R3D  CdTe

Wednesday, Nov. 4  16:30-18:05  California

Session Chair:  Loick Verger, CEA-LETI, France

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(16:30) R3D-1, invited, Non-Destructive Piping Inspection Apparatus by CdTe X-Ray Imager

T. Aoki1,2,3, A. Koike2,3, K. Kimura1,2, H. Morii1,3, T. Okunoyama3

1Research Institute of Electronics, Shizuoka University, Hamamatsu, Japan
2Graduate School of Informatics, Shizuoka University, Hamamatsu, Japan
3ANSeeN Inc., Hamamatsu, Japan
The X-ray penetration imaging equipment to check corrosion has been developed in order to checking decrease wall thickness and/or generating rust of the pipes in a chemical plant and a petroleum plant. Penetration imaging by an X ray is performed because a heat insulator of glass wool and protective material of aluminum are wound on laying of the pipes. The edge check with which fleshiness of laying of the pipes is gauged was performed. CdTe detector has high-sensitivity for the high-energy X ray, therefore it seem to suitable for this kind of measurement and imaging. For example 150 mm diameter pipe has 7mm-thick wall, and an X-ray longest penetration length will be 70mm. We prepare the 200 kV of X-ray pulse source for this imaging system. The sensitivity of CdTe fits in with this energy well. The detected picture by CdTe is different from X-ray films, because energy-response and gamma-curve is very different between CdTe and X-ray films. We have developed small 100 um pitch 750 um-thick CdTe imager plate. The pictures by current mode CdTe imager was estimated by using Monte Carlo simulation, and X-ray irradiation and filter conditions were investigated and measuring system was produced experimentally. The image quality was different from an X-ray film, but an edge was observed and we improved advantage convenience substantially.

(16:50) R3D-2, Characterization of Large-Area Spectroscopic Imaging Array Fabricated Using Epitaxially Grown Thick Single Crystal CdTe Layer on Si Substrate

M. Niraula, K. Yasuda, S. Kouno, S. Sugimoto, Y. Ito, D. Yamazaki, M. Kozima, S. Kitagawa, Y. Agata

Graduate School of Engineering, Nagoya Institute of Technology, Nagoya, Japan
We are developing large-area x-ray, gamma ray spectroscopic imaging arrays for medical applications with working photon energies up to 100 keV, using metalorganic vapor-phase epitaxy grown thick single crystal CdTe epilayers on Si substrates. We have previously reported on the fabrication details of (8x8) pixel type imaging arrays using a 100 µm-thick single crystal CdTe epilayers grown on 12 mm x 12 mm sized Si substrate1. The p-CdTe/n-CdTe/n+-Si heterojunction diode type detector has a continuous gold electrode on the n+-Si side, while pixels of 1.12 mm x 1.12 mm size in a 1.27 mm pitch were patterned by making deep vertical cuts orthogonally using a diamond blade on the opposite side. The spectroscopic imaging performance of those arrays was promising. However, larger arrays with uniform performances among the pixels are needed for actual imaging applications. In this study, we optimized our crystal growth technique further to improve the material quality of the large area CdTe crystals grown on 25.4 mm x 25.4 mm sized Si substrates. With this improvement together with further optimization of array fabrication technique, we developed (20x20) pixel detector arrays. Evaluation of leakage currents from the entire pixels in the array revealed that most of the pixels had uniform leakage currents, which were less than 0.5 µA/cm2 at an applied reverse bias of 50V at room temperature. Detail results on the array evaluation will be presented. 1. M. Niraula et al., IEEE Trans. Nucl. Sci., vol. 59, p. 3201 (2012)

(17:20) R3D-4, Development of CdTe Pixel Detectors Combined with an Aluminum Schottky Diode Sensor and Photon-Counting ASICs

H. Toyokawa1, M. Kawase1, C. Saji1, S. Wu1, K. Kajiwara2, K. Osaka2, M. Sato2, T. Hirono3, T. Shobu4, A. Shiro4, A. Suenaga5, H. Ikeda6

