A. S. Hoover

Los Alamos National Laboratory, Лос-Аламос, California, United States

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Publications (74)93.97 Total impact

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    ABSTRACT: Calorimetric decay energy spectroscopy of electron-capture-decaying isotopes is a promising method to achieve the sensitivity required for electron neutrino mass measurement. The very low total nuclear decay energy \((Q_\mathrm{EC}\) \(<\) 3 keV) and short half-life (4570 years) of \(^{163}\)Ho make it attractive for high-precision electron-capture spectroscopy (ECS) near the kinematic endpoint, where the neutrino momentum goes to zero. In the ECS approach, an electron-capture-decaying isotope is embedded inside a microcalorimeter designed to capture and measure the energy of all the decay radiation except that of the escaping neutrino. We have developed a complete process for proton irradiation-based isotope production, isolation, and purification of \(^{163}\)Ho. We have developed transition-edge sensors for this measurement and methods for incorporating \(^{163}\)Ho into high-resolution microcalorimeters, and have measured the electron-capture spectrum of \(^{163}\)Ho. We present our work in these areas and discuss the measured spectrum and its comparison to current theory.
    Full-text · Article · Oct 2015 · Journal of Low Temperature Physics
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    ABSTRACT: The demonstration of a microwave superconducting quantum interference device (SQUID) multiplexed readout of transition-edge sensor (TES) microcalorimeters has the potential to dramatically expand the scale of arrays of TESs. In this paper, we discuss recent work to develop an instrument for high-resolution high-efficiency gamma-ray spectroscopy that integrates previously demonstrated high-resolution TES microcalorimeters with new lower noise microwave SQUID multiplexers. We will discuss the proposed instrument design and readout noise of the optimized microwave SQUID multiplexed readout. Finally, we will discuss the potential limits on the number of multiplexed TESs imposed by the analog-to-digital converter.
    No preview · Article · Jun 2015 · IEEE Transactions on Applied Superconductivity
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    ABSTRACT: We discuss sensor and method development for the analysis of alphaand beta-decaying radioisotopes encapsulated within superconducting transition-edge sensor microcalorimeter absorbers. For alpha-decaying isotopes, e.g., 238Pu, 241Am, and 210Po, this is a measurement of the total nuclear reaction energy (Q) and the spectra consist of sharp, narrow peaks. The primary peak is at the Q value, with secondary peaks corresponding to gamma-ray-escape peaks. In contrast, for beta-decaying isotopes, e.g., 241Pu, the spectrum is a low energy continuum ending at E=Q. We are developing transition edge-sensor (TES) microcalorimeters to measure these spectra simultaneously in a single sample, hence allowing quantitative analysis of all Pu isotopes from 238 to 242. We have developed and used TES microcalorimeter detectors for this purpose, and it represents a new quantitative analysis tool for nuclear forensics and safeguards. Due to the high efficiency of the embedded source geometry, measurement times can be minimized. The high dynamic range of our sensors creates the opportunity to measure the relatively low energy beta-decay spectrum of 241Pu (Q = 20.78 keV) simultaneously with the Q ~ 5-6 MeV of alpha-decaying actinides. Finally, the technique could also be effective for determining the time since chemical purification of Pu using the 241Pu/241Am isotopic ratio via simultaneous measurement of the low-energy 241Pu beta particles and the high-energy 241Am Q-value.
    No preview · Article · Jun 2015 · IEEE Transactions on Applied Superconductivity
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    ABSTRACT: We describe a laser-driven x-ray plasma source designed for ultrafast x-ray absorption spectroscopy. The source is comprised of a 1 kHz, 20 W, femtosecond pulsed infrared laser and a water target. We present the x-ray spectra as a function of laser energy and pulse duration. Additionally, we investigate the plasma temperature and photon flux as we vary the laser energy. We obtain a 75 μm FWHM x-ray spot size, containing ∼106 photons/s, by focusing the produced x-rays with a polycapillary optic. Since the acquisition of x-ray absorption spectra requires the averaging of measurements from >107 laser pulses, we also present data on the source stability, including single pulse measurements of the x-ray yield and the x-ray spectral shape. In single pulse measurements, the x-ray flux has a measured standard deviation of 8%, where the laser pointing is the main cause of variability. Further, we show that the variability in x-ray spectral shape from single pulses is low, thus justifying the combining of x-rays obtained from different laser pulses into a single spectrum. Finally, we show a static x-ray absorption spectrum of a ferrioxalate solution as detected by a microcalorimeter array. Altogether, our results demonstrate that this water-jet based plasma source is a suitable candidate for laboratory-based time-resolved x-ray absorption spectroscopy experiments.
