M. W. Rabin

We have measured the resistivity tensor in one nearly twin-free crystal and one twin-free crystal of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$. The first crystal was grown with a large twin-free region. We removed the twins in the second crystal by applying a uniaxial stress. Using a modified Montgomery technique, we measured the electrical resistivities in all three crystal directions and found them to be linear in temperature above the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$. Our results for ${\mathrm{\ensuremath{\rho}}}_{\mathit{a}}$ and ${\mathrm{\ensuremath{\rho}}}_{\mathit{b}}$ at room temperature are among the lowest that have been reported in the literature, indicating that our samples are of high quality. The ratio ${\mathrm{\ensuremath{\rho}}}_{\mathit{a}}$/${\mathrm{\ensuremath{\rho}}}_{\mathit{b}}$ is independent of temperature between 150 and 275 K, and its value, 2.2\ifmmode\pm\else\textpm\fi{}0.2, indicates that the Cu-O chains contribute 60% of the current when the electric field is parallel to them, with the rest of the current being contributed by the Cu-O planes. This result, at zero frequency, is in good agreement with the recent conductivity at infrared frequencies obtained by Schlesinger et al.

Improvements in superconductor device fabrication, detector hybridization techniques, and superconducting quantum interference device readout have made square-centimeter-sized arrays of gamma-ray microcalorimeters, based on transition-edge sensors (TESs), possible. At these collecting areas, gamma microcalorimeters can utilize their unprecedented energy resolution to perform spectroscopy in a number of applications that are limited by closely-spaced spectral peaks, for example, the nondestructive analysis of nuclear materials. We have built a 256 pixel spectrometer with an average full-width-at-half-maximum energy resolution of 53 eV at 97 keV, a useable dynamic range above 400 keV, and a collecting area of 5 cm(2). We have demonstrated multiplexed readout of the full 256 pixel array with 236 of the pixels (91%) giving spectroscopic data. This is the largest multiplexed array of TES microcalorimeters to date. This paper will review the spectrometer, highlighting the instrument design, detector fabrication, readout, operation of the instrument, and data processing. Further, we describe the characterization and performance of the newest 256 pixel array.

The authors present a prototype for a high-energy-resolution, high-count-rate, gamma-ray spectrometer intended for nuclear forensics and international nuclear safeguards. The prototype spectrometer is an array of 14 transition-edge-sensor microcalorimeters with an average energy resolution of 47eV (full width at half maximum) at 103keV. The resolution of the best pixel is 25eV. A cryogenic, time-division multiplexer reads out the array. Several important topics related to microcalorimeter arrays are discussed, including cross-talk, the uniformity of detector bias conditions, fabrication of the arrays, and the multiplexed readout. The measurements and calculations demonstrate that a kilopixel array of high-resolution microcalorimeters is feasible.

Non-Gaussian flux-flow noise was measured in the ``peak-effect'' regime of NbS${\mathrm{e}}_{2}$. The form of the second spectra showed that neither flux bundles with Poisson statistics nor simple flow channels switching on and off could account for the noise. More complicated persistent flow patterns were needed. These metastable flow states were shown to correspond to different pinned configurations.

We discuss recent developments in using cryogenic microcalorimeter detectors for x- and gamma-ray spectroscopy. We are currently operating a detector array consisting of thirteen pixels with time-domain multiplexed readout. With a single pixel from this detector, we have measured 97.43-keV gamma rays from 153-Gd with 22-eV resolution (FWHM). We have also made the first multiplexed array measurements of plutonium x- and gamma-rays with 45-eV resolution. We are currently testing a 66-pixel next-generation detector chip. Preliminary measurements with the new detector indicate improved energy linearity and single-pixel energy resolution of 50-100 eV at 100 keV. We present preliminary calibration data from this chip, and a high-statistics multiplexed 21-pixel spectrum of the Pu x-ray region between 90 and 130 keV.