura6, Y. Inoue1, T. Aoki6, T. Nakano1

1Dept. of Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu, Japan
2Dept. of Nanovision Technology, Shizuoka University, 3-5-1 Johoku, Hamamatsu, Japan
3Dept. of Engineering and Institute of Materials and Systems for Sustainability, Nagoya University, Furo-cho Chikusa-ku, Nagoya, Japan
4Akasaki Research Center, Nagoya University, Furo-cho Chikusa-ku, Nagoya, Japan
5Freiburg Materials Research Center, University of Freiburg, Stefan-Meier-Str. 21, Freiburg, Germany
6Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Hamamatsu, Japan

We focused B, which have large neutron cross-section area, to suggest a new GaN based semiconductor neutron detector. In this system, the characteristics of a-particle detection for GaN are important, but the evaluation of radiation detection property of GaN has not been sufficiently addressed so far. In this study, we estimate the range distance of a-particles in GaN by measurements and calculations. We fabricated GaN pin-diodes with two patterns of electrodes, which are planar type and mesh type. We used 241Am as a-particle source and the energy spectrum measurement was performed by using multi-channel analyser. We evaluated the characterization of radiation detection for a-particles by changing the distance from the diode to the a-particle source. a-particle energy, which attenuated with air dumping, was estimated in the simulation. In the result of the energy spectrum measurement using planar electrode, when the incident energy of a-particles is 3.25 MeV, the detected peak energy attains a maximum value. In the region of less than 3.25 MeV, the detection energy was attenuated by the reduction of a-particle energy. This result indicates that the Bragg peak of a-particles is present in the detection area. In the region of more than 3.25 MeV, the detection energy is slowly attenuated due to an increase of a-particles. This result indicates that Bragg peak of a-particles is not present in the detection area and that the a-particles with energies over 3.25 MeV penetrate through the GaN diodes. In the case of mesh pattern electrode, detection peak position becomes maximum value when the energy is 3.14 MeV. This phenomenon means that the incidence of the a-particles from the electrode-free part is affected. If the energy of a-particles from electrode-free part becomes dominant, the peak position changes. From these results, we calculated the range of 1.47 MeV-a-particles which are generated by B(n, a)Li reaction, and we estimated it to be about 0.9 m.

(10:00) R15-7, invited, Growth of TlBr Crystals by the Vertical Bridgman Method and the Traveling Molten Zone Method for Gamma-Ray Detector Applications

K. Hitomi1, T. Onodera2, N. Nagano1, K. Watanabe3, M. Matsumura3, S.-Y. Kim1, T. Ito1, K. Ishii1

1Tohoku University, Sendai, Japan
2Tohoku Institute of Technology, Sendai, Japan
3Nagoya University, Nagoya, Japan

Thallium bromide (TlBr) is a promising semiconductor material for gamma-ray detector fabrication. Growing large crystals is indispensable for using TlBr detectors for practical applications including gamma-ray spectrometers, gamma cameras and Compton cameras. In this study, two crystal growth methods including the vertical Bridgman method and the traveling molten zone method were employed for growing large TlBr crystals. The starting materials for both crystal growth methods were commercially available TlBr with 99.999% purity. Further purification of the starting materials was performed by the zone refining method. Growth of TlBr crystals by the traveling molten zone method was performed using the same system as the zone-purification method with a slower furnace movement speed. The advantage of the traveling molten zone method is that no extrinsic impurities are introduced in the crystal during the process because material purification and crystal growth can be performed in the same ampule. For growing TlBr crystals by the vertical Bridgman method, the pure part of the zone-purified ingot was loaded into another ampule. A TlBr crystal with 1-inch diameter was grown successfully in this study by the vertical Bridgman method. Pixelated detectors and strip detectors were fabricated from the TlBr crystals. Very large mobility-lifetime products for electrons and holes were observed from the crystals. The crystal growth methods, the detector fabrication processes, and the detector performance will be discussed in detail.