    Preview · Article · Mar 2015 · Structural Dynamics
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    ABSTRACT: We have developed a new category of sensor for measurement of the (240)Pu/(239)Pu mass ratio from aqueous solution samples with advantages over existing methods. Aqueous solution plutonium samples were evaporated and encapsulated inside of a gold foil absorber, and a superconducting transition-edge-sensor microcalorimeter detector was used to measure the total reaction energy (Q-value) of nuclear decays via heat generated when the energy is thermalized. Since all of the decay energy is contained in the absorber, we measure a single spectral peak for each isotope, resulting in a simple spectral analysis problem with minimal peak overlap. We found that mechanical kneading of the absorber dramatically improves spectral quality by reducing the size of radioactive inclusions within the absorber to scales below 50 nm such that decay products primarily interact with atoms of the host material. Due to the low noise performance of the microcalorimeter detector, energy resolution values of 1 keV FWHM at 5.5 MeV have been achieved, an order of magnitude improvement over alpha-spectroscopy with conventional silicon detectors. We measured the (240)Pu/(239)Pu mass ratio of two samples and confirmed the results by comparison to mass-spectrometry values. These results have implications for future measurements of trace samples of nuclear material.
    No preview · Article · Feb 2015 · Analytical Chemistry
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    T. Burr · A. Hoover · S. Croft · M. Rabin
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    ABSTRACT: High purity germanium (HPGe) currently provides the highest readily available resolution gamma detection for a broad range of radiation measurements, but microcalorimetry is a developing option that has considerably higher resolution even than HPGe. Superior microcalorimetry resolution offers the potential to better distinguish closely spaced X-rays and gamma-rays, a common challenge for the low energy spectral region near 100 keV from special nuclear materials, and the higher signal-to-background ratio also confers an advantage in detection limit. As microcalorimetry continues to develop, it is timely to assess the impact of uncertainties in detector and item response functions and in basic nuclear data, such as branching ratios and half-lives, used to interpret spectra in terms of the contributory radioactive isotopes. We illustrate that a new inference option known as approximate Bayesian computation (ABC) is effective and convenient both for isotopic inference and for uncertainty quantification for microcalorimetry. The ABC approach opens a pathway to new and more powerful implementations for practical applications than currently available.
    Full-text · Article · Jan 2015 · Nuclear Data Sheets
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    ABSTRACT: We are developing superconducting transition-edge sensor (TES) microcalorimeters that are optimized for rapid isotopic analysis of trace actinide samples by Q-spectroscopy. By designing mechanically robust TESs and simplified detector assembly methods, we have developed a detector for Q-spectroscopy of actinides that can be assembled in minutes. We have characterized the effects of each simplification and present the results. Finally, we show results of isotopic analysis of plutonium samples with Q-spectroscopy detectors and compare the results to mass spectrometry.
    No preview · Article · Dec 2014 · Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment
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    ABSTRACT: The application of cryogenic microcalorimeter detectors to γ-ray spectroscopy allows for measurements with unprecedented energy resolution. These detectors are ideally suited for γ-ray spectroscopy applications for which the measurement quality is limited by the spectral overlap of many closely spaced transitions using conventional detector technologies. The non-destructive analysis of mixed-isotope Pu materials is one such application where the precision can be potentially improved utilizing microcalorimeter detectors compared to current state-of-the-art high-purity Ge detectors (HPGe). The LANL-NIST γ-ray spectrometer, a 256-pixel microcalorimeter array based on transition-edge sensors (TESs), was recently commissioned and used to collect data on a variety of Pu isotopic standards to characterize the instrument performance. These measurements represent the first time the simultaneous readout of all 256 pixels for measurements of mixed-isotope Pu materials has been achieved. The LANL-NIST γ-ray spectrometer has demonstrated an average pixel resolution of 55 eV full-width-at-half-maximum at 100 keV, nearly an order of magnitude better than HPGe detectors. Some challenges of the analysis of many-channel ultra-high resolution data and the techniques used to produce quality spectra for isotopic analysis will be presented. The LANL-NIST γ-ray spectrometer has also demonstrated stable operation and obtained high resolution measurements at total array event rates beyond 1 kHz. For a total event rate of 1.25 kHz, approximately 5.6 cps/pixel, a 72.2 eV average FWHM for the 103 keV photopeak of 153Gd was achieved.
