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Synchrotron X-ray fluorescence (SXRF) and X-ray absorption spectroscopy (XAS) have become standard tools to measure element concentration, distribution at micrometer- to nanometer-scale, and speciation (e.g., nature of host phase: oxidation state) in inhomogeneous geomaterials. The new Maia X-ray detector system provides a quantum leap for the method in terms of data acquisition rate. It is now possible to rapidly collect fully quantitative maps of the distribution of major and trace elements at micrometer spatial resolution over areas as large as 1 x 5 [cm.sup.2]. Fast data acquisition rates also open the way to X-ray absorption near-edge structure (XANES) imaging, in which spectroscopic information is available at each pixel in the map. These capabilities are critical for studying inhomogeneous Earth materials. Using a 96-element prototype Maia detector, we imaged thin sections of an oxidized pisolitic regolith (2 x 4.5 [mm.sup.2] at 2.5 x 2.5 [micro][m.sup.2] pixel size) and a metamorphosed, sedimentary exhalative Mn-Fe ore (3.3 x 4 [mm.sup.2] at 1.25 x 5 [micro][m.sup.2]). In both cases, As K-edge XANES imaging reveals localized occurrence of reduced As in parts of these oxidized samples, which would have been difficult to recognize using traditional approaches. Keywords: Arsenic, oxidation state, XANES, element distribution, imaging, X-ray fluorescence DOI: 10.2138/am.2010.3469

Commercial 15 x 15 x 7.5 [mm.sup.3] coplanar grid CdZnTe detectors were studied on the micron-scale using a collimated high-energy X-ray beam provided by Brookhaven's National Synchrotron Light Source. This powerful tool enables simultaneous studies of detector response uniformity, electronic properties of the material, and effects related to the device's contact pattern and electric field distribution. The availability of a front-end Application Specific Integrated Circuit, developed at Brookhaven's Instrumentation Division, providing low noise amplification of grids and cathode signals, corresponding timing signal and adjustable relative gain, allowed to correlate performance mapping and fluctuations in collected charge. We observed the effect of the strip contacts comprising the coplanar grids on the energy resolution of the coplanar-grid device.

A new detector for EXAFS experiments is being developed. It is based on a multi-element Si sensor and dedicated readout application specific integrated circuit (ASIC). The sensor is composed of 384 pixels, each having 1 [mm.sup.2] area, arranged in four quadrants of 12 x 8 elements and it is wire-bonded to 32-channel ASICs. Each channel implements low-noise preamplification with self-adaptive continuous reset, high-order shaper, bandgap referenced baseline stabilizer, one threshold comparator, and two digital-analog converter (DAC) adjustable window comparators, each followed by a 24-bit counter. Fabricated in 0.35 [micro]m CMOS, the ASIC dissipates about 8 mW per channel. First measurements show at room temperature a resolution of 14 [e.sup.-] rms without the detector and 40 [e.sup.-] rms (340 eV) with the detector connected and biased. Cooling to -35 C a full width at half maximum (FWHM) of 205 eV (167 eV from electronics) was measured at the Mn-K[alpha] line. A resolution of about 300 eV was measured for rates approaching 100 kc/s per channel, corresponding to an overall rate in excess of 10 Mc/s/[cm.sup.2]. Channel-to channel threshold dispersion after DAC adjustment 2.5 was [e.sup.-] root mean square. Index Terms--EXAFS, readout ASIC, SI pixel sensor.

The concept of X-ray active matrix pixel sensor (XAMPS) is introduced. XAMPSs are direct illumination, position sensitive X-ray detectors with the possibility of containing 1 000 000 pixels. They count the number of diffracted X-rays in each pixel by measuring the total charge released by converted X-rays in the body of the sensor. Readout is accomplished with a relatively small number of channels equal to the square root of the number of pixels. The estimated readout time can be about 1 ms. Noise of the readout electronics can be so low that practically no additional fluctuations in the number of incident X-rays per pixel are added and, therefore, the XAMPS performance is very close to that of an ideal detector for X-ray crystallography. Index Terms--Active pixel sensors, protein crystallography, silicon radiation detectors, X-ray imaging.

