The thermoelectric inhomogeneity of wires is one of the main components of the measurement uncertainty when using thermocouples. During calibration, it is therefore important to determine how much the inhomogeneities affect the measurement result. Thermoelectric inhomogeneity is normally assessed by gradual, or stepwise, insertion of a thermocouple into a furnace or liquid bath. With this type of equipment, the length that can be scanned is typically limited to about half a meter. To assess thermoelectric inhomogeneity over greater lengths, it is necessary to adopt a different technique. Therefore, an apparatus with a short, movable heating zone has been set up and evaluated. The apparatus produces a short, well-defined heating zone that is moved along the thermocouple while both the measuring and reference junctions are kept at 0 • C. Heating is done by means of a hot-air fan that produces a temperature-controlled heating zone up to 700 • C. Two directionally controllable cold-air fans, one on each side of the heating zone, make it possible to vary the slopes of both temperature gradients of the heating zone. Two temperature gradients influence the thermocouple when performing measurements of inhomogeneity with this setup. The results are, therefore, not directly comparable to the results from measurements taken in a bath or furnace, where only one temperature gradient is present. The resulting curve obtained with the two gradients is approximately equivalent to the derivative of the curve obtained with one gradient. It is possible to convert the two-gradient curve to a single-gradient curve by numerical integration, as shown in this work. Comparisons with inhomogeneity measurements obtained using salt baths show good agreement with the calculated results.123 916 Int J Thermophys (2008) 29:915-925
The longevity of high gain GaAs photoconductive semiconductor switches (PCSS) has been extended to well over 10 million pulses by reducing the density of carriers at the semiconductp to metal interface. This was achieved by reducing the density in the vertical and lateral directions. The first was achieved by varying the spatial distribution of the trigger light thereby widening the current filaments that are characteristic of the high gain switches. We reduced the carrier density in the vertical direction by using ion implantation. These results were obtained for currents of about 10 A, current duration of 3.5 ns, and switched voltage of -2 kV. At currents of -70 A, the switches last for 0.6 million pulses. In order to improve the performance at high currents new processes such as deep diffusion and epitaxial growth of contacts are being pursued. To guide this effort we measured a carrier density of 6 x 10 l8 electrons (or holes)/ cm in filaments that carry a current of 5 A.
NUCAM3 is the latest generation of solid-state pixelated gamma cameras developed at Soreq NRC. The NUCAM3 head is based on segmented pad monolithic CdZnTe detectors that currently provide a useful field of view of 18.5 cm 20.1 cm. The camera is designed for cardiac SPECT, breast scintimammography, thyroid, and other small organ evaluation. The camera contains 528 detectors. Each detector is 5 mm thick and is divided to a matrix of 16 square pixels, with a pitch of 2.1 mm. The use of pixelated CdZnTe detectors and low-noise electronics provides a camera with an average energy resolution of 4.5% full width half maximum (FWHM) at 140.5 keV and 9.5% FWHM at 59.6 keV and an intrinsic spatial resolution of 2.1 mm, regardless of the photon energy. We present the analysis of over 1300 CdZnTe monolithic detectors, the physical and imaging characteristics of the NUCAM3 camera and their comparison to state of the art Anger cameras. We show the advantages of CdZnTe technology, which are due to the camera pixel structure and superior energy resolution. These advantages lead to better detectability of small size cold and hot lesions in a scatter environment.
A numerical model for optically triggered switching in bulk GaAs is presented. First, a one-dimensional model is described and calculated behavior compared to experimental observations. Results from the one-dimensional model are not consistent with observed switching behavior. The model is then modified to include filament formation. Results from the modified model agree qualitatively and quantitatively with experimental data. Details of the dynamic behavior of the device are shown and a unified picture of the switching phenomenon presented. On the basis of the agreement of the numerical model and experimental observation it is concluded that switching is a result of localized impact ionization creating a conductive filament channel through the bulk material.
High gain GaAs photoconductive semiconductor switches (PCSS) are being used in a variety of electrical and optical short pulse applications. The highest power application, which we are developing, is a compact, repetitive, short pulse linear induction accelerator. The array of PCSS, which drive the accelerator, will switch 75 kA and 250 kV in 30 ns long pulses at 50 Hz. The accelerator will produce a 700 kV, 7kA electron beam for industrial and military applications. In the low power regime, these switches are being used to switch 400 A and 5 kV to drive laser diode arrays which produce 100 ps optical pulses. These short optical pulses are for military and commercial applications in optical and electrical range sensing, 3D laser radar, and high speed imaging. Both types of these applications demand a better understanding of the switch properties to increase switch lifetime, reduce jitter, optimize optical triggering, and improve overall switch performance. These applications and experiments on the fundamental behavior of high gain GaAs switches will be discussed. Open shutter, infra-red images and time-resolved Schlieren images of the current filaments, which form during high gain switching, will be presented. Results from optical triggering experiments to produce multiple, diffuse filaments for high current repetitive switching will be described.
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