    No preview · Article · Oct 2014 · Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment
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    ABSTRACT: Microcalorimeter detectors with embedded radioactive material offer many possibilities for new types of measurements and applications. We will discuss the designs and methods that we are developing for precise deposition of radioactive material and its encapsulation in the absorber of transition-edge sensor (TES) microcalorimeter detectors for two specific applications. The first application is total nuclear reaction energy (Q) spectroscopy for nuclear forensics measurements of trace actinide samples, where the goal is determination of ratios of isotopes with Q values in the range of 5–7 MeV. Simplified, rapid sample preparation and detector assembly is necessary for practical measurements, while maintaining good energy resolution. The second application is electron capture spectroscopy of isotopes with low Q values, such as \(^{163}\) Ho, for measurement of neutrino mass. Detectors for electron capture spectroscopy are designed for measuring energies up to approximately 6 keV. Their smaller heat capacity and physical size present unique challenges. Both applications require precise deposition of radioactive material and encapsulation in an absorber with optimized thermal properties and coupling to the TES. We have made detectors for both applications with a variety of designs and assembly methods, and will present their development.
    No preview · Article · Sep 2014 · Journal of Low Temperature Physics
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    ABSTRACT: The nondestructive assay (NDA) of plutonium-bearing materials using gamma-ray spectroscopy supports global nuclear nonproliferation and safeguards efforts. High-purity germanium (HPGe) detectors have been used for this application for decades, but the uncertainty limit remains around 1% relative error for measured isotope ratios, an order of magnitude larger than destructive assay. To lower NDA uncertainty limits, we are pursuing new measurement technology using superconducting microcalorimeter detectors, and assessing the sources of current uncertainty limits. We compare results from analysis of plutonium isotopic standards using HPGe and microcalorimeter detectors, and find lower random error for the microcalorimeter data. Uncertainties in the reference values of constants of nature contribute to the total measurement error. For one particular set of constants, the gamma-ray energies, we find that microcalorimeter analysis is much less sensitive (more than a factor of ten) to the uncertainty in nuclear data than HPGe.
    No preview · Article · Aug 2014 · IEEE Transactions on Nuclear Science
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    ABSTRACT: Microcalorimeter sensors operated near 0.1 K can measure the energy of individual x- and gamma-ray photons with significantly more precision than conventional semiconductor technologies. Both microcalorimeter arrays and higher per pixel count rates are desirable to increase the total throughput of spectrometers based on these devices. The millisecond recovery time of gamma-ray microcalorimeters and the resulting pulse pileup are significant obstacles to high per pixel count rates. Here, we demonstrate operation of a microcalorimeter detector at elevated count rates by use of convolution filters designed to be orthogonal to the exponential tail of a preceding pulse. These filters allow operation at 50% higher count rates than conventional filters while largely preserving sensor energy resolution.
    No preview · Article · May 2013 · The Review of scientific instruments

  • No preview · Article · Apr 2013 · IEEE Transactions on Nuclear Science
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    ABSTRACT: Microcalorimeter detectors provide unprecedented energy resolution for gamma-ray spectroscopy. One application is measuring the isotopic composition of plutonium-bearing samples by non-destructive gamma-ray spectroscopy to support nuclear safeguards and nonproliferation efforts. When measured with conventional high-purity germanium (HPGe) detectors, data from these samples contain significant peak overlaps requiring spectral deconvolution for analysis. The improved energy resolution of the microcalorimeter detector reduces peak overlaps leading to improvement in the statistical error component of the total measurement uncertainty. In this paper, we describe analysis code that was developed for spectral peak fitting and isotopic content determination from microcalorimeter and HPGe data. We apply the code to data collected from several plutonium standards to quantify the improvement of the statistical error derived from the improved energy resolution.
