David V. Jordan

MaGe-a Geant4-Based Monte Carlo Application Framework for Low-Background Germanium Experiments

We describe a physics simulation software framework, MaGe, that is based on the Geant4 simulation toolkit. MaGe is used to simulate the response of ultra-low radioactive background detectors to ionizing radiation, specifically the Majorana and Gerda neutrinoless double-beta decay experiments. Majorana and Gerda use high-purity germanium detectors to search for the neutrinoless double-beta decay of 76Ge and MaGe is jointly developed between these two collaborations. The MaGe framework contains the geometry models of common objects, prototypes, test stands and the actual experiments. It also implements customized event generators, Geant4 physics lists and output formats. All of these features are available as class libraries that are typically compiled into a single executable. The user selects the particular experimental setup implementation at run-time via macros. The combination of all these common classes into one framework reduces duplication of efforts, eases comparison between simulated data and experiment and simplifies the addition of new detectors to be simulated. This paper focuses on the software framework, custom event generators and physics lists.

Isotope identification in the GammaTracker handheld radioisotope identifier

GammaTracker is a portable handheld radioisotope identifier using position sensitive CdZnTe crystals. The device uses a peak-based method for isotope identification implemented on an embedded computing platform within the device. This paper presents the run-time optimized algorithms used in this peak-based method of analysis. Performance of the algorithms is presented using measured data from gamma-ray sources.

A Simulation Framework for Evaluating Detector Performance in Cargo Screening Applications

Deployed radiation portal monitor systems (RPMs) screen gamma-ray signatures of cargo at international border crossings with the goal of detecting illicit radiological materials. Estimating the detection sensitivity of these systems requires an in-depth understanding, and quantification, of RPM response to both benign and illicit sources. Benign sources of radioactivity include background, alterations of the background due to the presence of vehicles and cargo, as well as sources that frequently cause nuisance alarms. These nuisance sources, for example those consisting of naturally occurring radioactive materials (NORM) and medical isotopes, frequently limit system performance. Advanced detector technology promises to increase the capability of deployed systems to discriminate illicit from nuisance sources. Presented here is a framework developed to assess the performance of these passive detection technologies. Due to the difficulty in obtaining empirical data for emerging technologies, the foundation of this comparison framework lies on a simulated benign source population to create a comprehensive set of data representing cargo vehicles driving through the RPM. Quantification of performance stems from injecting simulated signatures from illicit sources and comparing probabilities of detection via case and population studies.

Estimating radiological background using imaging spectroscopy

Optical imaging spectroscopy is investigated as a method to estimate radiological background by spectral identification of soils, sediments, rocks, minerals and building materials derived from natural materials and assigning tabulated radiological emission values to these materials. Radiological airborne surveys are undertaken by local, state and federal agencies to identify the presence of radiological materials out of regulatory compliance. Detection performance in such surveys is determined by (among other factors) the uncertainty in the radiation background; increased knowledge of the expected radiation background will improve the ability to detect low-activity radiological materials. Radiological background due to naturally occurring radiological materials (NORM) can be estimated by reference to previous survey results, use of global 40K, 238U, and 232Th (KUT) values, reference to existing USGS radiation background maps, or by a moving average of the data as it is acquired. Each of these methods has its drawbacks: previous survey results may not include recent changes, the global average provides only a zero-order estimate, the USGS background radiation map resolutions are coarse and are accurate only to 1 km - 25 km sampling intervals depending on locale, and a moving average may essentially low pass filter the data to obscure small changes in radiation counts. Imaging spectroscopy from airborne or spaceborne platforms can offer higher resolution identification of materials and background, as well as provide imaging context information. AVIRlS hyperspectral image data is analyzed using commercial exploitation software to determine the usefulness of imaging spectroscopy to identify qualitative radiological background emissions when compared to airborne radiological survey data.

The Multi-Isotope Process (MIP) Monitor Project: FY13 Final Report

The Multi-Isotope Process (MIP) Monitor provides an efficient approach to monitoring the process conditions in reprocessing facilities in support of the goal of “… (minimization of) the risks of nuclear proliferation and terrorism.” The MIP Monitor measures the distribution of the radioactive isotopes in product and waste streams of a nuclear reprocessing facility. These isotopes are monitored online by gamma spectrometry and compared, in near-real-time, to spectral patterns representing “normal” process conditions using multivariate analysis and pattern recognition algorithms. The combination of multivariate analysis and gamma spectroscopy allows us to detect small changes in the gamma spectrum, which may indicate changes in process conditions. By targeting multiple gamma-emitting indicator isotopes, the MIP Monitor approach is compatible with the use of small, portable, relatively high-resolution gamma detectors that may be easily deployed throughout an existing facility. The automated multivariate analysis can provide a level of data obscurity, giving a built-in information barrier to protect sensitive or proprietary operational data. Proof-of-concept simulations and experiments have been performed in previous years to demonstrate the validity of this tool in a laboratory setting for systems representing aqueous reprocessing facilities. However, pyroprocessing is emerging as an alternative to aqueous reprocessing techniques.

