R2A  Detector Systems and Applications

Tuesday, Nov. 3  08:30-10:05  California

Session Chair:  Jan Iwanczyk, DxRay, Inc., United States

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(08:30) R2A-1, invited, Advanced Compton Imagers: from Universal Exploration down to Earth Investigation and Medical Application

W. Lee, T. Lee, H. Lee

Bio-convergence Engineering, Korea University, Seoul, Korea
Since Compton imaging technology was first introduced about 40 years ago, it has been applied to many fields such as astrophysics, industrial and medical application. In the early days, Compton camera started to be used to search for signals from space in which the high energy radiation comes from point sources. The application in astrophysics was followed by the investigation for nuclear material in which advanced systems such as hybrid collimation method were applied. The Compton cameras were also utilized to monitor the environmental radiations caused by nuclear accidents. In medical application, both conventional and recoiled electron tracking method was applied for emission tomography. Recent researches presented Monte Carlo studies of multiple scattering Compton imaging methods for the detection of neutron induced gamma rays and scattered gamma rays in 3D radiation therapy. In summary, we present the historical trend of Compton camera and advanced Compton imagers made by us.

(08:50) R2A-2, Enabling Coded-Aperture and Compton Imaging in a Standalone CZT Gamma Camera

S. Stanchina, G. Montemont, O. Monnet, L. Verger

DTBS/STD/LDET, CEA, LETI, Grenoble, France
Today, there is a growing demand for gamma cameras targeting applications like homeland security, decommissioning, decontamination and safeguard. On the performance side, these applications are very challenging. They require at the same time a high energy resolution and a wide energy range for source identification, a high spatial resolution and a wide field of view for source localisation and a high sensitivity for source detection. Moreover, they also require for practicable reasons on-field measurement and analysis and thus require the cameras to be portable and autonomous. In previous works, we showed how to approach these goals by introducing a CZT coded aperture gamma-camera using subpixelated CZT detectors. With a 40x40x5 mm^3 detection volume, and 128x128 subpixels, this system shows good performances at low to middle energy range, but its efficiency decreases with the energy. In this paper, we further integrate this system to fulfil constraint of real field conditions and add Compton imaging capabilities for a better performance at high energy. Thanks to a novel embedded hardware and software data processing, this new Compton imaging system is autonomous, battery-powered, self-contained and can deliver ready images to the operator's smartphone via a WIFI connection. The paper is structured as follows: First, we review existing architectures. Then, we introduce the architecture of the gamma camera and discuss its key features. After that, we describe the main building blocks of the camera: readout ASIC and readout principles, event sorting and grouping, photon parameter extraction (in particular Compton-related processing), image processing and finally image transmission. Finally, we present results obtained with our prototype in various operating conditions.

(09:05) R2A-3, CdZnTe Gamma-Ray Spectroscopy in High Flux Environments Using Digital Pulse Processing Techniques

M. W. Streicher, Y. Zhu, Z. He

Nuclear Engineering & Radiological Sciences, University of Michigan, Ann Arbor, MI, USA
CdZnTe imaging gamma-ray spectrometers using digital pulse processing techniques have shown great promise in recent years. The energy resolution of 15 mm thick CdZnTe spectrometers has improved to around 0.4 % FWHM at 662 keV for single pixel events and near 0.6 % FWHM at 662 keV using all events, regardless of the number of pixels triggered. Additionally, the position resolution through subpixel position sensing has improved to less than 300 µm FWHM for 662 keV gamma-rays. These excellent detection properties have encouraged the use of CdZnTe in challenging environments with high gamma-ray fluxes such as nuclear power plants. CdZnTe is also being studied for use in medical imaging applications as well as nuclear safeguards and treaty verification where high count rate environments could exist. The waveforms generated from charge motion in CdZnTe were studied at high count rates to learn how the material responds in high gamma-ray fluxes. The relationship between average preamplifier decay slope and dose rate is shown to be monotonic. This relationship could be used in a real system to predict the dose rate on the detector surface. Minimal energy resolution degradation was observed up to 50 mR/hr. Beyond 50 mR/hr, the energy resolution degrades more substantially for multiple pixel events due to chance coincidence Compton scattering events. Changes in the electric field due to positive space charge accumulation were observed which likely contributes to the energy resolution degradation. Preamplifier instability is the other largest contributor to energy resolution degradation.

