Robert D. Horansky

14-pixel, multiplexed array of gamma-ray microcalorimeters with 47eV energy resolution at 103keV

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.

A high resolution gamma-ray spectrometer based on superconducting microcalorimeters

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.

Array-compatible transition-edge sensor microcalorimeter γ-ray detector with 42eV energy resolution at 103keV

The authors describe a microcalorimeter γ-ray detector with measured energy resolution of 42eV full width at half maximum for 103keV photons. This detector consists of a thermally isolated superconducting transition-edge thermometer and a superconducting bulk tin photon absorber. The absorber is attached with a technique compatible with producing arrays of high-resolution γ-ray detectors. The results of a detailed characterization of the detector, which includes measurements of the complex impedance, detector noise, and time-domain pulse response, suggest that a deeper understanding and optimization of the thermal transport between the absorber and thermometer could significantly improve the energy resolution of future detectors.

The dielectric response of chloromethylsilyl and dichloromethylsilyl dipolar rotors on fused silica surfaces

We have measured the dielectric response of monolayer films of surface mounted chloromethyl- and dichloromethylsilyl dipolar rotors on fused silica at frequencies in the 1 kHz range and temperatures from 4 to 300 K. The torsional potentials, calculated from molecular mechanics, show an asymmetrical three-fold barrier to rotation with a barrier height sufficient to hinder motion of the rotor at experimental temperatures. A broad distribution of barrier heights is observed experimentally, consistent with calculated results showing that the intrinsic barrier of the rotor is modified by interactions with the underlying substrate. For a series of samples with differing concentrations of the rotor, the observed signal strength varies in proportion to the rotor coverage measured by Auger spectroscopy; however, the absolute strength of the signal is about three times larger than expected.

High-resolution gamma-ray spectroscopy with a microwave-multiplexed transition-edge sensor array

We demonstrate very high resolution photon spectroscopy with a microwave-multiplexed two-pixel transition-edge sensor (TES) array. We measured a 153Gd photon source and achieved an energy resolution of 63 eV full-width-at-half-maximum at 97 keV and an equivalent readout system noise of 86 pA/Hz at the TES. The readout circuit consists of superconducting microwave resonators coupled to radio-frequency superconducting-quantum-interference-devices and transduces changes in input current to changes in phase of a microwave signal. We use flux-ramp modulation to linearize the response and evade low-frequency noise. This demonstration establishes one path for the readout of cryogenic X-ray and gamma-ray sensor arrays with more than 103 elements and spectral resolving powers R=λ/Δλ>103.

A Near-Infrared 64-pixel Superconducting Nanowire Single Photon Detector Array with Integrated Multiplexed Readout

We demonstrate a 64-pixel free-space-coupled array of superconducting nanowire single photon detectors optimized for high detection efficiency in the near-infrared range. An integrated, readily scalable, multiplexed readout scheme is employed to reduce the number of readout lines to 16. The cryogenic, optical, and electronic packaging to read out the array, as well as characterization measurements are discussed.

High-efficiency superconducting nanowire single-photon detectors fabricated from MoSi thin-films

We report on MoSi SNSPDs which achieved high system detection efficiency (87.1 ± 0.5% at 1542 nm) at 0.7 K and we demonstrate that these detectors can also be operated with saturated internal efficiency at a temperature of 2.3 K in a Gifford-McMahon cryocooler. We measured a minimum system jitter of 76 ps, maximum count rate approaching 10 MHz, and polarization dependence as low as 3.3 ± 0.1%. The performance of MoSi SNSPDs at 2.3 K is similar to the performance of WSi SNSPDs at < 1 K. The higher operating temperature of MoSi SNSPDs makes these devices promising for widespread use due to the simpler and less expensive cryogenics required for their operation.

Superconducting calorimetric alpha particle sensors for nuclear nonproliferation applications

Identification of trace nuclear materials is usually accomplished by alpha spectrometry. Current detectors cannot distinguish critical elements and isotopes. We have developed a detector called a microcalorimeter, which achieves a resolution of 1.06 keV for 5.3 MeV alphas, the highest resolving power of any energy dispersive measurement. With this exquisite resolution, we can unambiguously identify the P240u/P239u ratio in Pu, a critical measurement for ascertaining the intended use of nuclear material. Furthermore, we have made a direct measurement of the P209o ground state decay.

High-efficiency WSi superconducting nanowire single-photon detectors operating at 2.5 K

We investigate the operation of WSi superconducting nanowire single-photon detectors (SNSPDs) at 2.5 K, a temperature which is ∼70% of the superconducting transition temperature (TC) of 3.4 K. We demonstrate saturation of the system detection efficiency at 78 ± 2% at a wavelength of 1310 nm, with a jitter of 191 ps. We find that the jitter at 2.5 K is limited by the noise of the readout and can be improved through the use of cryogenic amplifiers. Operation of SNSPDs with high efficiency at temperatures very close to TC appears to be a unique property of amorphous WSi.

Large-Area Microcalorimeter Detectors for Ultra-High-Resolution X-Ray and Gamma-Ray Spectroscopy

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.