Stephen E. Mitchell

Networked gamma radiation detection system for tactical deployment

A networked gamma radiation detection system with directional sensitivity and energy spectral data acquisition capability is being developed by the National Security Technologies, LLC, Remote Sensing Laboratory to support the close and intense tactical engagement of law enforcement who carry out counterterrorism missions. In the proposed design, three clusters of 2″ × 4″ × 16″ sodium iodide crystals (4 each) with digiBASE-E (for list mode data collection) would be placed on the passenger side of a minivan. To enhance localization and facilitate rapid identification of isotopes, advanced smart real-time localization and radioisotope identification algorithms like WAVRAD (wavelet-assisted variance reduction for anomaly detection) and NSCRAD (nuisance-rejection spectral comparison ratio anomaly detection) will be incorporated. We will test a collection of algorithms and analysis that centers on the problem of radiation detection with a distributed sensor network. We will study the basic characteristics of a radiation sensor network and focus on the trade-offs between false positive alarm rates, true positive alarm rates, and time to detect multiple radiation sources in a large area. Empirical and simulation analyses of critical system parameters, such as number of sensors, sensor placement, and sensor response functions, will be examined. This networked system will provide an integrated radiation detection architecture and framework with (i) a large nationally recognized search database equivalent that would help generate a common operational picture in a major radiological crisis; (ii) a robust reach back connectivity for search data to be evaluated by home teams; and, finally, (iii) a possibility of integrating search data from multi-agency responders.

Scintillator efficiency study with MeV x-rays

We have investigated scintillator efficiency for MeV radiographic imaging. This paper discusses the modeled detection efficiency and measured brightness of a number of scintillator materials. An optical imaging camera records images of scintillator emission excited by a pulsed x-ray machine. The efficiency of various thicknesses of monolithic LYSO:Ce (cerium-doped lutetium yttrium orthosilicate) are being studied to understand brightness and resolution trade-offs compared with a range of micro-columnar CsI:Tl (thallium-doped cesium iodide) scintillator screens. The micro-columnar scintillator structure apparently provides an optical gain mechanism that results in brighter signals from thinner samples. The trade-offs for brightness versus resolution in monolithic scintillators is straightforward. For higher-energy x-rays, thicker materials generally produce brighter signal due to x-ray absorption and the optical emission properties of the material. However, as scintillator thickness is increased, detector blur begins to dominate imaging system resolution due to the volume image generated in the scintillator thickness and the depth of field of the imaging system. We employ a telecentric optical relay lens to image the scintillator onto a recording CCD camera. The telecentric lens helps provide sharp focus through thicker-volume emitting scintillators. Stray light from scintillator emission can also affect the image scene contrast. We have applied an optical light scatter model to the imaging system to minimize scatter sources and maximize scene contrasts.

MCNP estimate of ZLS lens sensitivity in an x-ray field

The telecentric zoom lens system (ZLS) has proven to be invaluable in flash x-ray field operations and recent successful experiments pertaining to stockpile stewardship. The ZLS contains 11 custom-manufactured lenses, a turning mirror (pellicle), and an x-ray-to-visible-light converting scintillator. Images are recorded on a fully characterized CCD. All hardware is supported by computerized, programmable, electro-mechanical mounts and alignment apparatus. Seven different glass material types varying in chemical stoichiometry comprise the 11 ZLS lenses. All lenses within the ZLS are out of the path of direct x-ray radiation during normal operation. However, any unshielded scattered x-ray radiation can result in energy deposition into the lenses, which may generate some scintillating light that can couple into the CCD. This extra light may contribute to a decrease in signal-to-noise ratio (SNR) and lower the overall fidelity of the radiograph images. An estimate of the scintillation generation and sensitivities for each of the seven types of glass used as lenses in the ZLS is presented. This report also includes estimates of the total observed background decoupling that each of the lens material types contribute.

Detector blur associated with MeV radiographic imaging systems

We are investigating scintillator performance in radiographic imaging systems at x-ray endpoint energies of 0.4 and 2.3 MeV in single-pulse x-ray machines. The effect of scene magnification and geometric setup will be examined along with differences between the detector response of radiation and optical scatter. Previous discussion has reviewed energy absorption and efficiency of various imaging scintillators with a 2.3 MeV x-ray source. The focal point of our study is to characterize scintillator blur to refine system models. Typical detector geometries utilize thin tiled LYSO:Ce (cerium-doped lutetium yttrium orthosilicate) assembled in a composite mosaic. Properties of individual tiles are being studied to understand system resolution effects present in the experimental setup. Comparison of two different experiments with different geometric configurations is examined. Results are then compared to different scene magnifications generated in a Monte-Carlo simulation.