(09:20) R2A-4, First Flight Models of Caliste-SO Hard X-Ray Spectrometers for the STIX Instrument on-Board Solar Orbiter

A. Meuris1, O. Limousin1, O. Gevin1, C. Blondel1, M. Donati1, I. Le Mer1, J. Martignac1, M.-C. Vassal2, D. Blain2, M. Boussadia2, N. Fiant2, F. Soufflet2, M. Bednarzik3, G. Birrer3, S. Stutz3, C. Wild3, O. Grimm4, V. Commichau4, M. Billot5

1IRFU, CEA Saclay, Gif-sur-Yvette, France
23D Plus, Buc, France
3LMN, Paul Scherrer Institute, Villigen, Switzerland
4Institute for particle physics, ETH Zurich, Zurich, Switzerland
5CNES, Toulouse, France
ESA adopted in 2011 the Solar Orbiter mission to make significant breakthroughs in our understanding of the inner heliosphere and the solar activity. The probe will embed a suite of in-situ and remote-sensing instruments. Among them, the Spectrometer Telescope Imaging X-rays (STIX) will do spectrally-resolved and time-resolved imaging of solar flares in hard X-rays. For that purpose, the telescope uses a Fourier-imaging technique with 32 subcollimators to measure visibilities of the image. CEA-Irfu is responsible for the design and delivery of the spectrometer units to measure energies of photons from 4 to 150 keV with rather high resolution (< 1 keV FWHM at 6 keV). The device called Caliste-SO and fitting in a 12 mm x 14 mm x 18 mm volume is the hybridization of a 12-pixel CdTe Schottky detector produced by Acrorad and patterned by the Paul Scherrer Institute with a full custom front-end ASIC IDeF-X HD designed at CEA, in a packaging based on the 3D Plus company technology. The design inherits of a R&D program of CdTe hybrid imaging spectrometers with small pixels for hard X-ray astronomy with focusing optics (Caliste 64, Caliste 256 and Caliste HD devices). The main challenges for the solar flares observations on board Solar Orbiter is to maintain the initial spectral performance over 7 years mission despite the high temperature environment (up to 300�C on the X-ray entrance window), the high proton flux (1E10 10 MeV equivalent protons/cm2) and the high X-ray count rate during intense events (up to 100.000 counts/s/cm2 at the detector level). Thirty flight-quality Caliste-SO units have been produced so far and the production of twenty other detectors will be achieved before summer 2015. They will equip the STIX qualification and flight models. The paper presents the design, the production, the test set-ups and the performance results of Caliste-SO spectrometers. It focuses on the spectral response (transfer function, energy resolution) in various temperature and count rate conditions and concludes on some optimal flight operation configurations.

(09:35) R2A-5, A Material Base Decomposition Algorithm for High Resolution Spectroscopic X-Ray Imaging Detectors

A. Brambilla, A. Gorecki, A. Potop, C. Paulus, V. Rebuffel, V. Moulin, L. Verger

CEA-LETI,MINATEC Campus, F-38054 Grenoble, France
Energy sensitive photon counting detectors (PCDs) provide energy dependent information which can be exploited for material identification. The attenuation of an x-ray beam as a function of energy depends on the effective atomic number Zeff and the electron density. However, the attenuated spectra are degraded by the imperfections of the detector response such as charge sharing or pile-up, which leads to nonlinearities in the measured attenuation functions. This work aims to implement a basis material decomposition method which overcomes this problem. It consists in finding a combination of thicknesses from chosen basis materials that reproduces exactly the attenuation of the analysed object. The attenuated spectra are compared to those of a calibration base using a maximum likelihood criterion assuming a Poisson law distribution of count for each energy bin. The method has been tested with a ME100 linear array detector with 128 CdTe pixels of 800 µm pitch which is able to measures transmitted X-ray spectra in the 20-160 keV range on 64 energy bins. The images acquired from different plastic materials such as PMMA, POM or PTFE were decomposed into PE and PVC obtaining standard deviations lower 1 mm when using a photon statistic of 10^4 or more. The effective atomic number was deduced from the estimated equivalent lengths. It is independent of the sample thickness and can also be estimated in simple cases of material overlaps. The method can also be used for medical applications such as bone mineral densitometry and can be extended to 3 basis materials for contrast enhanced k-edge imaging.

(09:50) R2A-6, Challenges and Opportunities in the Development of Fieldable Imaging Spectrometers

C. G. Wahl, W. Kaye, F. Zhang, Y. A. Boucher, W. Wang, J. M. Jaworski, Z. He

H3D, Inc., Ann Arbor, MI, USA
In recent decades, room-temperature semiconductor detectors have made great strides, such that portable and reliable detectors can be designed with both good spectroscopy and gamma-ray imaging performance. In our experience adapting pixelated-anode laboratory technology to fieldable instruments, we have identified five areas in which further research and development can improve the current state of the art in fieldable instruments: reduced crystal cost, lower-power readout, higher count rate, better position resolution, and better high-energy response. Advances in any of these areas will allow new applications of these detectors. In this work, we describe the challenges and opportunities of each task and then describe some of our recent work in that area.