Dean J. Mitchell

Gamma-ray response functions for scintillation and semiconductor detectors

Abstract A gamma-ray detector response function model has been developed which enables computation of spectra for a variety of scintillation or high purity germanium detectors over the energy range 50 keV to 3 MeV. A set of 32 empirically determined parameters are used to describe the detector efficiency and interaction event probabilities. The model also computes the effects of scattered radiation, attenuation, coincidence event detection, and random pileup. The accuracy and general applicability of this model for a variety of detector types and configurations surpasses capabilities existing heretofore. The ability to compute accurate spectral templates is a requirement for radiochemical analysis of low resolution spectra using linear regression. An example analysis of a sodium iodide measurement of fallout from the Chernobyl nuclear reactor accident is presented. The detector response model also provides a means of computing the response of a detector to an arbitrary photon flux profile such as leakage spectrum computed by a radiation transport code.

A Framework for the Solution of Inverse Radiation Transport Problems

Radiation sensing applications for SNM detection, identification, and characterization all face the same fundamental problem: each to varying degrees must infer the presence, identity, and configuration of a radiation source given a set of radiation signatures. This is a problem of inverse radiation transport: given the outcome of a measurement, what was the source and transport medium that caused that observation? This paper presents a framework for solving inverse radiation transport problems, describes its essential components, and illustrates its features and performance.

A framework for the solution of inverse radiation transport problems

Radiation sensing applications for SNM detection, identification, and characterization all face the same fundamental problem: each to varying degrees must infer the presence, identity, and configuration of a radiation source given a set of radiation signatures. This is a problem of inverse radiation transport: given the outcome of a measurement, what source terms and transport medium caused that observation? This paper presents a framework for solving inverse radiation transport problems, describes its essential components, and illustrates its features and performance. The framework implements an implicit solution to the inverse transport problem using deterministic neutron, electron, and photon transport calculations embedded in a Levenberg-Marquardt nonlinear optimization solver. The solver finds the layer thicknesses of a one-dimensional transport model by minimizing the difference between the gamma spectrum calculated by deterministic transport and the measured gamma spectrum. The fit to the measured spectrum is a full-spectrum analysis-all spectral features are modeled, including photopeaks and continua from spontaneous and induced photon emissions. An example problem is solved by analyzing a high-resolution gamma spectrometry measurement of plutonium metal.

Irregularities in helium release rates from metal ditritides

Helium release rates from erbium and zirconium ditritide films were measured with 3‐s time resolution. All films exhibited some variability in 3He release rates on this time scale, but the nonuniformity of the release rate was greatest for samples undergoing the transition into accelerated release, which occurs when the occluders approach the maximum quantities of helium that they can retain. The 3He release rate variability appeared to be associated with the spontaneous release of helium in bursts of about 109 atoms. The release of bursts of helium was also stimulated by vibrating or flexing the film substrates. Although these effects are not expected based on the diffusion of isolated helium atoms of the lattice, the effects of temperature on helium release rates were consistent with the diffusion model. These observations lead to the conclusion that two distinct mechanisms are responsible for the release of helium from the metal ditritide films.

Neutron Detection With Gamma-Ray Spectrometers for Border Security Applications

Development of technologies for neutron detection that do not require 3He is important because the supply of 3He is very limited, and the cost of the gas is becoming prohibitive for many applications. This study evaluates the ability to detect neutron sources with gamma-ray spectrometers that are already present in many radiation measurement systems. Detection is based on count rates for gamma rays in the 3 to 8 MeV range, which are produced by the emission of fission gamma rays and neutron capture reactions in vehicles and their cargo. For materials in the normal stream of commerce, gamma rays above 3 MeV are produced only by sources that also emit neutrons. Therefore, unless the gamma-ray count rate is high enough to produce excessive random pileup, the detection of high-energy gamma rays provides an unambiguous indication of the presence of a neutron source. As part of this investigation, several shields that are suitable for use in radiation portals were constructed and characterized for their abilities to produce additional high-energy, neutron-capture gamma rays. A shield (composed of alternating layers of polyethylene and steel) enhances the ability to detect neutrons without producing detrimental effects for gamma-ray measurements. Calculations show that when shielded by neutron-detection-enhancing materials, NaI detectors can be as sensitive to the presence of a concealed neutron source as moderated 3He detectors.

