Engineering topics

Mitchell, Dean James

Publications and source records attributed to Mitchell, Dean James.

3 records

DOE OSTI2012

Design and Characterization of a Sodium Iodide (NaI) Array for Coincidence Measurements

An array of sodium iodide detectors (NaI) was constructed to record time-correlated gamma ray measurements. The NaI Array consists of twelve 2×4×16-inch detectors. When the array is placed close to an inspected object, this arrangement provides high efficiency for gamma ray detection, particularly for high-energy gamma rays. Data are collected in time-tagged, list-mode format, which is suitable for the application of a variety of coincidence analysis methods. Automated energy calibration of each detector is accomplished by utilizing gamma radiation from thoriated welding rods that are embedded in the system. The energy resolution of spectra that are produced by the NaI array is 7.3% at 662 keV. The data are processed to sum gamma-ray energies for simultaneous detections in adjacent detectors, which are dominated by the escape of scattered photons from one detector and subsequent absorption in an adjacent detector. When data are processed in this way, the photopeak efficiency exceeds 20% over the range 400 keV to 10 MeV; the total efficiency, which includes the Compton continuum and single and double escape peaks, exceeds 70% over this energy range. Utilization of time correlations enables isolation of spectral features that are associated with the same decay cascade or fission event. Characterization of the response of the NaI Array to both gamma rays and neutrons with the Gamma Detector Response and Analysis Software (GADRAS) enables accurate computation for virtually any radiation sources.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2012

Summary of Alpha-Neutron Sources in GADRAS

A common source of neutrons for calibration and testing is alpha-neutron material, named for the alpha-neutron nuclear reaction that occurs within. This material contains a long-lived alpha-emitter and a lighter target element. When the alpha particle from the emitter is absorbed by the target, neutrons and gamma rays are released. Gamma Detector Response and Analysis Software (GADRAS) includes built-in alpha-neutron source definitions for AcC, AmB, AmBe, AmF, AmLi, CmC, and PuC. In addition, GADRAS users may create their own alpha-neutron sources by placing valid alpha-emitters and target elements in materials within their one-dimensional models (1DModel). GADRAS has the ability to use pre-built alpha-neutron sources for plotting or as trace-sources in 1D models. In addition, if any material (existing or user-defined) specified in a 1D model contains both an alpha emitter in conjunction with a target nuclide, or there is an interface between such materials, then the appropriate neutron-emission rate from the alpha-neutron reaction will be computed. The gamma-emissions from these sources are also computed, but are limited to a subset of nine target nuclides. If a user has experimental data to contribute to the alpha-neutron gamma emission database, it may be added directly or submitted to the GADRAS developers for inclusion. The gadras.exe.config file will be replaced when GADRAS updates are installed, so sending the information to the GADRAS developers is the preferred method for updating the database. This is also preferable because it enables other users to benefit from your efforts.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2012

PVT-NG Sensor Final Report

This document is a final report for the polyvinyl toluene (PVT) neutron-gamma (PVT-NG) project, which was sponsored by the Domestic Nuclear Detection Office (DNDO). The PVT-NG sensor uses PVT detectors for both gamma and neutron detection. The sensor exhibits excellent spectral resolution and gain stabilization, which are features that are beneficial for detection of both gamma-ray and neutron sources. In fact, the ability to perform isotope identification based on spectra that were measured by the PVT-NG sensor was demonstrated. As described in a previous report, the neutron sensitivity of the first version of the prototype was about 25% less than the DNDO requirement of 2.5 cps/ng for bare 252 Cf. This document describes design modifications that were expected to improve the neutron sensitivity by about 50% relative to the PVT-NG prototype. However, the project was terminated before execution of the design modifications after portal vendors demonstrated other technologies that enable neutron detection without the use of 3 He. Nevertheless, the PVT-NG sensor development demonstrated several performance goals that may be useful in future portal designs.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