1Controls and Computing Division, Japan Synchrotron Radiation Research Institute, Hyogo, Japan
2Industrial Application Division, Japan Synchrotron Radiation Research Institute, Hyogo, Japan
3Universitaet Bonn, Universitaet Bonn, Bonn, Germany
4Kansai Photon Science Institute, Japan Atomic Energy Agency, Hyogo, Japan
5Physikalisches Institut, Howa Sangyo Co., Ltd., Tokyo, Japan
6Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Kanagawa, Japan
We have been developing CdTe pixel detectors combined with a Schottky diode sensor and photon-counting ASICs. On the CdTe sensor device the front side was deposited with aluminum to form pixelated electrodes and the back side was covered with a single platinum-electrode. This electrode configuration has the advantage of providing a high Schottky barrier formed on the Al/CdTe interface, and, hence, a benefit to operate the CdTe as an electron-collecting pixelated diode. The 2D-detector module was designed with a pixel pitch of 0.2 mm x 0.2 mm and an area of 19 mm x 20 mm or 38.2 mm x 40.2 mm. The SP8-04 ASIC has a preamplifier, a shaper, 3-level window-type discriminators and a 24-bits counter in each pixel. The prototype detector with 20 × 50 pixels successfully operated with a photon-counting mode selecting X-ray energy with the window comparator. Excellent energy linearity was achieved between 15 and 120 keV. We have performed a feasibility study for white X-ray microbeam experiment with the prototype 2D detector. The 1D-type CdTe sensor assembly was designed as it is compatible with the MYTHEN Si-strip sensor. While the silicon strip sensor could directly connect to an ASIC with wires, the CdTe strip sensor couldn’t employ the wire-bonding technique because the CdTe crystal easily gave due to mechanical stress during the bonding procedure. In our technical choice the CdTe strip sensor was once bump-bonded to an interposer board with an In/Au-stud bonding method, and, then, the interposer board was wire-bonded to the ASIC. 638 strip-shaped electrodes formed 50 µm in pitch on the CdTe sensor and two sensors was adapted on one interposer board with 200 µm dead area in between two sensors.

(17:35) R3D-5, Space and Ground Applications of CdTe Double-Sided Strip Detectors for Hard X-Ray Imaging

M. Katsuragawa1,2, S. Takeda1, A. Harayama1, G. Sato1, S. Saito3, S.-N. Ishikawa1, S. Watanabe1,2, T. Takahashi1,2, M. Tampo4, K. Hamada4, Y. Miyake4

1Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Kaagawa, Japan
2Dept. of Physics, University of Tokyo, Tokyo, Japan
3Dept. of Physics, Rikkyo University, Tokyo, Japan
4Institute of Materials Structure Science, High Energy Acceleration Research Organization, Ibaraki, Japan
Recently, we developed an imaging device, cadmium telluride double-sided strip detector (CdTe-DSD) by forming strip electrodes on both sides of the CdTe device, based on the technologies of high-resolution Schottky CdTe diode and low-noise ASICs. This compact imaging device was developed for the ASTRO-H mission to be launched in 2015, where this device is for the hard X-ray spectroscopic imaging of astronomical objects. Because of its capability of fine energy and position resolutions as well as the possibility of fabricating compact imaging systems, the device is now being utilized for other space and ground applications, including NASA-funded FOXSI rocket experiment to observe X-rays from the Sun, and applications to accelerator-based physics experiments. In these circumstances, it is quite important to understand the properties of the CdTe-DSDs. For ground applications, we can consider use of pinhole collimators or coded aperture masks, which we reported in the previous conference, dependent on photon flux and signal-to-noise ratio. In high flux situation, the former may be ideal to obtain images free from coding ghost noise. Here, we develop an imager using a conic pinhole collimator to detect muonic X-rays, where high flux muon beam is irradiated on a target and muonic X-rays with characteristic energy of the material are emitted after de-excitation. Because of the high flux of muon beam, muonic X-ray and electrons from surrounding materials are also generated. To reduce such background noise components, we adopt a conic pinhole collimator made of 8 mm-thick tungsten. The detector enclosure is placed in graded-Z shields constructed by lead, tin and copper. Even with this configuration, high energy gamma-rays may be transparent to the shields, and incident on the detector. However, further background rejection is possible by examining detected energy on each strip. In this presentation, we describe implementation of the imaging system.

