Proceedings Volume 5540

Hard X-Ray and Gamma-Ray Detector Physics VI

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Proceedings Volume 5540

Hard X-Ray and Gamma-Ray Detector Physics VI

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Volume Details

Date Published: 21 October 2004
Contents: 7 Sessions, 27 Papers, 0 Presentations
Conference: Optical Science and Technology, the SPIE 49th Annual Meeting 2004
Volume Number: 5540

Table of Contents

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Table of Contents

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  • CZT Detectors
  • CZT Detectors and Characterization
  • Scintillators and Hgl2
  • Imagers
  • Novel Devices
  • CdTe and CdZnTe I
  • Posters - Monday
  • Novel Devices
  • CdTe and CdZnTe I
CZT Detectors
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Thick CZT detectors for spaceborne x-ray astronomy
Henric S. Krawczynski, Ira Jung, Jeremy S. Perkins, et al.
Cadmium Zinc Telluride (CZT) detectors are having a major impact on the field of hard X-ray astronomy. Without the need for cryogenic cooling they achieve good spatial and energy resolutions over the broad energy range from 10 keV to ~600 keV. In this paper, we briefly review the historical development of detectors used in X-ray astronomy. Subsequently, we present an evaluation of CZT detectors from the company Imarad. The standard 2x2x0.5 cm detectors, contacted with 8x8 In pixels and an In cathode, exhibit FWHM energy resolutions of 7 keV at 59 keV, and 10 keV at 662 keV. A direct measurement of the 662 keV photopeak efficiency gives 67%. We have started a detailed study of the performance of Imarad detectors depending on surface preparation, contact materials, contact deposition, post-deposition detector annealing, and detector passivation techniques. We present first results from contacting detectors with Cr, Ag, Au, and Pt.
Coplanar grid CdZnTe detectors for space science applications
Benjamin W. Sturm, Zhong He, Edgar A. Rhodes, et al.
The characteristics and performance for two of the latest coplanar grid CdZnTe detectors, which use the third-generation coplanar grid design, will be discussed. These detectors, with dimensions of 1.5x1.5x0.9 cm3 and 1.5x1.5x0.95 cm3, were fabricated by Baltic Scientific Instruments, Ltd., using crystals from Yinnel Tech, Inc. The high electron mobility-lifetime product measured for these crystals will lead to improved charge collection efficiency and better energy resolution. The spectroscopic performance obtained from the detectors, employing various methods such as depth sensing, radial sensing, and relative gain compensation, will be reported. Results from these measurements will give us insight into the material properties as well as the charge induction uniformity of the detector.
Multipixel characterization of imaging CZT detectors for hard x-ray imaging and spectroscopy
We report our in-depth study of Cd-Zn-Te (CZT) crystals to determine an optimum pixel and guard band configuration for Hard X-ray imaging and spectroscopy. We tested 20x20x5mm crystals with 8x8 pixels on a 2.46mm pitch. We have studied different types of cathode / anode contacts and different pixel pad sizes. We present the measurements of leakage current as well as spectral response for each pixel. Our I-V measurement setup is custom designed to allow automated measurements of the I-V curves sequentially for all 64 pixels, whereas the radiation properties measurement setup allows for interchangeable crystals with the same XAIM3.2 ASIC readout from IDEAS. We have tested multiple crystals of each type, and each crystal in different positions to measure the variation between individual crystals and variation among the ASIC channels. We also compare the same crystals with and without a grounded guard band deposited on the crystal side walls vs. a floating guard band and compare results to simulations. This study was carried out to find the optimum CZT crystal configuration for prototype detectors for the proposed Black-Hole Finder mission, EXIST.
