Mitchell L. Woodring

Pacific Northwest National Laboratory, Richland, Washington, United States

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Publications (33)15.16 Total impact

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    ABSTRACT: Thousands of radiation detection systems have been deployed over the last decade to domestic and international locations, primarily at points of entry or departure. An essential ingredient in supporting sensor deployment is the capability to develop and test emerging systems and replicate the behavior of deployed systems. Pacific Northwest National Laboratory recently designed and constructed a laboratory for developing and testing these large-scale radiation detection systems. The Large Detector Laboratory is capable of housing up to 30 fully integrated radiation portal monitoring systems and allows for complex testing scenarios that include background suppression, equipment shielding, and source cross-talk between systems. The laboratory's design also allows implementation of applications ranging from pedestrian screening to baggage or package inspection to vehicle and cargo inspection to standoff detection. This manuscript describes the unique attributes of this laboratory, including the ability to reproduce operational conditions faced by integrated systems, realistic source configurations, and measurements across multiple systems.
    IEEE Transactions on Nuclear Science 01/2013; 60(2):1151-1155. · 1.22 Impact Factor
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    ABSTRACT: The level of detail discernible in imaging techniques has generally excluded them from consideration as verification tools in inspection regimes. An image will almost certainly contain highly sensitive information, and storing a comparison image will almost certainly violate a cardinal principle of information barriers: that no sensitive information be stored in the system. To overcome this problem, some features of the image might be reduced to a few parameters suitable for definition as an attribute, which must be non-sensitive to be acceptable in an Information Barrier regime. However, this process must be performed with care. Features like the perimeter, area, and intensity of an object, for example, might reveal sensitive information. Any data-reduction technique must provide sufficient information to discriminate a real object from a spoofed or incorrect one, while avoiding disclosure (or storage) of any sensitive object qualities. Ultimately, algorithms are intended to provide only a yes/no response verifying the presence of features in the image. We discuss the utility of imaging for arms control applications and present three image-based verification algorithms in this context. The algorithms reduce full image information to non-sensitive feature information, in a process that is intended to enable verification while eliminating the possibility of image reconstruction. The underlying images can be highly detailed, since they are dynamically generated behind an information barrier. We consider the use of active (conventional) radiography alone and in tandem with passive (auto) radiography. We study these algorithms in terms of technical performance in image analysis and application to an information barrier scheme.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2012; 662(1):81–89. · 1.14 Impact Factor
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    ABSTRACT: Multiplicity counters are an example of a neutron detection system affected by the shortage of 3He. Efforts are underway to identify potential 3He neutron detector replacements for use in multiplicity counters. Boron-10 and 6Li based systems are two of the options being explored as near-term 3He alternatives for neutron multiplicity counters. Simulations of BF3 filled proportional counters, 10B-lined proportional counters and 6Li/ZnS(Ag) sheets in various configurations have been performed with the Monte Carlo particle transport code MCNPX and compared to simulations of existing 3He counters. The system performances are being compared using figure of merits that are the square or cube of the total system efficiency divided by the die-away time (the average lifetime of neutrons in the counter). Design considerations include developing a system with enough neutron detection material to achieve the highest possible efficiency, while simultaneously minimizing system size. Adequate moderation is required to thermalize the incident neutrons for increased counting efficiency, but as the system size increases so will the die-away time. The optimal moderator configuration is one for which the increase in neutron detection efficiency is not off-set by an increase in die-away time. Thus, the entire system performance must be evaluated with every configuration change. The simulation results will be validated against a bench-top demonstrator unit design based on the system identified through simulations as having the highest performance potential. Presented here are the simulation results with various configurations of BF3 filled proportional counters, 10B lined proportional counters and 6Li/ZnS(Ag) sheets, and preliminary measurements with the initial bench-top system.
    Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC), 2012 IEEE; 01/2012
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    ABSTRACT: Radiation detection systems for homeland security applications must possess the capability of detecting both gamma rays and neutrons. The radiation portal monitor systems that are currently deployed use a plastic scintillator for detecting gamma rays and 3He gas-filled proportional counters for detecting neutrons. Proportional counters filled with 3He are the preferred neutron detectors for use in radiation portal monitor systems because 3He has a large neutron cross-section, is relatively insensitive to gamma-rays, is neither toxic nor corrosive, can withstand extreme environments, and can be operated at a lower voltage than some of the alternative proportional counters. The amount of 3He required for homeland security and science applications has depleted the world supply and there is no longer enough available to fill the demand. Thus, alternative neutron detectors are being explored.Two possible temporary solutions that could be utilized while a more permanent solution is being identified are reducing the 3He pressure in the proportional counters and using boron trifluoride gas-filled proportional counters. Reducing the amount of 3He required in each of the proportional counters would decrease the rate at which 3He is being used; not enough to solve the shortage, but perhaps enough to increase the amount of time available to find a working replacement. Boron trifluoride is not appropriate for all situations as these detectors are less sensitive than 3He, boron trifluoride gas is corrosive, and a much higher voltage is required than what is used with 3He detectors. Measurements of the neutron detection efficiency of 3He and boron trifluoride as a function of tube pressure were made. The experimental results were also used to validate models of the radiation portal monitor systems.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 10/2011; 652(1):347–350. · 1.14 Impact Factor
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    ABSTRACT: The shortage of 3He has triggered the search for effective alternative neutron detection technologies for national security and safeguards applications. Any new detection technology must satisfy two basic criteria: (1) it must meet a neutron detection efficiency requirement, and (2) it must be insensitive to gamma-ray interference at a prescribed level, while still meeting the neutron detection requirement. It is the purpose of this paper to define measureable gamma ray sensitivity criteria for neutron detectors. Quantitative requirements are specified for: intrinsic gamma ray detection efficiency and gamma ray absolute rejection. The gamma absolute rejection ratio for neutrons (GARRn) is defined, and it is proposed that the requirement for neutron detection be 0.9
    Nuclear Instruments and Methods 01/2011; 654:412-416.
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    ABSTRACT: Radiation portal monitors used for interdiction of illicit materials at borders include highly sensitive neutron detection systems. The main reason for having neutron detection capability is to detect fission neutrons from plutonium. Most currently deployed radiation portal monitors (RPMs) use neutron detectors based upon 3He-filled gas proportional counters, which are the most common large area neutron detector. This type of neutron detector is used in the TSA and other RPMs installed in international locations and in the Ludlum and Science Applications International Corporation RPMs deployed primarily for domestic applications. There is a declining supply of 3He in the world and, thus, methods to reduce the use of this gas in RPMs with minimal changes to the current system designs and sensitivity to cargo-borne neutrons are being investigated. Four technologies have been identified as being currently commercially available, potential alternative neutron detectors to replace the use of 3He in RPMs. These technologies are: 1) Boron trifluoride-filled proportional counters, 2) Boron-lined proportional counters, 3) Lithium-loaded glass fibers, and 4) Coated wavelength-shifting plastic fibers. Reported here is a summary of the testing carried out at Pacific Northwest National Laboratory on these technologies to date, as well as measurements on 3He tubes at various pressures. Details on these measurements are available in the referenced reports. Sponsors of these tests include the Department of Energy (DOE), Department of Homeland Security (DHS), and the Department of Defense (DoD), as well as internal Pacific Northwest National Laboratory funds.
    04/2010;
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    ABSTRACT: Skyshine reflection, or the scattering off of the air above a source, is frequently observed in situations where large sources are incompletely shielded, such as radiography sources in shielding pits that are open to the sky. Originally, concern about skyshine regarded limiting the dose from the source. However, even in situations where dose is minimal, the contribution of skyshine may interfere with sensitive measurement instruments operating near background limits, such as with border security applications. To help determine effective methods for shielding sensitive detection systems from skyshine interference, a series of measurements and model simulations have been conducted using a specially configured, portable collimated detector and an iridium-192 source. This paper will report these results, and also show their similarity when compared to other measurements using different sources. )
    02/2010;
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    ABSTRACT: Neutron detection is an essential aspect of interdiction of radiological threats for national security purposes, since plutonium, a material used for nuclear weapons, is a significant source of fission neutrons. Radiation portal monitoring systems, of which there are thousands deployed for homeland security and non-proliferation purposes, currently use 3He gas-filled proportional counters for detecting neutrons. Because of the high usage of 3He for neutron scattering science and national security, the supply has dwindled, and can no longer meet the demand. Consequently, a replacement technology for neutron detection is required in the very near future.
    01/2010;
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    ABSTRACT: Reported here are the results of measurements performed to determine the efficiency of 3He filled proportional counters as a function of gas pressure in the SAIC system. Motivation for these measurements was largely to validate the current model of the SAIC system. Those predictions indicated that the neutron detection efficiency plotted as a function of pressure has a simple, logarithmic shape. As for absolute performance, the model results indicated the 3He pressure in the current SAIC system could not be reduced appreciably while meeting the current required level of detection sensitivity. Thus, saving 3He by reducing its pressure was predicted not to be a viable option in the current SAIC system.