(17:50) R3D-6, High Sensitivity, Thin-Film CdTe Detector for Charged Particles and Neutrons

A. D. Compaan1, F. B. Cauffiel2, V. Plotnikov1, A. Shan3

1Lucintech Inc, Toledo, USA
2Lithium Innovations Co., LLC, Toledo, USA
3consultant, Toledo, USA
High sensitivity, thin-film CdTe detector for charged particles and neutrons Alvin Compaan,1 Ford Cauffiel,2 Victor Plotnikov,1 and Ambalanath Shan3 Lucintech Inc, 1510 N. Westwood Ave, Toledo, OH 43606 Lithium Innovations Co, LLC, 3171 N. Republic Blvd, Toledo, OH 43615 2324 Cheltenham Rd., Toledo, OH 43606 Lithium Innovations in collaboration with Lucintech Inc has developed a polycrystalline thin-film CdTe semiconductor detector with high sensitivity to energetic charged particles such as hydrogen and helium nuclei. The detector operates without external bias due to its heterojunction structure with n-CdS. This thin-film detector is an ideal component of a low-cost thermal neutron detector when paired with an isotopically enriched Li-6 sensitizer layer. The Li-6 isotope has a high cross-section for neutron conversion into alpha particles and tritons, through the reaction: n + Li-6 ? H-3 (2.735 MeV) + He-4 (2.05 MeV). High sensitivity of the detector to alpha and triton particles will be shown as well as its response to gamma ray sources. Neutron detection sensitivity with “thin” and “thick” Li-6 layers will be discussed.

(18:05) R3D-7, Sub-Pitch Spatial Resolution in CZT Detectors: Simulation Study

X. Zheng1, Z. Cheng1, J. Deen1, H. Peng2

1Electrical and Compute Engineering, McMaster University, Hamilton, ON, Canada
2Medical Physics, McMaster University, Hamilton, ON, Canada
Cadmium Zinc Telluride (CZT) semiconductor detectors have recently attracted extensive research interests in the development of novel PET imaging systems. Due to direct charge carrier generation/collection rather than indirect process of photon emission as in scintillator based detectors, it is able to demonstrate significantly improved energy resolution. In addition, by incorporating the depth of interaction (DOI) capability, a CZT detector is able to perform 3D positioning of high spatial resolution [1-3]. With high voltage applied between anode and cathode, electron-hole pairs excited by gamma ray photons would migrate within CZT bulk. By deploying single polarity sensing techniques [4] and fine electrodes (also known as small pixel effect) [5], both pixelated and cross-strip detector configurations can be used to improve charge collection and energy resolution. In a CZT detector, the spatial resolution is primarily dependent on the pitch of electrodes. However, as the pitch decreases, a number of challenges arise, such as the large number of readout channels, leakage current, multiple interactions, and charge sharing across adjacent electrodes. The major motivation of our study is to answer the question to what extent one can achieve sub-pitch spatial resolution (i.e., less than the pitch of electrodes) in CZT detectors using the following two techniques: charge sharing effect and transient charge analysis. As shown in Fig.1 (a), charge sharing occurs when the interactions locate near the electrode edge or within the gap, and electrons are collected by at least two adjacent electrodes. As a result, it becomes more noticeable as the sizes of electrodes and/or the gap are reduced. If not accounted for properly, it degrades the detector performances with respect to spatial resolution, energy resolution, as well as counting efficiency of 511 keV photons [6-9].

(11:45) R2B-5, invited, Novel Non-Metallic Contact on CdTe-Based Materials

U. N. Roy1, A. E. Bolotnikov1, G. S. Camarda1, Y. Cui1, R. Gul1, A. Hossain1, G. Yang1, R. B. James1, A. K. Pradhan2, R. M. Mundle2

1NN, Brookhaven National Laboratory, Upton, USA
2Department of Engineering & Center for Materials Research, Norfolk State University, Norfolk, USA
For the present technology of CdTe-based contacts, the major long-standing problem with the conventional metallic electrode is its degradation over time. The contact interfacial layer plays a vital role in detector performance and the detector durability. One major problem for CdTe-based materials with conventional metallic contacts is the poor adhesion caused by the large difference of thermal expansion coefficients of the CdTe-based substrate and the commonly used electrode material. This large mismatch of thermal expansion coefficients exerts a large amount of stress and is the root cause for detector failure over time, in addition to the poor adhesion. In order to address this long-standing issue for radiation detector applications, BNL has developed a novel non-metallic conductive contact on CdTe-based materials. The new novel contact provides better adhesion with close matching of the thermal expansion coefficients for the CdTe-based material and the contact. The better adhesion and close match of the thermal expansion coefficients between the contact layer and CdTe/CZT has tremendous potential to assure the long-term durability of the contact properties, hence more stability of the detector performance. In addition to the above advantages, the new contact provides 8-20 times higher hardness compared to most conventional metals used today on CdTe/CZT for fabrication of radiation detectors. The higher hardness of the new contact enhances the ease of handling and fabrication of detectors without scratching/punching the contact layers, especially for bump-bonded pixelated detectors. The new non-metallic contact on CZT and the effect on detector performance will be discussed in detail.