CZT Detectors and Characterization
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New results from performance studies of Frisch-grid CdZnTe detectors
New data regarding performance studies of Frish-grid CdZnTe (CZT) detectors are presented. The Frisch-grid detector configuration under investigation is a bar shaped CZT crystal with teh side surfaces coated with an insulating layer. A Frisch grid is fashioned by inserting the CZT bar into a metallic sleeve, or by depositing the metal directly upon the insulator; hence the semiconductor material does not come in contact with the metal grid. The simple design operates well as a single-carrier-sensitive device. Despite the simplicity of this device, its performance depends on the balanced combinations of several factors, including the bulk and surface conductivity, μτ product, and geometrical aspect ratio. Described are several effects that determine charge collection in such drift devices and, consequently, the performance of the non-contacting Frisch-grid configuration.
Study of growth defects in CZT and their influence on detector uniformity
The main problems involved in applying Cadmium Zinc Telluride (CZT) to detectors are the crystal perfection required and the difficulty in making reliable surface electrical contacts to the material. Our efforts have focused on the development of interconnect techniques and testing methods which will allow us to explore the interaction of defects with detector properties. Local stoichiometry variations and other local disordering make it very hard to find a systematic correlation between performance and material defects in the macroscopic scale. In order to understand the factors limiting the energy resolution of CZT detectors, our efforts were directed to the area of material characterization and detector testing using the National Synchrotron Light Source (NSLS). NSLS provides us with a highly collimated high intensity X-ray beam, which is employed to investigate micron-scale detector performance mapping and the correlation between microscopic defects and fluctuations in collected charge. Some results were already published and more are presented and correlated to X-ray diffraction topography (XDT) measurements. XDT at the beamline X17B1 is used to investigate more systematically the origins of the mosaicity that can give us information about the defect distribution and strains in bulk CZT crystals.
Characterization of coplanar grid CZT detectors with highly collimated x-ray beam
CdZnTe detectors demonstrated great potentials for detection of gamma radiation. However, energy resolution of CdZnTe detectors is significantly affected by uncollected holes which have low mobility and short lifetime. To overcome this deleterious effects upon energy resolution special detector designs have to be implemented. The most practical of them are the small pixel effect device, the co-planar grid device, and the virtual Frisch-grid device. We routinely use a highly collimated high-intensity X-ray beams provided by National Synchrotron Light Source (NSLS) facility at Brookhaven National Laboratory to study of CdZnTe material and performances of the different types of devices on the micron-scale. This powerful tool allows us to evaluate electronic properties of the material, device performance, uniformity of the detector responses, effects related to the device's contact pattern and electric field distribution, etc. In particular, in this paper we present new results obtained from the performance studies of 15 x 15 x 7.5 mm3 coplanar-grid devices coupled to readout ASIC. We observed the effect of the strip contacts comprising the grids on the energy resolution of the coplanar-grid device.
Laboratory coded-aperture imaging experiments: radial hole coded masks and depth-sensitive CZT detectors
Jae Sub Hong, Santosh V. Vadawale, Minhua Zhang, et al.
The proposed black-hole finder mission EXIST will consist of multiple wide-field hard X-ray coded-aperture telescopes. The high science goals set for the mission require innovations in telescope design. In particular, wide energy band coverage and fine angular resolution require relatively thick coded masks and thick detectors compared to their pixel size, which may introduce mask self-collimation and depth-induced image blurring with conventional design approaches. Previously we proposed relatively simple solutions to these potential problems: radial hole for mask selfcollimation and cathode depth sensing detector for image blurring. We have now performed laboratory experiments to explore the potential of these two techniques. The experimental results show that the radial hole mask greatly alleviates mask self-collimation and a ~1 mm resolution depth-sensitive detector scheme can be relatively easily achieved for the large scale required for EXIST.
Scintillators and Hgl2
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Cerium-activated rare-earth orthophosphate and double-phosphate scintillators for x- and gamma-ray detection
Lynn A. Boatner, L. A. Keefer, J. Matt Farmer, et al.