    No preview · Article · Apr 2013 · IEEE Transactions on Nuclear Science
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    ABSTRACT: We introduce a filter-construction method for pulse processing that differs in two respects from that in standard optimal filtering, in which the average pulse shape and noise-power spectral density are combined to create a convolution filter for estimating pulse heights. First, the proposed filters are computed in the time domain, to avoid periodicity artifacts of the discrete Fourier transform, and second, orthogonality constraints are imposed on the filters, to reduce the filtering procedure's sensitivity to unknown baseline height and pulse tails. We analyze the proposed filters, predicting energy resolution under several scenarios, and apply the filters to high-rate pulse data from gamma-rays measured by a transition-edge-sensor microcalorimeter.
    Full-text · Article · Dec 2012
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    ABSTRACT: 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.
    No preview · Article · Sep 2012 · The Review of scientific instruments
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    ABSTRACT: Microcalorimeter detectors provide unprecedented energy resolution for the measurement of X-rays and soft gamma-rays. Energy resolution in the 100 keV region can be up to an order of magnitude better than planar high-purity germanium (HPGe) detectors. The technology is well-suited to analysis of materials with complex spectra presenting closely spaced photopeaks. One application area is the measurement and assay of nuclear materials for safeguards and fuel cycle applications. In this paper, we discuss the operation and performance of a 256-pixel array, and present results of a head-to-head comparison of isotopic determination measurements with high-purity germanium using a plutonium standard. We show that the uncertainty of a single measurement is smaller for the microcalorimeter data compared to the HPGe data when photopeak areas are equal. We identify several key areas where analysis codes can be optimized that will likely lead to improvement in the microcalorimeter performance.
    No preview · Article · Oct 2011 · Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment
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    ABSTRACT: According to one embodiment, generating image data includes receiving coded aperture imaging sensor data collected according to coded aperture imaging and receiving Compton imaging sensor data collected according to Compton imaging. The coded aperture imaging sensor data and the Compton imaging sensor data are generated by a sensor system sensing radiation from a radiation source. A coded aperture imaging pixel value and a Compton imaging pixel value are determined for each pixel of an image. A combining function comprising addition is applied to the coded aperture imaging pixel value and the Compton imaging pixel value to yield a combined pixel value for each pixel. Combined image data is generated from the combined pixel values. The combined image data is configured to yield a combined image of the radiation source.
    No preview · Patent · Jan 2011
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    ABSTRACT: High-resolution superconducting gamma-ray sensors show potential for the more accurate analysis of nuclear material. These devices are part of a larger class of microcalorimeters and bolometers based on transition edge sensors (TESs) that have two distinct thermal bodies. We derive the time domain behavior of the current and temperature for compound TES devices in the small signal limit and demonstrate the utility of these equations for device design and characterization. In particular, we use the model to fit pulses from our gamma-ray microcalorimeters and demonstrate how critical damping and electrothermal stability can be predicted.
    Full-text · Article · Oct 2010 · Applied Physics Letters
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    ABSTRACT: Microcalorimeters have been shown to yield unsurpassed energy resolution for alpha spectrometry, up to 1.06 keV FWHM at 5.3 MeV. These detectors use a superconducting transition‐edge sensor (TES) to measure the temperature change in an absorber from energy deposited by an interacting alpha particle. Our system has four independent detectors mounted inside a liquid nitrogen∕liquid helium cryostat. An adiabatic demagnetization refrigerator (ADR) cools the detector stage to its operating temperature of 80 mK. Temperature regulation with ∼15‐μK peak‐to‐peak variation is achieved by PID control of the ADR. The detectors are voltage‐biased, and the current signal is amplified by a commercial SQUID readout system and digitized for further analysis. This paper will discuss design and operation of our microcalorimeter alpha‐particle spectrometer, and will show recent results.
    No preview · Article · Dec 2009
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    ABSTRACT: Issues regarding the energy calibration of high dynamic range microcalorimeter detector arrays are presented with respect to new results from a minor actinide‐mixed oxide radioactive source. The need to move to larger arrays of such detectors necessitates the implementation of automated analysis procedures, which turn out to be nontrivial due to complex calibration shapes and pixel‐to‐pixel variability. Some possible avenues for improvement, including a more physics‐based calibration procedure, are suggested.
    No preview · Article · Dec 2009