This work aims to study the feasibility of the integration, on the same chip, of GaAs pixel detectors and front-end electronics employing GaAs metal semiconductor FET's (MESFET's) or high electron mobility transistors (HEMT's). The interest of fully integrated GaAs systems lies in X and [Gamma]-ray spectroscopy and imaging for scientific, industrial, and medical applications. The system design criteria and the prediction of the performance have been derived on the basis of recent experimental results on semi-insulating GaAs pixel detectors. Measurements of the relevant parameters of GaAs FET's suitable for the stringent requirements of a spectroscopy-grade front-end amplifier are analyzed. It is shown that an optimized GaAs integrated system can reach an electronic noise level below 100 electrons rms (< 1 keV FWHM) even at room temperature. Some open problems regarding the detector-electronics integration are highlighted and discussed. Index Terms - Gamma-ray spectroscopy detectors, MESFET, MODFET, semiconductor radiation detectors, X-ray detectors.

The traditional method for electron lifetime measurements of CdZnTe (CZT) detectors relies on using the Hecht equation. The procedure involves measuring the dependence of the detector response on the applied bias to evaluate the μτ product, which in turn can be converted into the carrier lifetime. Despite general acceptance of this technique, which is very convenient for comparative testing of different CZT materials, the assumption of a constant electric field inside a detector is unjustified. In the Hecht equation, this assumption means that the drift time would be a linear function of the distance. This condition is not fulfilled in practice at low applied biases, where the Hecht equation is most sensitive to the μs product. As a result, researchers usually take measurements at relatively high biases, which work well in the case of the low μτ-product material, <10-3 cm²/V, but give significantly underestimated values for the case of high μτ-product crystals. In this work, we applied the drift-time method to measure the electron lifetimes in long-drift-length (4 cm) standard-grade CZT detectors produced by the Redlen Technologies. We found that the electron μτ product of tested crystals is in the range 0.1-0.2 cm²/V, which is an order of the magnitude higher than any value previously reported for a CZT material. In comparison, using the Hecht equation fitting, we obtained μτ=2.3 × 10-2 cm²/V for a 2-mm thin planar detector fabricated from the same CZT material. [ABSTRACT FROM AUTHOR]

The future high-energy physics experiments, based on the new high-luminosity accelerators, will require a new generation of front-end monolithic electronics characterized, in particular, by high speed and low-power dissipation. In this perspective, the performances of Si and GaAs field effect transistors (FET's) are compared here in conditions of low-power dissipation. The advantages of solutions based on GaAs FET's, in applications requiring fast shaping times, are presented and experimental results are reported. The criteria for the optimum choice of the input transistor dimension and of its bias point are discussed. Index Terms - Charge preamplifier, GaAs, low power.

As a result of a cooperation between Brookhaven National Laboratory and eV Products a generation of high performance readout ASICs was developed. The ASICs, realized in CMOS 0.5 [micro]m technology, are available in several different versions, single or multi-channel and with unipolar or bipolar shaper, in view of their use in research, spectroscopy, medical, safeguard and industrial applications. Four innovative circuit solutions are implemented in the ASICs: (i) a high linearity compensated continuous reset system which self-adapts to a wide range of detector leakage currents with minimum noise contribution; (ii) a high order shaper realized by using the follow-the-leader feedback (FLF) approach; (iii) a class AB rail-to-rail output stage with high driving capability; (iv) a baseline holder (BLH) for the baseline stabilization. The basic structure of the ASICs, a description of the circuit solutions and the preliminary experimental results are reported.

