A. S. Hoover

Los Alamos National Laboratory, Los Alamos, California, United States

Are you A. S. Hoover?

Claim your profile

Publications (66)72.01 Total impact

  • [Show abstract] [Hide abstract]
    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.
    10/2014;
  • [Show abstract] [Hide abstract]
    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.
    Journal of Low Temperature Physics 09/2014; 176(5-6). · 1.18 Impact Factor
  • [Show abstract] [Hide abstract]
    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.
    IEEE Transactions on Nuclear Science 01/2014; 61(4):2365-2372. · 1.22 Impact Factor
  • [Show abstract] [Hide abstract]
    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.
    The Review of scientific instruments 05/2013; 84(5):056107. · 1.52 Impact Factor
  • IEEE Transactions on Nuclear Science 04/2013; 60(2):681-688. · 1.22 Impact Factor
  • [Show abstract] [Hide abstract]
    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.
    IEEE Transactions on Nuclear Science 01/2013; 60(2):681-688. · 1.22 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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.
    12/2012;
  • [Show abstract] [Hide abstract]
    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.
    The Review of scientific instruments 09/2012; 83(9):093113. · 1.52 Impact Factor
  • [Show abstract] [Hide abstract]
    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.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 10/2011; 652(1):302-305. · 1.14 Impact Factor
  • [Show abstract] [Hide abstract]
    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.
    Ref. No: US 7863567 B1, Year: 01/2011
  • Source
    [Show abstract] [Hide abstract]
    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.
    Applied Physics Letters 10/2010; · 3.52 Impact Factor
  • [Show abstract] [Hide abstract]
    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.
    AIP Conference Proceedings. 12/2009; 1185(1):741-744.
  • [Show abstract] [Hide abstract]
    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.
    AIP Conference Proceedings. 12/2009; 1185(1):711-714.
  • [Show abstract] [Hide abstract]
    ABSTRACT: A Stand-Off Radiation Detection System (SORDS) is being developed through a joint effort by Raytheon, Los Alamos National Laboratory, Bubble Technology Industries, Radiation Monitoring Devices, and the Massachusetts Institute of Technology, for the Domestic Nuclear Detection Office (DNDO). The system is a mobile truck-based platform performing detection, imaging, and spectroscopic identification of gamma-ray sources. A Tri-Modal Imaging (TMI) approach combines active-mask coded aperture imaging, Compton imaging, and shadow imaging techniques. Monte Carlo simulation and modeling using the GEANT4 toolkit was used to generate realistic data for the development of imaging algorithms and associated software code.
    Nuclear Science Symposium Conference Record (NSS/MIC), 2009 IEEE; 12/2009
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present experimental data describing cross-talk within an array of gamma-ray microcalorimeters during gamma-ray irradiation. The microcalorimeters consist of Mo/Cu transition-edge sensors (TESs) with attached Sn absorbers. We observe both thermal and electrical cross-talk with peak cross-talk amplitudes as large as 0.4%. We have developed an analytical model for thermal cross-talk and make a preliminary comparison to data. Cross-talk must be understood and minimized for high resolution spectroscopy at high input count rates.
    12/2009;
  • [Show abstract] [Hide abstract]
    ABSTRACT: GEANT4 is a versatile Monte Carlo code for simulating the interactions of radiation with matter. GEANT4 has proven to be an effective toolkit for the simulation of a wide variety of detectors. We are interested in the application of GEANT4 to a new type of sensor technology being developed for X-ray and gamma-ray measurements. Microcalorimeter detectors based on transition-edge sensors coupled to bulk absorbers are an emerging technology for hard X-ray and soft gamma-ray measurements with unprecedented energy resolution. In this work, we assess the ability of the GEANT4 electromagnetic physics package to reproduce measured microcalorimeter data. We also use the simulations to explore the design space of absorber materials and cryostat design.
    IEEE Transactions on Nuclear Science 09/2009; · 1.22 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Gamma-ray spectrometry systems based on High-Purity Germanium (HPGe) have been the long-standing leader in terms of resolution since their introduction many years ago. The application of this technology to the spectroscopic assay of special nuclear material led to the development of several isotopic analysis tools, including the advanced software package FRAM, which was, and continues to be, developed at Los Alamos National Laboratory. Although FRAM can be applied over a wide range of energies, the significantly higher intensity of the x-ray region in the neighborhood of 100 keV makes analysis of this area of the spectrum advantageous, especially in the case of plutonium. However, even with HPGe, the multitude of gamma-ray and x-ray peaks that exist in the 100-keV region are sufficiently convoluted so as to preclude determination of plutonium isotopic composition without the introduction of some error. The novel technology of cryogenic microcalorimeter detectors, shown to have an order of magnitude better spectral resolution than HPGe, has recently opened new doors with respect to these difficulties. Now, for the first time, the powerful capabilities of FRAM have been paired with the unparalleled resolution of microcalorimetry in the analysis of plutonium spectra. Preliminary results of these analyses, as well as an outlook for future measurements, heretofore unobtainable with HPGe, will be presented.
    IEEE Transactions on Nuclear Science 09/2009; · 1.22 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: 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.
    IEEE Transactions on Nuclear Science 09/2009; · 1.22 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The Gamma-Ray Burst Monitor (GBM) will significantly augment the science return from the Fermi Observatory in the study of gamma-ray bursts (GRBs). The primary objective of GBM is to extend the energy range over which bursts are observed downward from the energy range of the Large Area Telescope (LAT) on Fermi into the hard X-ray range where extensive previous data sets exist. A secondary objective is to compute burst locations onboard to allow re-orienting the spacecraft so that the LAT can observe delayed emission from bright bursts. GBM uses an array of 12 sodium iodide scintillators and two bismuth germanate scintillators to detect gamma rays from ~8 keV to ~40 MeV over the full unocculted sky. The onboard trigger threshold is ~0.7 photons cm–2 s–1 (50-300 keV, 1 s peak). GBM generates onboard triggers for ~250 GRBs per year.
    The Astrophysical Journal 08/2009; 702(1):791. · 6.73 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: 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 mm<sup>2</sup>. 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 <sup>240</sup>Pu 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.
    IEEE Transactions on Applied Superconductivity 07/2009; · 1.20 Impact Factor