R1B  Opening RTSD Symposium

Monday, Nov. 2  10:30-12:35  California

Session Chair:  Ralph James, Savannah River National Laboratory, United States

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(10:30) R1B-1, Introductory and Welcoming Remarks

R. B. James1, M. Fiederle2

1Nonproliferation and National Security, Brookhaven National Laboratory, Upton, New York, USA
2University of Freiburg, Freiburg, Germany
The talk will provide a welcome to the RTSD attendees and brief introductory comments about the conference.

(10:38) R1B-2, invited, X-Ray Energy Dispersive Diffraction Detector System for Screening Checked Baggage at Airports

J. S. Iwanczyk1, E. Nygard2, D. Kosciesza3

1DxRay, Inc., Northridge, CA, USA
2Interon AS, Asker, Norway
3Morpho Detection, Hamburg, Germany
CdTe detectors operating at room temperature are viable candidates to replace cryogenically cooled Ge detectors that are currently used in commercial energy dispersive x-ray diffraction (EDXRD) baggage scanning systems. This paper reports on our development of a detector system for the screening of checked baggage at airports. Due to usually larger dimensions of checked baggage compared to carry-on baggage the field of view (FOV) and absorption length of the detectors has to be increased in EDXRD systems for checked baggage. We have constructed modules containing two-dimensional arrays using edge-on CdTe detectors. The edge-on detector configuration allows us to create long pixels to assure a sufficient absorption length for x-ray photons up to 180 keV. The edge-on configuration is specifically advantageous with the use of CdTe detectors employing Schottky barrier structures. These structures offer advantages over thick area detectors due to very low leakage currents and a short distance for collecting charges and can provide exquisite spectroscopic performance. The edge-on CdTe detectors combine high detection efficiency (along the x-ray beam direction) and at the same time preserve an excellent charge collection.There are 26 linear detector arrays in each module. Each linear detector array has dimensions of 22mm by 4mm by 0.75mm. Each linear array has 15 pixels connected to the corresponding amplification/processing channels of our custom application specific integrated circuit (ASIC) electronics. The linear arrays are positioned to exactly match the output of a scatter collimator in the XDi baggage screener from Morpho Detection, GmbH. The module has been specifically designed to minimize the cost of materials and assembly. We will present detailed performance characteristics for the detectors. Energy resolution as good as 3% (1.8 keV) FWHM at 60 keV with a very minimal low energy tailing in the spectra have been measured for the individual pixels in the array.

(10:58) R1B-3, invited, Photon Counting Medipix Chips with Thin Si, GaAs and CdTe Sensors for Mammography

S. Procz1, F. Fischer1, A. Fauler1, E. Hamann2, M. Fiederle1

1FMF Universität Freiburg, Freiburg im Breisgau, Germany
2KIT Karlsruhe Institute of Technology, Karlsruhe, Germany
The high spatial resolution and high efficiency of photon counting detectors with semiconductor sensor materials like Si, GaAs or CdTe are able to improve the quality of medical examination methods. Mammography is of peculiar interest because the typical X-ray energy (~25 keV) used for diagnosis here is very low in comparison to other medical diagnosis types. This allows for use of thin semiconductor sensors, which minimizes charge-sharing and gives optimum spatial resolution without a major loss of detection efficiency.

The Medipix3RX is a pixelated photon counting semiconductor detector which features up to eight adjustable energy thresholds and a correction for charge sharing effects. The Medipix3RX chip is under continued development by the “Medipix3 Collaboration” at CERN and offers 256x256 pixels with a native pixel pitch of 55x55 µm² and can be bump bonded to different semiconductor sensor materials.

The use of thin Si, CdTe and GaAs sensors with Medipix3RX chips for mammography using a mammography phantom will be presented here. A quantitative comparison of MTF, efficiency and sensor stability will be shown as well as the influence of X-ray fluorescence within the sensor and effects of improved flatfield correction [1] on image quality.

[1] S. Procz et al., Flatfield Correction Optimization for Energy Selective X-Ray Imaging With Medipix3, IEEE TNS Vol. 58, Issue 6, De. 2011

(11:18) R1B-4, invited, Feasibility Study on Fission Neutron Detection Using 3-D CdZnTe Detectors

Z. He, Y. Zhu, M. Streicher

Nuclear Engineering and Radiological Sciences, The University of Michigan, Ann Arbor, Michigan, USA
Three-dimensional position-sensitive (3-D) CdZnTe detector technology has made steady progress since 1998. Gamma-ray energy resolution close to 0.6% FWHM at 662 keV has been demonstrated on 2×2×1.5 cm^3 CdZnTe detectors at ambient temperatures. Real-time gamma-ray imaging and isotope identification capabilities have been implemented on research and commercial systems. Thermal neutron source detection and location have been experimentally demonstrated, and thermal neutron imaging is under development at University of Michigan. This work is to explore fission (fast) neutron detection capability using 3-D CdZnTe detectors. Conventional fission neutron detection has mostly relied on the use of low-Z scintillators or thermal neutron detectors surrounded by low-Z moderators. This new technology is based on elastic scatterings between incident fast neutrons and detector material, proposed previously by our group. The success of this method requires the detection of very small, such as less than 5 keV, electron-equivalent signals recorded in CdZnTe detectors. The feasibility of fast neutron detection using 3-D CdZnTe detectors will be reported.