    01/2010;
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    ABSTRACT: The time series of data from a radiation portal monitor (RPM) system are evaluated for the presence of point sources by isolating the contribution of anomalous radiation. Energy-windowed background spectra taken from the RPM are compared with the observed spectra at each time step during a vehicle drive-through. The total signal is turned into a ¿spectral distance¿ index using this method. This provides a time series with reduced systematic fluctuations due to background attenuation by the vehicle, and allows for point source detection by time-series analyses. The anomalous time series is reanalyzed by using a wavelet filter function of similar size to the expected source profile. A number of real drive-through data sets taken at a U.S. port of entry are analyzed in this way. A set of isotopes are injected into the data set, and the resultant ¿benign¿ and ¿injected¿ data sets are analyzed with gross-counting, spectral-ratio, and time-based algorithms. Spectral and time methods together offer a significant increase to detection performance.
    IEEE Transactions on Nuclear Science 01/2010; · 1.22 Impact Factor
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    ABSTRACT: One of the main uses for 3He is in gas proportional counters for neutron detection. Large radiation detection systems deployed for homeland security and proliferation detection applications use such systems. Due to the large increase in use of 3He for homeland security and basic research, the supply has dwindled, and can no longer meet the demand. This has led to the search for an alternative technology to replace the use of 3He-based neutron detectors. In this paper, we review the testing of currently commercially available alternative technologies for neutron detection in large systems used in various national security applications.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2010; 623(3):1035-1045. · 1.14 Impact Factor
  • Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2010; A 623. · 1.14 Impact Factor
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    ABSTRACT: Skyshine, the radiation scattered from the air above a source, such as radiography or x-ray imaging systems, can be a problematic source of background gamma rays that interfere with radiation detection systems even at very large distances. The effects of skyshine have been studied for many years, but almost entirely within the context of nuclear power and radioactive waste storage. These earlier studies show that modeling of skyshine is made difficult because it must take into account huge volumes of space in the models that slow analysis significantly. This paper reports on results from various modeling and measurement efforts that examine shielding approaches to the skyshine interference problem with radiation detection systems.
    Nuclear Science Symposium Conference Record (NSS/MIC), 2009 IEEE; 12/2009
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    ABSTRACT: Skyshine, the radiation scattered in the air above a high-activity gamma-ray source, can produce interference with radiation portal monitor (RPM) systems at distances up to even many hundred meters. Pacific Northwest National Laboratory (PNNL) has been engaged in a campaign of measurements, design work and modeling that explore methods of mitigating the effects of skyshine on outdoor measurements with sensitive instruments. An overview of our work with shielding of skyshine is being reported by us in another paper at this conference. This paper will concentrate on two topics: measurements and modeling with Monte Carlo transport calculations to characterize skyshine from an iridium-192 source, and testing of a prototype louver system, designed and fabricated at PNNL, as a shielding approach to limit the impact of skyshine interference on RPM systems.
    Nuclear Science Symposium Conference Record (NSS/MIC), 2009 IEEE; 12/2009
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    ABSTRACT: Spectroscopic gamma-ray detectors are used for many research applications as well as Homeland Security screening applications. Sodium Iodide (Nal) scintillator crystals coupled with photomultiplier tubes provide medium-resolution spectral data about the surrounding environment. Nal-based detectors, paired with spectral identification algorithms, are often effective in identifying sources of interest by isotope. However, intrinsic limitations exist for Nal systems because of gain shifts and spectral marring (e.g., loss of resolution and count-rate saturation) at high count rates. These effects are hardware dependent and have strong effects on the radioisotopic identification capability of these systems. In this work, the effects of high count rate on the response of isotope-identification algorithms are explored. It is shown that a small gain shift of a few tens of keV is sufficient to disturb identification. The onset of this and other spectral effects is estimated for Nal crystals, and a mechanism for mitigating these effects by estimating and correcting for them is implemented and evaluated.
    Nuclear Science Symposium Conference Record, 2007. NSS '07. IEEE; 11/2009
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    ABSTRACT: Radiation portal monitors used for interdiction of illicit materials at borders include highly sensitive neutron detection systems. The main reason for having neutron detection capability is to detect fission neutrons from plutonium. The currently deployed radiation portal monitors (RPMs) from Ludlum and Science Applications International Corporation (SAIC) use neutron detectors based upon 3He-filled gas proportional counters, which are the most common large neutron detector. There is a declining supply of 3He in the world; thus, methods to reduce the use of this gas in RPMs with minimal changes to the current system designs and detection capabilities are being investigated. Reported here are the results of tests of the efficiency of BF3 tubes at a pressure of 800 torr. These measurements were made partially to validate models of the RPM system that have been modified to simulate the performance of BF3-filled tubes. While BF3 could be a potential replacement for 3He, there are limitations to its use in deployed systems.