As a result of a cooperation between Brookhaven National Laboratory and eV Products a generation of high performance readout ASICs was developed. One of the novel circuit solutions implemented in the ASICs is a fully compensated continuous reset system capable of handling detector leakage currents up to several tens of nA with high linearity and with noise contribution always lower than the shot noise contribution of the detector. The reset is based on the use of a MOSFET operating above threshold and in saturation. The full compensation is obtained by using a suitable replica of the reset MOSFET. The experimental results are reported. Evaluation and measurement of the noise contribution and of the dependence of the gain on the leakage current are also discussed.

Position-sensitive virtual Frisch-grid (VFG) CdZnTe (CZT) detectors offer a unique capability for correcting the response nonuniformities caused by crystal defects. This allowed us to achieve high energy resolution, while using typical-grade commercial CZT crystals with relaxed requirements to their quality, thus reducing the overall cost of detectors. Another advantage of the VFG detectors is that they can be integrated into arrays and used in small compact hand-held instruments or large-area gamma cameras that will enhance detection capability for many practical applications, including nonproliferation, medical imaging, and gamma-ray astronomy. Here, we present the results from testing small array prototypes coupled with front-end application-specified integrated circuit. Each detector in the array is furnished with 5-mm-wide charge-sensing pads placed near the anode. The pads signals are converted into XY coordinates, which combined with the cathode signals (for Z coordinates) provide 3-D position information of all interaction points. The basic array consists of a number of detectors grouped into $2\times 2$ subarrays, each having a common cathode made by connecting together the cathodes of the individual detectors. These features can significantly improve the performance of detectors while using typical-grade low-cost CZT crystals to reduce the overall cost of the proposed instrument. [ABSTRACT FROM PUBLISHER]

We investigated the feasibility of long-drift-time CdZnTe (CZT) gamma-ray detectors, fabricated from CZT material produced by Redlen Technologies. CZT crystals with cross-section areas of 5 × 5mm² and 6 × 6mm² and thicknesses of 20-, 30-, 40-, and 50-mm were configured as 3D position-sensitive drift detectors and were read out using a front-end ASIC. By correcting the electron charge losses caused by defects in the crystals, we demonstrated high performance for relatively thick detectors fabricated from unselected CZT material. [ABSTRACT FROM AUTHOR]

The longer electron lifetime of today’s CdZnTe (CZT) crystals allows for free carriers to travel longer distances in the crystals, which means that, in principle, thicker devices could be fabricated. These thicker CZT devices would offer greater detection efficiency for high-energy gamma-ray detectors. However, up to now, the thicknesses and sizes of actual detectors have still been limited by the nonuniform detector response, and the biggest devices reported in the literature are (20\times 20\times 15)\-mm^3 pixelated detectors with a drift distance of 15 mm. Although thicker and bigger single crystals are becoming available today, the high requirements on their crystal quality drastically reduce their acceptance yield and increase their cost. Fortunately, in many cases, the inhomogeneity in response can be corrected by segmenting the active volumes of the detectors and correcting the responses generated from each of the voxels. Such high-granularity position-sensitive detectors open up the opportunity for using thicker and less-expensive CZT crystals. The goal of this work is to demonstrate that today’s commercial high electron mobility-lifetime CZT material is suitable for a new class of detectors with 20-25-mm drift distances and even larger in the near future, provided that the detectors’ response nonuniformities can be corrected on a scale comparable to or larger than the sizes of the electron clouds, which is \sim 100~\mu\m. [ABSTRACT FROM AUTHOR]

We have constructed a pixelated germanium detector using a technique which has been shown to provide good isolation between adjacent pixels. In this work we present initial tests of the application of a low-noise CMOS ASIC to read out this detector. The detector has 64 pixels, each 0.5~\mm\times 5~\mm, arranged as a series of strips. It is connected by wire-bonds to two 32-channel application-specific integrated circuits (ASICs) which provide a complete photon-counting chain for every channel. Since the size of the pixel array is no longer restricted by the difficulties of instrumenting large channel-count conventional electronics, this development will open up the possibility of even larger arrays, similar to those offered by silicon detectors. [ABSTRACT FROM PUBLISHER]