When activated with an appropriate rare-earth ion (e.g., Ce or Nd), rare-earth orthophosphates of the form REPO4 (where RE = a rare-earth cation) and alkali rare-earth double phosphates of the form A3RE(PO4)2 (where A = K, Rb, or Cs) are characterized by light yields and decay times that make these materials of interest for radiation-detection applications. Crystals of the compound Rb3Lu(PO4)2 when activated with ~0.1 mol % Ce exhibit a light yield that is ~250% that of BGO with a decay time on the order of ~40 nsec. The cerium-activated rare-earth orthophosphate LuPO4:Ce is also characterized by a high light yield and a relatively fast decay time of ~25 nsec. Additionally, the rare-earth orthophosphates are extremely chemically, physically, and thermally durable hosts that recover easily from radiation damage effects. The properties of the rare-earth orthophosphates and double phosphates that pertain to their use as X- and gamma-ray detectors are reviewed. This review includes information related to the use of Nd-doped LuPO4 as a scintillator with a sufficiently energetic, short-wavelength output (λ=90 nm) so that it can be used in conjunction with appropriately activated proportional counters. Information is presented on the details of the synthesis, structure, and luminescence properties of lanthanide double phosphates that, when activated with cerium, are efficient scintillators with output wavelengths that are sufficiently long to be well matched to the response of silicon photodiode detectors.
Scintillator and photodetector array optimization for gamma-ray imaging
Aboubakr Bakkali, Najia Tamda, Michel Parmentier, et al.
Needs of improved medical diagnostics, specially for early and reliable breast cancer detection, lead us to consider developments in scintillation crystals and position sensitive photomultiplier tubes (PSPMT) in order to develop a high resolution medium field g-ray imaging device. However, gamma rays detector need to find a compromise between many conflicting requirements. In order to optimize different parameters involved in the detection process, we have developed a Monte Carlo simulation software. Its aims were to optimize a gamma ray imaging system based on pixellated scintillation crystal coupled to a PSPMT array. Several crystal properties were taken into account as well as the measured intrinsic response of PSPMTs. Images obtained by simulations are compared with experimental results. Agreement between simulation and experimental results validate our simulation model.
A physics model of lutetium oxyorthosilicate detectors: theory and experimental validation
Michael J. Berninger, Thomas J.T. Kwan, Lin Yin, et al.
Detector physics is an important element in the simulation of X-ray radiography. In conjunction with the radiographic chain model (RCM) developed at Los Alamos National Laboratory (LANL), we have built a high-fidelity model of the Lu2SiO5:Ce3+ (LSO) detector system for use with the Cygnus rod-pinch X-ray source. In the RCM, the two-dimensional (2D) fully electromagnetic and relativistic particle-in-cell (PIC) code MERLIN is used to model the Cygnus electron diode. The electron distributions from PIC calculations are used in the Monte Carlo N-Particle (MCNP) code to model the generation of the X-rays via the bremsstrahlung process and subsequent transport through dense objects to detectors. Radiographs are calculated in conjunction with empirically measured scintillation efficiencies for light yields. To model detector blur, MCNP calculates the point-spread functions (PSF) of X-ray scattering in the LSO. Two length scales in the PSFs can account for correlated short-range (< 0.4 mm) and long-range (uncorrelated) blur. By employing a detector model methodology, we can examine detector parameters such as the detector quantum efficiency (DQE), blur, and photon statistics. The calculations are validated in juxtaposition with experimental radiographic data on step wedges, rolled edges, and static objects. In this paper, we focus on characterizing the detector performance.
Characterization of strain and crystallographic defects in HgI2 single crystals
We investigated bulk-grown HgI2 crystals to better understand the nature of crystallographic defects and strain/stress in different growth regions of the crystal and their affect on the performance of HgI2-based radiation detectors. Double-axis and triple-axis high-resolution x-ray diffraction were used to characterize the mosaic structure and strain in HgI2. Rocking curves revealed significant mosaic spreading in <110> growth regions exhibiting X-defects versus X-defect-free <100> growth regions. Both <110> and <100> growth regions exhibited little strain (~0.01%). We report the narrowest rocking curves (~ 9 arcsec) to date on HgI2 as a result of the resolution of the instrument (~ 6 arcsec). Raman spectroscopy was used collaboratively to confirm little residual stress in the crystals. We developed a growth rate ratio (chi) and show this geometric model used to describe crystal shape and regions of <100> and <110> growth. Optical characterization of X-defects are presented and discussed. Further the influence of crystallographic defects and strain on radiation detector performance are discussed.