(11:38) R1B-5, invited, The Domestic Nuclear Detection Office and the Role of Semiconductor-Based Radiation Detectors

A. Janos

Domestic Nuclear Detection Office, Washington, DC, USA
The Domestic Nuclear Detection Office (DNDO) within the U.S. Department of Homeland Security (DHS) was established to improve the Nation’s capability to detect and report unauthorized attempts to import, possess, store, develop, or transport nuclear or radiological material for use against the Nation, and to integrate federal nuclear forensics programs. Within DNDO, the R&D function is mainly performed by the Transformational and Applied Research (TAR) Directorate. Materials research is an important component of that R&D. In particular, semiconductor-based gamma and or neutron detection materials comprise an important component of the materials research. This presentation will provide a brief overview of the TAR Directorate, the mission challenges, the rationale behind pursuing semiconductor research, and priorities for R&D within TAR. Examples will be given for past, present, and future R&D programs involving semiconductors.

(11:58) R1B-6, invited, Feasibility of CdZnTe Gamma-Ray Detectors with Thicknesses >15 mm

A. E. Bolotnikov1, G. S. Camarda1, E. Chen2, Y. Cui1, G. De Geronimo1, C. Finfrock1, J. Fried1, A. Hossain1, G. Mahler1, M. Petryk3, U. Roy1, S. Taherion2, E. Vernon1, G. Yang1, R. B. James1

1Brookhaven National Laboratory, Upton, NY, USA
2Redlen Technologies Inc., Saanichton, BC, Canada
3SUNY Binghamton, Vestal, NY, USA
The long electron lifetime in today’s CdZnTe (CZT) crystals allows for making CZT detectors with much greater thicknesses than those of existing ones. However, inhomogeneity in their response, limits the thicknesses and sizes of the actual detectors. The biggest single-crystal device reported in the literature so far is a 20x20x15 mm3 pixelated detector with a drift distance of 15 mm. Since the variations of the detector responses are due to random distribution of the interaction points, such variations can be corrected by segmenting the active volumes of the detectors and normalizing the responses generated from each of the voxels. Such high-granularity position-sensitive detectors open up the opportunity for using thicker, less expensive CZT crystals. The goal of this work was to demonstrate that today’s commercial high mu-tau CZT material is suitable for detectors with up to 30-mm drift distances, provided that the detectors have the ability to correct their response non-uniformities on a scale comparable to the sizes of electron clouds, which is ~100 µm. The samples of the detectors used in these measurements were fabricated by Redlen Technologies Inc. using the Travelling Heater Method (THM). THM feed stock was synthesized using 6N purity starting metals using a proprietary in-house compounding method. Tellurium-rich CZT alloy used for the solvent zone was prepared in a similar manner. All Cd0.9Zn0.1Te crystals were grown on single crystal (111) seeds in a direction and annealed as oriented wafers. Detectors were cut from areas of the annealed wafers free from defects such as dislocation walls and mosaic structures. We employed the ASIC and data-acquisition system developed by BNL’s Instrumentation Division for arrays of virtual Frisch-grid detectors. For each detector, we used 6 ASIC channels to read the negative signals from the cathode and from four position-sensing pads and the positive signals from the anode.

(12:18) R1B-7, Investigation of the Performance of CdZnTe X-Ray Detectors by Studying the IV Characteristics under High Flux Irradiation Conditions.

G. Prekas1, A. Densmore1, U. El-Hanany1, V. Perumal1, E. Johnson2

1Redlen Technologies Inc., Saanichton, BC, Canada
2Surescan Corporation, Endicott, NY, USA
We have demonstrated that Redlen’s CdZnTe detectors can perform well, without polarizing, in high flux X-ray environments. We have shown that by minimizing the concentration of deep level hole traps this material can operate at incoming X-ray fluxes exceeding the 100MCps.mm-2 with highly uniform and stable response. Furthermore, the detectors do not exhibit polarization effects such as electric field collapse that result in ballistic deficit and incomplete charge collection. In this paper, we present our findings studying detectors of different quality and in particular their associated electrical characteristics at different biases and irradiation conditions. We present the IV characteristics of different samples and we discuss the role of extended crystal defects (i.e twins, sub-grain boundaries etc.) and the effect of point defects, on the detector dark current, photo-current and photoconductive gain. We show correlations between the dark currents, resistivity and photocurrent of various samples that polarize and do not polarize under high flux conditions and explain their relationship and effect on the electron transport, signal collection and output count rate linearity. Finally, we show that the IV characteristics, dark current and photocurrent mapping can be used as a reliable screening tool for assessing the quality of CdZnTe wafers for the fabrication of high flux X-ray detectors.