    01/2009;
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    ABSTRACT: Time series of data from radiation portal monitors are evaluated for radioactive sources by comparing background to vehicle spectra over time with a “spectral-distance” metric, isolating the contribution of anomalous radiation. This may diminish systematic fluctuations from vehicular background attenuation and allow time-shape filtering for discriminating compact sources. To examine this, a wavelet function similar in size to the expected source profile filters the spectral-distance output. Spectra from chosen isotopes are injected into data from a U.S. port of entry. The resulting data are analyzed with gross-counting, spectral-distance, and spatial algorithms. Combined spectral/spatial filtering is shown to enhance sensitivity and discrimination of compact versus distributed sources.
    Journal of Radioanalytical and Nuclear Chemistry 01/2009; 282(3):883-887. · 1.47 Impact Factor
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    ABSTRACT: We are developing a medium-resolution autonomous in situ gamma detection system for marine and coastal waters. The system is designed to extract and preconcentrate isotopes of interest from natural waters prior to detection in order to eliminate signal attenuation of the gamma rays traveling through water and lower the overall background from the presence of naturally occurring radioactive isotopes (40K and U–Th series radionuclides). Filtration is used to preconcentrate target isotopes residing on suspended particles, while chemosorption is employed to preferentially extract truly dissolved components from the water column. Used filter and chemosorbent media will be counted autonomously using two LaBr3 detectors in a near 4-π configuration around the samples. A compact digital pulse processing system, developed in-house and capable of running in coincidence mode, is used to process the signal from the detectors to a small on-board computer. The entire system is extremely compact (9″ dia. × 30″ len.) and platform independent, but designed for initial deployment on a research buoy. A variety of commercial and in-house nano-porous chemosorbents have been selected, procured or produced, and these and filter and detector components have been tested.
    Journal of Radioanalytical and Nuclear Chemistry 01/2009; 282(3):889-895. · 1.47 Impact Factor
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    ABSTRACT: Radiation portal monitor (RPM) systems perform cargo screening at U.S. and other ports of entry. The data from these systems are routinely evaluated in regard to energy spectrum and total count rate above background. However, few analyses are performed on the time progression of data. In this work, the time series of data from an RPM system are evaluated for the presence of sources of interest by isolating the contribution of anomalous radiation. Source contributions are isolated by comparing the background spectra (or energy-windowed spectral information from low energy-resolution systems) to the spectrum at each successive time step. At every time in the data sequence, the total gross-count signal is turned into a “spectral distance” index using this method. This has the potential to dramatically reduce systematic fluctuations due to background attenuation by a vehicle (the so-called “shadow shielding” effect), and allow for time-series shape fitting for source size discrimination. The anomalous time series is reanalyzed for the presence of compact sources by using a wavelet filter function of similar size to the expected source profile. This may allow a dramatic reduction in gross-count alarm thresholds, leading to a corresponding sensitivity increase. This increase is shown by analysis of a number of real drive-through data sets taken at a U.S. port of entry. A set of isotopes of interest are injected into the data set, and the resultant “benign” and “injected” data sets are analyzed with gross-counting, spectral-ratio, and spatial algorithms. Spatial analysis alone was not sufficient to increase overall sensitivity given this data set, but both methods together gave a significant increase to detection performance.
    Nuclear Science Symposium Conference Record, 2008. NSS '08. IEEE; 11/2008
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    ABSTRACT: Fission neutron spectra from neutron induced fission in 235U and 239Pu for energies below that of the neutron inducing fission have been measured. The spectra were obtained for 1.5 MeV and 2.5 MeV incident neutrons. Previous accelerator-based fission neutron spectra measurements have been seriously complicated by time-correlated gamma rays and scattered neutrons from the fission sample. Three barium fluoride detectors were placed near the sample undergoing induced fission and used to identify fission gamma rays. A coincidence of fission gamma rays was used to gate a liquid scintillator neutron detector to distinguish fission events from other events. The fission neutron spectral shape and average energy measured in this experiment compare well to both previous measurements and prior theory and also suggest a dependence on incident neutron energy and mass of the fissioning nucleus. An overview of the experiment, a discussion of the results, and the importance of this work to homeland security are given.
    Journal of Radioanalytical and Nuclear Chemistry 06/2008; 276(3). · 1.47 Impact Factor