Imagers
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A high-resolution gamma camera based on array of R8520-00-C12 PSPMTs
Najia Tamda, Aboubakr Bakkali, Michel Parmentier, et al.
Although gamma cameras have emerged in the sixties, their spatial resolution is still not sufficient to detect small tracer concentration abnormalities. Examinations like scintimammography requires high spatial resolution and then the possibility to position the detector as close to the explored organ as possible . The emergence of the new position sensitive photomultipliers tubes(PSPMT), from HAMAMATSU, permitted us to develop a compact gamma ray imaging probe which fulfils these requirements. The major interest of the new R8520-00-C12 PSPMT generation is their very low height (27mm) which allows to build a very compact and relatively light gamma ray detector. Their square shape (25.7x25.7mm2) and their very thin dead edges (1.85mm) authorize their juxtaposition in order to obtain a large detection area. In this study we investigate the characteristics of a prototype using a square 2x2 array of HAMAMATSU R8520 position sensitive photomultiplier tubes coupled to a pixelated NaI(Tl) crystal array containing 24x24 pixels each made of 2 x 2 x 5 mm3 crystals with 2.2 mm centre to centre spacing. We present the first results regarding intrinsic spatial resolution, energy resolution and homogeneity . Illuminating the detector, without scintillating crystal, with a light source simulating a scintillation at 140kev, we obtain an intrinsic spatial resolution better than 1mm on the whole field of view also including dead areas between PSPMTs. By coupling this detector to the crystal scintillator previously described, an energy resolution better than 10% FWHM at 140kev is obtained in PSPMT centers. These performances and the inherent scalability of detectors built using arrays of square tubes, make it an attractive choice for use in dedicated nuclear medicine instruments, including small animal imaging.
A Compton imaging device for radioactive material detection
Andrew S. Hoover, William Baird, R. Marc Kippen, et al.
The most serious terrorist threat we face today may come from radiological dispersion devices and unsecured nuclear weapons. It is imperative for national security that we develop and implement radiation detection technology capable of locating and tracking nuclear material moving across and within our borders. Many radionuclides emit gamma rays in the 0.2 -- 3 MeV range. Unfortunately, current gamma ray detection technology is inadequate for providing precise and efficient measurements of localized radioactive sources. Common detectors available today suffer from large background rates and have only minimal ability to localize the position of the source without the use of mechanical collimators, which reduces efficiency. Imaging detectors using the Compton scattering process have the potential to provide greatly improved sensitivity through their ability to reject off-source background. We are developing a prototype device to demonstrate the Compton imaging technology. The detector consists of several layers of pixelated silicon detectors followed by an array of CsI crystals coupled to photodiodes. Here we present the concept of our detector design and results from Monte Carlo simulations of our prototype detector.
3D position-sensitive CdZnTe gamma-ray spectrometers: improved performance with new ASICs
A 3-dimensional position sensitive CdZnTe gamma-ray spectrometer based on VAS3.1/TAT3 ASICs was developed and tested. The 3-D CZT spectrometer employs a 1.5 cm x 1.5 cm x 1 cm3 CdZnTe crystal with 11 by 11 pixelated anodes wire-bonded to the readout electronics. The signals from the anode pixels and the cathode were both read out through the ASICs. The pixel position provides the lateral 2-D coordinates, while the third coordinate can be determined by using depth-sensing techniques. With the help of 3-D position sensitivity, the variation in weighting potential, electron trapping and material non-uniformity can be mitigated to the scale of the position resolution, estimated to be 1.27 mm x 1.27 mm x 0.2 mm. The energies and 3-D coordinates can be reconstructed for multiple interaction events from a single incident gamma ray. The third-generation ASICs - VAS3.1/TAT3 has been developed to improve the electronic noise, uniformity, linearity and stability. Energy resolution of 0.93% FWHM and 1.52% FWHM have been achieved for single-pixel events and two-pixel events, respectively, including ~4.5 keV FWHM electronic noise.