Deuterium permeation through copper with trapping impurities

The time dependence of the deuterium permeation rate through impurity‐doped copper membranes was measured in the temperature range 300–700 °C. Copper membranes that were doped with Er, Zr, and Ti all exhibited permeabilities that were nearly equal to pure copper, but the apparent diffusivities were smaller than those for pure copper by factors of 10–100 over the experimental temperature range. The permeation characteristics of these alloys appear to be altered from those for pure copper due to trapping of deuterium at sites that are associated with the impurity atoms. It is shown that the deuterium permeation rate through the copper alloys can be expressed in an analytical form that is analogous to that for pure copper, except that the apparent diffusivity takes on a value which depends on the trap concentration and binding energy for deuterium. The binding energies that are calculated for the alloys are used to determine the lag time which is required for deuterium or hydrogen to permeate through initial...

Implementation and testing of a multivariate inverse radiation transport solver.

Detection, identification, and characterization of special nuclear materials (SNM) all face the same basic challenge: to varying degrees, each must infer the presence, composition, and configuration of the SNM by analyzing a set of measured radiation signatures. Solutions to this problem implement inverse radiation transport methods. Given a set of measured radiation signatures, inverse radiation transport estimates properties of the source terms and transport media that are consistent with those signatures. This paper describes one implementation of a multivariate inverse radiation transport solver. The solver simultaneously analyzes gamma spectrometry and neutron multiplicity measurements to fit a one-dimensional radiation transport model with variable layer thicknesses using nonlinear regression. The solvers essential components are described, and its performance is illustrated by application to benchmark experiments conducted with plutonium metal.

Permeation characteristics of some iron and nickel based alloys

The permeation characteristics of deuterium in several iron and nickel based alloys were measured by the gas phase breakthrough technique in the temperature range 100 to 500 °C with applied pressures ranging from 10 Pa to 100 kPa. The restriction of the gas flux imposed by surface oxides was modeled in order to evaluate the effects of surface oxide retardation of the gas flux on the effective values of the deuterium permeabilities and diffusivities in the alloys. The most permeable alloys were 430 and 431 stainless steels. The next most permeable alloy was Monel K‐500, which exceeded the permeability of pure Ni by more than a factor of five at room temperature. The alloys with permeabilities less than pure Ni were, in order of decreasing permeability: the Inconels 625, 718, and 750, the Fe‐Ni‐Co glass‐sealing alloys Kovar and Ceramvar, and the 300‐series stainless steels. Deuterium trapping within the alloys appeared to influence the values of bulk diffusivities, which were not correlated with either the ...

Benchmarks for GADRAS performance validation.

The performance of the Gamma Detector Response and Analysis Software (GADRAS) was validated by comparing GADRAS model results to experimental measurements for a series of benchmark sources. Sources for the benchmark include a plutonium metal sphere, bare and shielded in polyethylene, plutonium oxide in cans, a highly enriched uranium sphere, bare and shielded in polyethylene, a depleted uranium shell and spheres, and a natural uranium sphere. The benchmark experimental data were previously acquired and consist of careful collection of background and calibration source spectra along with the source spectra. The calibration data were fit with GADRAS to determine response functions for the detector in each experiment. A one-dimensional model (pie chart) was constructed for each source based on the dimensions of the benchmark source. The GADRAS code made a forward calculation from each model to predict the radiation spectrum for the detector used in the benchmark experiment. The comparisons between the GADRAS calculation and the experimental measurements are excellent, validating that GADRAS can correctly predict the radiation spectra for these well-defined benchmark sources.

GADRAS Detector Response Function

The Gamma Detector Response and Analysis Software (GADRAS) applies a Detector Response Function (DRF) to compute the output of gamma-ray and neutron detectors when they are exposed to radiation sources. The DRF is fundamental to the ability to perform forward calculations (i.e., computation of the response of a detector to a known source), as well as the ability to analyze spectra to deduce the types and quantities of radioactive material to which the detectors are exposed. This document describes how gamma-ray spectra are computed and the significance of response function parameters that define characteristics of particular detectors.