Barry L. Zink

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.

Thermal conductivity of micromachined low-stress silicon-nitride beams from 77 to 325 K

We present thermal conductivity measurements of micromachined 500 nm thick silicon-nitride (Si–N) beams suspended between two Si–N islands, in the temperature range from 77 to 325 K. The measured thermal conductivity, k, of Si–N at high temperatures is in good agreement with previously measured values for Si–N grown by low-pressure chemical vapor deposition, but behaves much differently as temperature is lowered, showing a dependence more similar to polycrystalline materials. Preliminary structural characterization by x-ray diffraction suggests that the material is likely nano- or polycrystalline. The micromachined suspended platform structure is designed to allow highly accurate measurements of the thermal conductivity of deposited metallic, semiconducting, or insulating thin films. As a demonstration, we present measurements of a 200 nm thick sputtered molybdenum film. In the entire temperature range the measured thermal conductivity matches the prediction of the Wiedemann–Franz thermal conductivity det...

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.

Large n- and p-type thermoelectric power factors from doped semiconducting single-walled carbon nanotube thin films

Lightweight, robust, and flexible single-walled carbon nanotube (SWCNT) materials can be processed inexpensively using solution-based techniques, similar to other organic semiconductors. In contrast to many semiconducting polymers, semiconducting SWCNTs (s-SWCNTs) represent unique one-dimensional organic semiconductors with chemical and physical properties that facilitate equivalent transport of electrons and holes. These factors have driven increasing attention to employing s-SWCNTs for electronic and energy harvesting applications, including thermoelectric (TE) generators. Here we demonstrate a combination of ink chemistry, solid-state polymer removal, and charge-transfer doping strategies that enable unprecedented n-type and p-type TE power factors, in the range of 700 μW m−1 K−2 at 298 K for the same solution-processed highly enriched thin films containing 100% s-SWCNTs. We also demonstrate that the thermal conductivity appears to decrease with decreasing s-SWCNT diameter, leading to a peak material zT ≈ 0.12 for s-SWCNTs with diameters in the range of 1.0 nm. Our results indicate that the TE performance of s-SWCNT-only material systems is approaching that of traditional inorganic semiconductors, paving the way for these materials to be used as the primary components for efficient, all-organic TE generators.

Long-distance spin transport in a disordered magnetic insulator

A demonstration of long-distance spin transport through an amorphous magnetic insulator shows that magnetic order is not required, and may not even be desirable, in materials for magnonic and spintronic applications.

Dependence of Excess Noise on the Partial Derivatives of Resistance in Superconducting Transition Edge Sensors

We have measured the relation between sensor noise and the steepness (α = TR⋅ and β = IR⋅dRdI) of the superconducting transition for various superconducting transition edge sensors. The measurements are performed on square Mo/Cu bilayer thermistors of three sizes (125 μm, 250 μm and 400 μm), which are fabricated for X‐ray detection. We have identified the devices in two regimes: a) α>100 and b) α<100. The devices with α smaller than 100 strictly follow non‐linear bolometer and micro‐calorimeter noise theory while in most devices with α greater than 100, we see an unexplained noise contribution.

Time-Division SQUID Multiplexers With Reduced Sensitivity to External Magnetic Fields

Time-division SQUID multiplexers are used in many applications that require exquisite control of systematic error. One potential source of systematic error is the pickup of external magnetic fields in the multiplexer. We present measurements of the field sensitivity figure of merit, effective area, for both the first stage and second stage SQUID amplifiers in three NIST SQUID multiplexer designs. These designs include a new variety with improved gradiometry that significantly reduces the effective area of both the first and second stage SQUID amplifiers.

Application of calorimetry on a chip to high-pressure materials

Silicon micromachined calorimeters (“calorimeter on a chip”) are used to measure heat capacities and phase transition enthalpies for thin film, single crystal, and powder samples (5–500 μg). The technology is thus compatible with the small samples produced in multianvil and large diamond anvil cells. Techniques for handling small samples and attaching them to the calorimetric devices have been developed. Initial data illustrate application to CoO and to Fe2SiO4 olivine and spinel, a quenched high pressure phase metastable at ambient conditions. The calorimetric entropy of the olivine - spinel transition in Fe2SiO4 (−16 ± 5 J/mol·K) is in good agreement with that calculated from phase equilibrium data (−14 ± 3 J/mol·K). A magnetic transition in iron silicate spinel, detected previously by Mossbauer spectroscopy, is seen in the calorimetric signal.

Identification and elimination of anomalous thermal decay in gamma-ray microcalorimeters

Microcalorimeter detectors rely on superconducting components and cryogenic temperatures to provide over an order-of-magnitude improvement in energy resolution compared to semiconducting sensors. Resolution improvements impact fields from gamma-ray astrophysics to nuclear safeguards. The temporal response of these detectors has been much slower than predicted from the known device parameters. This discrepancy has been attributed to the dynamics of quasiparticles and phonons in the bulk absorber used for absorbing photons. We will show that long-lived states in the glue used for absorber attachment have been the dominant cause of the slow response. Also, we have fabricated microcalorimeters using metal-to-metal diffusion bonding to attach the absorber. These detectors show a significant improvement in their recovery after gamma-ray events and will now enable study of the internal scattering dynamics of the bulk absorber.

Microcalorimeter Nuclear Spectrometers

We present results from the initial testing of arrays of cryogenic microcalorimeter gamma-ray detectors. The successful fabrication and multiplexed operation of these arrays allow us to make pixel-to-pixel performance comparisons. The relationship between operating conditions and array performance is investigated. Advanced refrigerators for these detectors work without liquid cryogens, achieve temperatures below 100 mK, and operate continuously for many days.