Lynn S. Wood

High-Purity Germanium Spectroscopy at Rates in Excess of

In gamma spectroscopy, a compromise must be made between energy resolution and event-rate capability. Some foreseen nuclear material safeguards applications require a spectrometer with energy resolution typical of high purity germanium (HPGe) detectors, operated at event rates up to and exceeding 106 per second. We report the performance of an HPGe spectrometer system adapted to run under such conditions. Our system consists of a commercial semi-coaxial HPGe detector, a modified high-voltage-rail, resistive-feedback, charge-sensitive preamplifier and a continuous waveform digitizer. Digitized waveforms are analyzed offline with a novel time-variant trapezoidal filter algorithm. Several time-invariant trapezoidal filters are run in parallel and the slowest one not rejected by instantaneous pileup conditions is used to measure each pulse height. We have attained full-width-at-half-maximum energy resolution approximately 8 keV measured at 662 keV with 1.03 ×106 per second incoming event rate and 39% throughput. An additional constraint on the width of the fast trigger filter removes a significant amount of rising edge pileup that passes the first pileup cut, reducing throughput to 25%. While better resolution has been reported by other authors, our throughput is an order of magnitude higher than any other reported HPGe system operated at such an event rate.

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A novel, multi-point contact high-purity germanium detector has been developed for applications in high-rate gamma environments. The planar detector was fabricated with seven point contacts, a high-voltage steering grid and bias electrode using amorphous germanium technology. We have characterized this detector and report herein on the depletion profile, leakage current, energy resolution, and charge-sharing behavior.

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The Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) is a double-sided Time Projection Chamber (TPC) with micromegas readout designed to measure the energy-dependent neutron-induced fission cross sections of the major and minor actinides with unprecedented accuracy. The NIFFTE project addresses the challenge of minimizing major sources of systematic uncertainties from previous fission chamber measurements such as: target and beam non-uniformities, misidentification of alpha and light charged particles as fission fragments, and uncertainties inherent to the reference standards used. In-beam tests of the NIFFTE TPC at the Los Alamos Neutron Science Center (LANSCE) started in 2010 and have continued in 2011, 2012 and 2013. An overview of the NIFFTE TPC status and performance at LANSCE will be presented.

A Multi-Point Contact HPGe Detector

Many applications require the generation of gamma spectra at event rates in excess of 106 s−1 as well as very good energy resolution, e.g., safeguards, emergency response, and nondestructive assay. Good energy resolution is especially important when lower activity isotopes are sought among a large background (or foreground) that would otherwise dominate the spectrum, such as the minor actinides present in spent fuel after a long cool down time. To this end, we anticipate that high-energy-resolution detectors, such as high-purity germanium, can be adapted to high rates at a small cost to energy resolution, rather than starting with a detector with high-rate capability and medium energy resolution, e.g., LaBr3. Here, we present recent design improvements of the ultra high-rate germanium (UHRGe) detection system to allow for a 24-channel spectrum generation output. Further, we present a sensitivity study to determine how uncertainties in parameters of the detection system response affect the resulting spectra. A preamplifier simulator is developed that can emulate the output of the system at various event rates, including very high rates in excess of 106 s−1. Here, we show how various levels of uncertainty in the DC offset of the preamplifier output can affect the full width at half max (FWHM) of the resulting spectrum.

The NIFFTE project

For many nuclear material safeguards inspections, spectroscopic gamma detectors are required which can achieve high event rates (in excess of 106 s-1) while maintaining very good energy resolution for discrimination of neighboring gamma signatures in complex backgrounds. Such spectra can be useful for non-destructive assay (NDA) of spent nuclear fuel with long cooling times, which contains many potentially useful low-rate gamma lines, e.g., Cs-134, in the presence of a few dominating gamma lines, such as Cs-137. Detectors in use typically sacrifice energy resolution for count rate, e.g., LaBr3, or vise-versa, e.g., CdZnTe. In contrast, we anticipate that beginning with a detector with high energy resolution, e.g., high-purity germanium (HPGe), and adapting the data acquisition for high throughput will be able to achieve the goals of the ideal detector. In this work, we present quantification of Cs-134 and Cs-137 activities, useful for fuel burn-up quantification, in fuel that has been cooling for 22.3 years. A segmented, planar HPGe detector is used for this inspection, which has been adapted for a high-rate throughput in excess of 500k counts/s. Using a very-high-statistic spectrum of 2.4 × 1011 counts, isotope activities can be determined with very low statistical uncertainty. However, it is determined that systematic uncertainties dominate in such a data set, e.g., the uncertainty in the pulse line shape. This spectrum offers a unique opportunity to quantify this uncertainty and subsequently determine required counting times for given precision on values of interest.

High-rate germanium gamma spectroscopy: A sensitivity study

Fine cell granularity in modern climate models can produce terabytes of data in each snapshot, causing significant I/O overhead. To address this issue, a method of reducing the I/O latency of high-resolution climate models by identifying and selectively outputting regions of interest is presented. Working with a global cloud-resolving model and running with up to 10,240 processors on a Cray XE6, this method provides significant I/O bandwidth reduction depending on the frequency of writes and the size of the region of interest. The implementation challenges of determining global parameters in a strictly core-localized model and properly formatting output files that only contain subsections of the global grid are addressed, as well as the overall bandwidth impact and benefits of the method. The gains in I/O throughput provided by this method allow dual output rates for high-resolution climate models: a low-frequency global snapshot as well as a high-frequency regional snapshot when events of particular interest occur.

Systematic uncertainties in high-rate germanium data

Neutron multiplicity counters are used in safeguards to provide rapid assay of samples which contain an unknown amount of plutonium in a potentially unknown configuration. Alternatives to the use of 3He for the detection of thermal neutrons are being investigated. With appropriate detector design, the neutron single, double, and triple coincidence events can be used to extract information of three unknown parameters such as the 240Pu-effective mass, the sample self-multiplication, and the (α,n) rate. A project at PNNL has investigated replacing 3He-based tubes with LiF/ZnS neutron-scintillator sheets and wavelength shifting plastic for light pipes. A demonstrator system was constructed and is being used to test potential data acquisition system options and neutron/gamma-ray discrimination algorithms for a larger-scale system. A full-scale system has been extensively modeled to better understand the impact of all the various components. The results of the current testing and modeling effort will inform the final design of a full scale system which is expected to match the performance of existing 3He-based neutron multiplicity counters. A review of the current effort and the most recent findings will be presented.

A global climate model agent for high spatial and temporal resolution data

High-resolution high-purity germanium (HPGe) spectrometers are needed for Safeguards applications such as spent fuel assay and uranium hexafluoride cylinder verification. In addition, these spectrometers would be applicable to other high-rate applications such as non-destructive assay of nuclear materials using nuclear resonance fluorescence. Count-rate limitations of todays HPGe technologies, however, lead to concessions in their use and reduction in their efficacy. Large-volume, very high-rate HPGe spectrometers are needed to enable a new generation of nondestructive assay systems. The Ultra-High Rate Germanium (UHRGe) project is developing HPGe spectrometer systems capable of operating at unprecedented rates, 10 to 100 times those available today. This report documents current status of developments in the analog electronics and analysis software.