Radiation anomaly detection algorithms for field-acquired gamma energy spectra

The Remote Sensing Laboratory (RSL) is developing a tactical, networked radiation detection system that will be agile, reconfigurable, and capable of rapid threat assessment with high degree of fidelity and certainty. Our design is driven by the needs of users such as law enforcement personnel who must make decisions by evaluating threat signatures in urban settings. The most efficient tool available to identify the nature of the threat object is real-time gamma spectroscopic analysis, as it is fast and has a very low probability of producing false positive alarm conditions. Urban radiological searches are inherently challenged by the rapid and large spatial variation of background gamma radiation, the presence of benign radioactive materials in terms of the normally occurring radioactive materials (NORM), and shielded and/or masked threat sources. Multiple spectral anomaly detection algorithms have been developed by national laboratories and commercial vendors. For example, the Gamma Detector Response and Analysis Software (GADRAS) a one-dimensional deterministic radiation transport software capable of calculating gamma ray spectra using physics-based detector response functions was developed at Sandia National Laboratories. The nuisance-rejection spectral comparison ratio anomaly detection algorithm (or NSCRAD), developed at Pacific Northwest National Laboratory, uses spectral comparison ratios to detect deviation from benign medical and NORM radiation source and can work in spite of strong presence of NORM and or medical sources. RSL has developed its own wavelet-based gamma energy spectral anomaly detection algorithm called WAVRAD. Test results and relative merits of these different algorithms will be discussed and demonstrated.

Neutron multiplicity measurements with 3He alternative: Straw neutron detectors

Counting neutrons emitted by special nuclear material (SNM) and differentiating them from the background neutrons of various origins is the most effective passive means of detecting SNM. Unfortunately, neutron detection, counting, and partitioning in a maritime environment are complex due to the presence of high-multiplicity spallation neutrons (commonly known as “ship effect”) and to the complicated nature of the neutron scattering in that environment. A prototype neutron detector was built using 10B as the converter in a special form factor called “straws” that would address the above problems by looking into the details of multiplicity distributions of neutrons originating from a fissioning source. This paper describes the straw neutron multiplicity counter (NMC) and assesses the performance with those of a commercially available fission meter. The prototype straw neutron detector provides a large-area, efficient, lightweight, more granular (than fission meter) neutron-responsive detection surface (to facilitate imaging) to enhance the ease of application of fission meters. Presented here are the results of preliminary investigations, modeling, and engineering considerations leading to the construction of this prototype. This design is capable of multiplicity and Feynman variance measurements. This prototype may lead to a near-term solution to the crisis that has arisen from the global scarcity of 3He by offering a viable alternative to fission meters. This paper describes the work performed during a 2-year site-directed research and development (SDRD) project that incorporated straw detectors for neutron multiplicity counting. The NMC is a two-panel detector system. We used 10B (in the form of enriched boron carbide: 10B4C) for neutron detection instead of 3He. In the first year, the project worked with a panel of straw neutron detectors, investigated its characteristics, and developed a data acquisition (DAQ) system to collect neutron multiplicity information from spontaneous fission sources using a single panel consisting of 60 straws equally distributed over three rows in high-density polyethylene moderator. In the following year, we developed the field-programmable gate array and associated DAQ software. This SDRD effort successfully produced a prototype NMC with ˜33% detection efficiency compared to a commercial fission meter.

Lithium Fluoride TLD dose quality

The use of Lithium Fluoride (LiF) Thermoluminescent Dosimeters (TLD) have been in use at a radiographic facility for over three years. The facility consists of two radiographic sources each with a dose rating of ~4-rad at 1 m. The calibration and fielding of LiF TLDs will be examined for accuracy and long term standard deviation of these measurements. LiF TLDs will be evaluated in single point measurements and multi-point arrays. Improved multi-point arrays will be compared to previous array data. The LiF TLDs will also be compared to Pin Diodes for routine measurements and evaluation of shielding around the sources.

X-ray pinhole camera measurements

The development of the rod pinch diode has lead to high resolution radiography used on contained explosive experiments. The rod pinch diodes use a small diameter anode rod, which extends through a cathode aperture. Electrons borne off the aperture edge can self-insulate and pinch onto the tip of the rod, creating an intense, small x-ray source. This source is utilized as the primary diagnostic on numerous experiments that include high-value, single-shot events. In such applications there is an emphasis on machine reliability, x-ray reproducibility, and x-ray quality. We have observed that an additional pinch occurs at the interface near the anode rod and the rod holder. This suggests that there are stray electrons emitted from the surfaces of the surrounding area. In this paper we present results of x-ray measurements using a pinhole camera. The camera geometry used is an upstream view 30° with respect to the diode centerline. This diagnostic will be employed to: (1) diagnose x-ray reproducibility and quality, and (2) investigate the effect of different diode configurations.

Deconvolving Current from a Faraday Rotation Measurement

Summary form only given. In this paper, a unique software program is reported which automatically decodes the Faraday rotation signal into a time-dependent current representation. System parameters, such as the Faraday fibers Verdet constant and number of loops in the sensor, are the only user-interface inputs. The central aspect of the algorithm utilizes a short-time Fourier transform, which reveals much of the Faraday rotation measurements implicit information necessary for unfolding the dynamic current measurement.