C. R. Rudy

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.

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.

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.

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.

High Resolution Alpha Particle Spectroscopy with Cryogenic Microcalorimeters

We present measurements from a novel cryogenic microcalorimeter designed to detect alpha particles. We demonstrate a spectral resolution of 4.4 keV FWHM for 5.3 MeV alpha particles from a 210Po source. In addition, we present an alpha spectrum from a mixture of Pu isotopes demonstrating the feasibility of resolving the individual contributions of 239Pu and 240Pu. The unmatched resolution of microcalorimeter alpha detectors will provide new capabilities for actinide analysis. We discuss potential applications and also sensor design.

Improved Isotopic Analysis With a Large Array of Gamma-Ray Microcalorimeters

We present results from the largest array of gamma-ray microcalorimeters operated to date. The microcalorimeters consist of Mo/Cu transition-edge sensors with attached Sn absorbers. The detector array contains 66 pixels each with an active area 2.25 mm2. Out of 66 pixels, 55 are active, and 31 were used to acquire a high statistics Pu gamma-ray spectrum. The energy resolution of the best 21 pixels was found to be 120 eV FWHM. The array is read out using time domain SQUID multiplexing. Here, we describe the multiplexing and present a high statistics Pu spectra. Because of the large collecting area of our array, the statistical error in the 240Pu line intensity is approximately 0.7%, which is comparable to the systematic error in a measurement with a 500 eV germanium sensor. Hence, we have reached an important threshold for demonstrating improved isotopic measurements with microcalorimeter sensors. With straightforward changes, we plan to achieve a resolution of about 50 eV FWHM with 256 multiplexed detectors. Finally, we present early estimates of on-chip heating within our sensor array.

Correlative assay of uranium with calorimetry, neutron counting, and mass spectrometry data

Abstract A methodology has been developed for assaying unirradiated enriched uranium that uses calorimetry, passive neutron counting, and historical mass spectrometry data. Calorimetry can be used to measure the thermal power of bulk uranium enriched in the isotope 235 U , and neutron counting can be used to determine its spontaneous-fission neutron emission rate. The thermal power and neutron-emission properties of uranium change in a regular way with increasing 235 U enrichment that can be quantified using mass spectrometry data. The measured ratio of the thermal power and spontaneous neutron fission rate can be used to determine the 235 U enrichment and the total mass of the 234 U , 235 U , and 238 U isotopes.

Simulating the response of ultra-high energy resolution x- and gamma-ray microcalorimeter detectors

Microcalorimeter detectors based on transition-edge sensors coupled to a bulk absorber are an emerging technology for hard X-ray and soft gamma-ray measurements. Monte Carlo simulations codes, like GEANT4, can be a valuable tool for evaluating detector design ideas, interpreting measured data, and predicting detector performance. We report on initial attempts to reproduce measured microcalorimeter data with GEANT4 simulations.

Monte Carlo Studies of High Resolution Microcalorimeter Detectors

World record energy resolution (DeltaE ~ 25 eV) has recently been demonstrated for soft gamma-ray detectors using small radiation absorbing elements coupled to transition-edge sensors (microcalorimeters). Initial results were obtained with a 1 mm wide by 250 mum thick tin absorber. We are proceeding with construction of arrays of sensors and exploring other absorber materials. Monte Carlo tools, e.g. - GEANT4, are useful for exploring the design space and performance expectations for these arrays of microcalorimeters. In this work, we use simulations to quantify some important characteristics of this emerging technology in the hard X- and gamma-ray regime, such as the effective area and effect of absorber material and thickness. We also explore, through simulations, how these detectors might be used in some interesting applications, like passive assay of spent nuclear fuel from X-ray fluorescence.

Steady-State Heat Transfer Modeling of a Calorimeter Measurement Chamber

The steady-state heat transfer in a calorimeter measurement chamber is modeled assuming that the sample is a generic nuclear material container filled with plutonium oxide powder. The measurement chamber is composed of nine solid materials and air. The heat transfer model includes natural convection in the plutonium oxide porous matrix, natural convection in the air space on top of the plutonium oxide powder, and heat conduction in the different solid parts of the measurement chamber and the heat flux sensor. The problem is treated as a conjugate heat transfer problem defined by a system of 24 partial differential equations coupled at the interfaces of the materials that form the measurement chamber. A computational fluid dynamics software is used to generate the grid and obtain the solution. The simulation results provide estimates of operational temperatures and heat losses that are of interest in the design of calorimeters.Copyright