4-pi Compton imaging with single 3D position-sensitive CdZnTe detector
Dan Xu, Zhong He, Carolyn E. Lehner, et al.
A 3D CdZnTe detector can provide 3D position information as well as energy information of each individual interaction when a gamma ray is scattered or absorbed in the detector. This unique feature provides the 3D CdZnTe detector the capability to do Compton imaging with a single detector. After detector calibration, real-time data acquisition and imaging are implemented with a single detector system. Because the detector has a finite size and any point in the detector can be the first scattering position, 3D gamma-ray imaging in near field is possible. In this work we will show the result of the 4π Compton imaging with a single 15mm × 15mm × 10mm CdZnTe detector. Different algorithms for sequence and imaging reconstruction will be addressed and compared. The angular uncertainty is estimated and the most recent results from measurements are presented.
Novel Devices
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Design of a multichannel ultra-high-resolution superconducting gamma-ray spectrometer
Stephan F. Friedrich, Stephane F. Terracol, Toshiyuki Miyazaki, et al.
Superconducting Gamma-ray microcalorimeters operated at temperatures around ~0.1 K offer an order of magnitude improvement in energy resolution over conventional high-purity Germanium spectrometers. The calorimeters consist of a ~1 mm3 superconducting or insulating absorber and a sensitive thermistor, which are weakly coupled to a cold bath. Gamma-ray capture increases the absorber temperature in proportion to the Gamma-ray energy, this is measured by the thermistor, and both subsequently cool back down to the base temperature through the weak link. We are developing ultra-high-resolution Gamma-ray spectrometers based on Sn absorbers and superconducting Mo/Cu multilayer thermistors for nuclear non-proliferation applications. They have achieved an energy resolution between 60 and 90 eV for Gamma-rays up to 100 keV. We also build two-stage adiabatic demagnetization refrigerators for user-friendly detector operation at 0.1 K. We present recent results on the performance of single pixel Gamma-ray spectrometers, and discuss the design of a large detector array for increased sensitivity.
Crystal growth, characterization, and fabrication of AgGaSe2 crystals as novel material for room-temperature radiation detectors
Utpal N. Roy, Michael Groza, Yunlong Cui, et al.
Silver gallium diselenide (AgGaSe2) is a semiconductor compound having an energy bangap of 1.7 eV, a value that is favorable for the room temperature radiation detection application. The starting material was synthesized from high purity elemental starting materials: 5N purity Se, 6N purity Ag, and 7N purity Ga. The crystals were grown at 880 °C in a three-zone semi-transparent gold-coated horizontal furnace. High resistivity (1.4 x 1011 ohm-cm) material was obtained and radiation detectors were fabricated. The response to gamma and alpha particles will be reported along with an analysis of the mobility - trapping time product for this novel material.
CdTe and CdZnTe I
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Large-size CdZnTe (Zn=10%) radiation detector materials and device performance
Longxia Li, Fengying Lu, Chun Lee, et al.
Large-size CZT single crystals of up to 300 cm3 have been grown at Yinnel Tech. These crystals were produced into radiation detectors with excellent performances, leading to enhancements in both energy resolution and detector efficiency.
Crystal growth, characterization, and testing of Cd0.9Zn0.1Te single crystals for radiation detectors
Krishna C. Mandal, Caleb Noblitt, Michael Choi, et al.
