Stephen Croft

Neutron imaging with the short-pulse laser driven neutron source at the Trident laser facility

Emerging approaches to short-pulse laser-driven neutron production offer a possible gateway to compact, low cost, and intense broad spectrum sources for a wide variety of applications. They are based on energetic ions, driven by an intense short-pulse laser, interacting with a converter material to produce neutrons via breakup and nuclear reactions. Recent experiments performed with the high-contrast laser at the Trident laser facility of Los Alamos National Laboratory have demonstrated a laser-driven ion acceleration mechanism operating in the regime of relativistic transparency, featuring a volumetric laser-plasma interaction. This mechanism is distinct from previously studied ones that accelerate ions at the laser-target surface. The Trident experiments produced an intense beam of deuterons with an energy distribution extending above 100 MeV. This deuteron beam, when directed at a beryllium converter, produces a forward-directed neutron beam with ∼5 × 109 n/sr, in a single laser shot, primarily due to ...

Uncertainty quantification for radiation measurements: Bottom-up error variance estimation using calibration information.

One example of top-down uncertainty quantification (UQ) involves comparing two or more measurements on each of multiple items. One example of bottom-up UQ expresses a measurement result as a function of one or more input variables that have associated errors, such as a measured count rate, which individually (or collectively) can be evaluated for impact on the uncertainty in the resulting measured value. In practice, it is often found that top-down UQ exhibits larger error variances than bottom-up UQ, because some error sources are present in the fielded assay methods used in top-down UQ that are not present (or not recognized) in the assay studies used in bottom-up UQ. One would like better consistency between the two approaches in order to claim understanding of the measurement process. The purpose of this paper is to refine bottom-up uncertainty estimation by using calibration information so that if there are no unknown error sources, the refined bottom-up uncertainty estimate will agree with the top-down uncertainty estimate to within a specified tolerance. Then, in practice, if the top-down uncertainty estimate is larger than the refined bottom-up uncertainty estimate by more than the specified tolerance, there must be omitted sources of error beyond those predicted from calibration uncertainty. The paper develops a refined bottom-up uncertainty approach for four cases of simple linear calibration: (1) inverse regression with negligible error in predictors, (2) inverse regression with non-negligible error in predictors, (3) classical regression followed by inversion with negligible error in predictors, and (4) classical regression followed by inversion with non-negligible errors in predictors. Our illustrations are of general interest, but are drawn from our experience with nuclear material assay by non-destructive assay. The main example we use is gamma spectroscopy that applies the enrichment meter principle. Previous papers that ignore error in predictors have shown a tendency for inverse regression to have lower error variance than classical regression followed by inversion. This paper supports that tendency both with and without error in predictors. Also, the paper shows that calibration parameter estimates using error in predictor methods perform worse than without using error in predictor methods in the case of inverse regression, but perform better than without using error in predictor methods in the case of classical regression followed by inversion. Both inverse and classical regression involve the ratio of dependent random variables; therefore, the assumed error distribution(s) will matter in parameter estimation and in uncertainty calculations. Mainly for that reason, calibration using a single predictor is distinct from simple regression, and it has not been thoroughly treated in the literature, nor in the ISO Guide to the Expression of Uncertainty in Measurements (GUM). Our refined approach is based on simulation, because we illustrate that analytical approximations are not adequate when there are, for example, 10 or fewer calibration measurements, which is common in calibration applications, each consisting of measured responses from known quantities.

Performance of a Boron-Coated-Straw- Based HLNCC for International Safeguards Applications

3He gas has been used in various scientific and security applications for decades, but it is now in short supply. Alternatives to 3He detectors are currently being integrated and tested in neutron coincidence counter designs, of a type which are widely used in nuclear safeguards for nuclear materials assay. A boron-coated-straw-based design, similar to the High-Level Neutron Coincidence Counter-II, was built by Proportional Technologies Inc., and has been tested by the Oak Ridge National Laboratory (ORNL) at both the JRC in Ispra and ORNL. Characterization measurements, along with nondestructive assays of various plutonium samples, have been conducted to determine the performance of this coincidence counter replacement in comparison with other similar counters. This paper presents results of these measurements.

K-shell fluorescence yields and their uncertainties for use in hybrid K-edge densitometry

Hybrid K-edge densitometry (HKED) is a non-destructive analytical assay technique used to provide rapid determination of actinide concentration in tank solutions. Of special interest for HKED is the estimation, along with associated uncertainties, of the ratio of the flouresence yeilds, ωK, of uranium and plutonium. Limited experimental data for ωK(Z) as a function of atomic number, Z, exist and the data are subject to experimental uncertainty. Previous studies have provided values for ωK(Z) with uncertainty estimates but have not included covariance information. We use a phenomenological model with a bootstrapping method to generate the ratio ωK(94)/ωK(92) and associated uncertainty.

A Critical Examination of Figure of Merit (FOM). Assessing the Goodness-of-Fit in Gamma/X-ray Peak Analysis

In this paper we develop and investigate several criteria for assessing how well a proposed spectral form fits observed spectra. We consider the classical improved figure of merit (FOM) along with several modifications, as well as criteria motivated by Poisson regression from the statistical literature. We also develop a new FOM that is based on the statistical idea of the bootstrap. A spectral simulator has been developed to assess the performance of these different criteria under multiple data configurations.