Computational assessment of the impact of gamma-ray detector material properties on spectroscopic performance

Pacific Northwest National Laboratory (PNNL) is performing a computational assessment of the impact of several important gamma-ray detector material properties (e.g. energy resolution and intrinsic detection efficiency) on the scenario-specific spectroscopic performance of these materials. The research approach combines 3D radiation transport calculations, detector response modeling, and spectroscopic analysis of simulated energy deposition spectra to map the functional dependence of detection performance on the underlying material properties. This assessment is intended to help guide formulation of performance goals for new detector materials within the context of materials discovery programs, with an emphasis on applications in the threat reduction, nonproliferation, and safeguards/ verification user communities. The research results will also provide guidance to the gamma-ray sensor design community in estimating relative spectroscopic performance merits of candidate materials for novel or notional detectors.

Data-Driven Exploration of the Ionization-Phonon Partitioning in Scintillating Radiation Detector Materials

An information-based approach to scintillating materials development has been applied to ranking the alkali halide and alkaline earth halide series in terms of their energy conversion efficiency. The efficiency of scintillating radiation detection materials can be viewed as the product of a consecutive series of electronic processes (energy conversion, transfer, and luminescence) as outlined by Lempicki and others. Relevant data are relatively sparse, but sufficient for the development of forward mapping of materials properties through materials signatures. These mappings have been used to explore the limits of the branching ratio between the ionization and phonons (K) in the Lempicki model with chemical composition, and examine its relationship with another common design objective, density. The alkali halides and alkaline earth halide compounds separate themselves into distinct behavior classes favoring heavier cations and anions for improved values of the K ratio. While the coupling of ionization is strongly related to the optical phonon modes, both dielectric and band gap contributions cannot be ignored. When applied as a candidate screen, the resulting model for K suggests design rules - simple structural restrictions - on scintillating radiation detector materials.

Simulated Performance of the GammaTracker CdZnTe Handheld Radioisotope Identifier

Abstract-The GammaTracker handheld radioisotope identifier currently under development at the Pacific Northwest National Laboratory uses a pixellated CdZnTe spectrometer array to measure the energy and incoming direction of gamma rays from 50 keV to 3 MeV. This instrument, which incorporates the Polaris technology developed at the University of Michigan, houses two 3times3 arrays of 2.25-cm3 CdZnTe detectors. In this work, we present Geant4 simulation results of gamma-ray detection, identification, and directionality performance of the GammaTracker instrument. Estimated minimum detectable activities (MDAs) are determined to be only 20-30% higher than the MDAs predicted for a comparable-efficiency HPGe system over a reasonable energy range. The simulated imaging resolution is determined to be ~20deg FWHM, and the imaging efficiency is evaluated as a function of gamma-ray energy.

Methods and Instruments for Fast Neutron Detection

Pacific Northwest National Laboratory evaluated the performance of a large-area (~0.7 m2) plastic scintillator time-of-flight (TOF) sensor for direct detection of fast neutrons. This type of sensor is a readily area-scalable technology that provides broad-area geometrical coverage at a reasonably low cost. It can yield intrinsic detection efficiencies that compare favorably with moderator-based detection methods. The timing resolution achievable should permit substantially more precise time windowing of return neutron flux than would otherwise be possible with moderated detectors. The energy-deposition threshold imposed on each scintillator contributing to the event-definition trigger in a TOF system can be set to blind the sensor to direct emission from the neutron generator. The primary technical challenge addressed in the project was to understand the capabilities of a neutron TOF sensor in the limit of large scintillator area and small scintillator separation, a size regime in which the neutral particle’s flight path between the two scintillators is not tightly constrained.

The Majorana 76Ge double-beta decay project

Abstract The interest and relevance of next-generation 0 v ββ-decay experiments is increasing. Even with nonzero neutrino mass strongly suggested by solar and atmospheric neutrino experiments sensitive to δm 2 , 0 v ββ-decay experiments are still the only way to establish the Dirac or Majorana nature of neutrinos by measuring the effective electron neutrino mass, 〈 m v 〉. In addition, the atmospheric neutrino oscillation experiments imply that at least one neutrino has a mass greater than about 50 meV. The Majorana Experiment expects to probe an effective neutrino mass near this critical value. Majorana is a next-generation 76 Ge double-beta decay search. It will employ 500 kg of Ge, isotopically enriched to 86% in 76 Ge, in the form of ∼ 200 detectors in a close-packed array. Each crystal will be electronically segmented and each segment fitted with pulse-shape analysis electronics. This combination of segmentation and pulse-shape analysis significantly improves our ability to discriminate neutrinoless double beta-decay from internal cosmogenic 68 Ge and 60 Co . The half-life sensitivity is estimated to be 4.2 × 10 27 y corresponding to a 〈 m v 〉 range of ≤ 20 − 70 meV, depending on the nuclear matrix elements used to interpret the data.