This paper describes our recent research in growing large single crystals of Cd0.9Zn0.1Te (CZT) by the vertical Bridgman technique using in-house processed zone refined precursor materials (Cd, Zn, and Te). The grown semi-insulating CZT crystals have shown high promise for high-resolution room-temperature radiation detectors due to their high dark resistivity (~1010 Ωcm), reasonably good charge transport properties [(μτ)e = (2-5) x 10-3 cm2/V] and low cost. The grown CZT single crystals (~2.5 cm diameter and up to 10 cm long) have demonstrated a very low radial Zn concentration deviation, low dislocation densities and Te precipitate/inclusions, and high infrared transmission. Details of the CZT single crystal growth, their physical and chemical analysis, surface processing, nuclear radiation detector fabrication, and testing of these devices are also presented.
Development of energy-discriminate CdTe imaging detector for hard x-ray
Toru Aoki, Yu Ishida, Daisuke Sakashita, et al.
Energy discriminate type CdTe imaging detector was developed for hard X-ray imaging. The device has 4 x 128 structured 512 semi-linear M-π-n CdTe pixels with 0.5 mm pixel pitch and 256 mm length. Each pixel was 2mm x 0.8 mm size and connected to photon-counting type data processing circuit integrated as 64ch ASIC. The ASIC could be operated at high speed over 1M cps and it has 5 levels of energy discriminated thresholds and 15bit counter with each thresholds levels. The imaging detector was designed for energy discriminated hard X-ray imaging using X-ray tube source, since its high incident rate correspondence by high speed operating. The detector was consisted by 512-CdTe detector chips, 8-ASICs with control digital circuits, system control MPU, interface device and high-voltage source in the detector unit, and connected to conventional laptop personal computer thorough USB2.0 interface. In this study, we build energy-discriminated X-ray penetrating imaging system with this CdTe 512 pixels imaging detector unit, 90keV micro-focus X-ray source, mechanical scanning system and imaging software. The energy discriminate X-ray penetrating imaging was carried out by this system.
High-pressure xenon electroluminescence detectors
We have investigated electroluminescence (EL) detectors with uniform and axial electric field configurations, filled with xenon gas pressurized up to 35 bar. Photomultipliers placed outside pressurized vessels and avalanche photodiodes placed directly inside the pressurized xenon have been used to detect scintillation and electroluminescence signals. A light-collection system based on a cylindrical array of wavelength shifting fibers has been used to collect light in the detector with axial electric field. The EL detectors with photomultiplier readout have demonstrated exceptionally low sensitivity to vibrations. An energy resolution of 10%FWHM was measured for 60 keV gamma rays. The results are discussed and measures that will be undertaken to improve performance of the detectors are considered.
Virtual Frisch-grid ionization chambers filled with high-pressure Xe
New approaches to the design of high-pressure Xe (HPXe) ionization chambers are described. HPXe ionization chambers represent a well-known technique for detecting gamma rays in the energy range between 50 keV and 3 MeV. Since the HPXe detector is an electron-only carrier device, its commonly accepted design includes a Frisch-grid-a metal mesh employed for the electrostatic shielding from the immobile positive ions. The grid is a key element of the device’s design which provides good energy resolution of the detector, typically 2-3% FWHM at 662 keV. However, the grid makes the design more complex and less rugged, especially for field applications. Recently, we developed several designs of HPXe ionization chambers without shielding grids. The results obtained from the testing of these devices are presented here.
Posters - Monday
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Studies of interaction between components in system ZnSe-ZnTe-Se-H2-C
Vladimir D. Ryzhikov, Leonid P. Gal'chinetskii, Sergey N. Galkin, et al.
A thermodynamic analysis has been carried out for interaction of the components in the ZnSe-ZnTe-Se-H2-C system. Studies of thermodesorption, thermodynamics and kinetics of interaction reactions in the ZnSe-ZnTe-Se-H2-C system indicate high probability of formation of fine-dispersed fullerene-like particles of hydrated carbon black in this system at the stage of ZnSe(Te) charge synthesis. It has been established that carbon black particles preserve their stability at the stage of ZnSe(Te) crystal growth and can substantially affect crystallophysical, optical and physico-chemical properties of these crystals.
