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The coefficients of thermal expansion (CTE) of the ternary borides, MoAlB, Cr2AlB2, Mn2AlB2, and Fe2AlB2, and the binary boride, CrB, were calculated from in situ high-temperature powder X-ray diffraction patterns. The order—from low to high—of the average linear thermal expansions was MoAlB (7.3 × 10−6 K−1), CrB (8.6 × 10−6 K−1), Fe2AlB2 (10.3 × 10−6 K−1), Cr2AlB2 (10.5 × 10−6 K−1), and Mn2AlB2 (14.0 × 10−6 K−1). Despite their structural and chemical similarities, the CTE anisotropies of these phases were different and could be grouped into two groups. In the first group, MoAlB and Fe2AlB2, the highest CTE values were along the stacking direction and the lowest were parallel to the B–B chains. In the second group, Cr2AlB2, Mn2AlB2, and CrB, the lowest CTE was along the stacking direction and the highest was normal to the chains. The thermal expansions parallel to the B–B chains were significantly lower (32% to 103%) than those perpendicular to the chains in all the ternaries except for MoAlB. In general, the relative CTEs parallel to the B–B chains in both the M2AlB2 ternaries and corresponding binary monoborides suggest that strong covalent character of the B–B bonds are at least partially responsible for the large thermal expansion anisotropies observed. [ABSTRACT FROM AUTHOR]

The coefficients of thermal expansion (CTE) of the ternary borides, MoAlB, Cr2AlB2, Mn2AlB2, and Fe2AlB2, and the binary boride, CrB, were calculated from in situ high-temperature powder X-ray diffraction patterns. The order—from low to high—of the average linear thermal expansions was MoAlB (7.3 × 10−6 K−1), CrB (8.6 × 10−6 K−1), Fe2AlB2 (10.3 × 10−6 K−1), Cr2AlB2 (10.5 × 10−6 K−1), and Mn2AlB2 (14.0 × 10−6 K−1). Despite their structural and chemical similarities, the CTE anisotropies of these phases were different and could be grouped into two groups. In the first group, MoAlB and Fe2AlB2, the highest CTE values were along the stacking direction and the lowest were parallel to the B–B chains. In the second group, Cr2AlB2, Mn2AlB2, and CrB, the lowest CTE was along the stacking direction and the highest was normal to the chains. The thermal expansions parallel to the B–B chains were significantly lower (32% to 103%) than those perpendicular to the chains in all the ternaries except for MoAlB. In general, the relative CTEs parallel to the B–B chains in both the M2AlB2 ternaries and corresponding binary monoborides suggest that strong covalent character of the B–B bonds are at least partially responsible for the large thermal expansion anisotropies observed. [ABSTRACT FROM AUTHOR]

Orthogonal strip detectors are single-carrier devices that allow reduction of the amount of read-out channels while obtaining a good spatial resolution. They are especially interesting for low count rate imaging applications like gamma-ray medical imaging. Nevertheless, applying a bias voltage between X and Y strips is necessary in order to localize correctly the interactions. Unfortunately, this bias voltage induces a leakage current, thus generating an unwanted shot noise. We propose in this paper a new setup using capacitive contacts, which suppresses leakage currents between collecting and non-collecting strips. In this paper, we first briefly present the state-of-the-art relative to coplanar grid and strip detectors, their advantages and their drawbacks. In a second part, we will focus on the theoretical background of the capacitive contact. In a third part, a simulation study of the electrode geometry is reported. Thanks to the spectral approach, we have computed the 3D applied and weighting electric fields, bulk resistances and capacitances for various geometries of the design. Afterwards, we have used these data to simulate pulse waveforms and study the associated electronics. Finally the fabrication of the devices is presented and our first experimental results are reported. As a conclusion, we will discuss the interests and the limitations of this kind of device. Index Terms--Capacitive contact, CdZnTe, coplanar grid detector, gamma-ray detectors, SPECT imager, strip detector.

A new cadmium zinc telluride (CZT) portable spectrometric system called OMEGA has been developed at CEA/LETI for high-energy applications like spent fuel assay in response to needs in the nuclear industry. The unit has beendesigned to operate at room temperature under water. The CZT OMEGA system is based on a single 10 x 10 x 10 [mm.sup.3] CZT detector associated with a single electronics channel. To improve performance, the system incorporates several technologies including: an electrode design named the 'Mixed' electrode which combines a noncontacting Frisch grid and the pixel field effect to obtain maximum sensitivity from the 10 x 10 x 10 [mm.sup.3] CZT detector, a single hybrid preamplifier designed to reduce noise, an electronics board configured to enhance count rate capabilities and finally, electron bi-parametric (BP) correction (pulse height corrected via measurement of rise time) to improve both energy resolution and detection efficiency. This paper reports the most recent results obtained with the CZT OMEGA spectrometric system at high energy ([sup.137]Cs and [sup.152]Eu sources) at both low and high count rate: spectrometric performance shows an energy resolution of 1% FWHM at 662 keV (0.8% at 1408 keV) from 1 kcps to 50 kcps, deteriorating to 2% FWHM at 200 kcps. Index Terms--[gamma]-ray detectors, bi-parametric correction, cadmium zinc telluride (CZT), charge sensitive preamplifier, count rate, energy resolution, noncontacting Frisch grid.

We present techniques and systems for a CZT gamma camera head based on an array of discrete CZT detectors and associated front-end electronics. In Section I, we describe the performance of a small 256 detector (16 x 16) CZT imager in terms of homogeneity, energy resolution, and detection efficiency. The array is composed of groups of 16 single detectors (4 x 4 x 6 [mm.sup.3] each), each group served by a 16-channel multiplexed ASIC preamplifier. The electron bi-parametric (BP) spectrum with the depth of interaction information is used to improve detection efficiency. The present work compares the performance (sensitivity, spatial resolution, and contrast) of this small prototype imager and an Anger NaI(TI) scintillation gamma camera using phantom images with small cold zones in a high-scatter environment. In Section II, we present an approach to the next generation of this technology, a gamma camera with a segmented monolithic CZT detector associated with the electron BP spectrum, which also allows correction for spectral tailing and dramatically improves both detection efficiency and energy resolution. Results with and without BP correction are given for a 10 x 10 x 5 [mm.sup.3] CZT monolithic detector (16 pixels, 2 x 2 mm pixel, and 2.5-mm pitch) associated with 16 individual ASIC preamplifiers: an energy resolution close to 2% FWHM at 122 keV has been obtained on 16 pixels after BP correction. Index Terms--ASIC, bi-parametric (BP) correction, CZT, depth of interaction (DOI), efficiency, energy resolution, solid-state gamma camera.