Spectrometric universality and reduction of nonstatistical noises in detectors with regular light collection
Sergey V. Naydenov, Vladimir D. Ryzhikov
Intrinsic resolution of the scintillator is one of the most important constituents of the full energy resolution of a detector. Intrinsic resolution contains components preserved at any ionizing radiation energy. A major role is played by the resolution of light collection. This is a component determined by geometric-optical non-uniformities of scintillation energy propagation and collection. In this work, theoretical studies of general light collection features have been carried out. A universal law has been predicted for light collection dispersion in detectors of regular beam dynamics. Such systems include scintillation blocks with mirror reflecting surface and regular geometry in the shape of cylinder, parallelepiped or sphere. An important regular collection feature is weak dependence of its dispersion on the scintillator material or shape. This allows to relate spectrometric efficiency and detection efficiency for any detector of the said type. The theoretically obtained law is confirmed by the available experimental data. The developed theory allows finding new ways to eliminate internal noises affecting the radiation measurements data. Such optimization is a necessary condition for creation of new detectors with improved characteristics.
Gamma detectors based on high-pressure xenon: their development and application
Sergey E. Ulin, Valery V. Dmitrenko, V. M. Grachev, et al.
Various modifications of xenon detectors and their parameters in comparison with gamma-detectors of other types are considered. Prospects of xenon detectors' applicatins in gamma-spectroscopy based on experimental results are discussed including detection and control of radioactive and fissile materials displacement, definition of uranium enrichment rate, and measurements of nuclear reactor radioactive gas waste concentration. Possibilities for xenon detector use for environmental control and measurement of cosmic gamma radiation on orbital stations are considered.
Novel Devices
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Innovations in low-temperature calorimeters: surface sensitive bolometers for background rejection and capacitive bolometers for higher energy resolution
Samuele Sangiorgio, Marco Barucci, Luca Foggetta, et al.
In this contribution, we describe two innovations of the structure of large mass bolometers, proposed by the cryogenic group of the Insubria University (Como) and developed in collaboration with the Firenze group. First, up to now, low temperature calorimeters do not have any sort of spatial resolution. This means that it is not possible to reject events coming from the material that faces the detectors (holder, refrigerators shields, ...). In order to cope this problem, we developed a new kind of composite bolometers able to discriminate, by means of active ultra-pure semiconductor shields, external surface events from those coming from the absorber bulk. A second innovation that we discuss here concerns the temperature sensors. Presently, neutron transmutation doped Ge thermistors are the most common kind of phonon sensors. Unfortunately, this kind of readout dissipates power on the detector because of the thermistor biasing and also introduces a Johnson noise term. To improve energy resolution we studied and test the application of capacitive sensors that in principle could allow us to achieve a better signal-to-noise ratio. Modeling, simulations and first encouraging measurements on surface sensitive bolometers will be discussed along with preliminary results on capacitive sensors.
CdTe and CdZnTe I
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Scintillation imaging: a technique to reduce coding noise in scanned coded aperture imagers
Coded aperture imagers provide the optimum means to generate an all-sky survey at gamma-ray energies from 10's of keV to a few MeV. Unfortunately, such imagers are plagued by systematic noise that limits their dynamic range. In par-ticular, spatial gradients in the background radiation across the detector and imperfectly coded signals from strong point sources in the field of view add artifacts to the images. Although theoretical signal-to-noise ratios of order 104 are possi-ble in perfectly coded images, real-world effects have limited performance of past imagers to significantly less than that. One technique to help remove these aberrations is the use of sequential exposures with a mask and it's inverse. However, for large instruments this is an impractical solution. In addition, it does not apply for scenes with rapidly varying sources. In a scanning instrument, one solution to this problem is to interleave the mask and anti-mask patterns in a sin-gle aperture in the direction of scan. A true source will oscillate between positive and negative images (i.e. scintillate) while spatially varying backgrounds are significantly suppressed.