Lee T. Harding

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.

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.

Environment Scattering in GADRAS

Radiation transport calculations were performed to compute the angular tallies for scattered gamma-rays as a function of distance, height, and environment. Greens Functions were then used to encapsulate the results a reusable transformation function. The calculations represent the transport of photons throughout scattering surfaces that surround sources and detectors, such as the ground and walls. Utilization of these calculations in GADRAS (Gamma Detector Response and Analysis Software) enables accurate computation of environmental scattering for a variety of environments and source configurations. This capability, which agrees well with numerous experimental benchmark measurements, is now deployed with GADRAS Version 18.2 as the basis for the computation of scattered radiation.

GADRAS-DRF 18.5 User's Manual

The Gamma Detector Response and Analysis Software - Detector Response Function (GADRAS-DRF) application computes the response of gamma-ray and neutron detectors to incoming radiation. This manual provides step-by-step procedures to acquaint new users with the use of the application. The capabilities include characterization of detector response parameters, plotting and viewing measured and computed spectra, analyzing spectra to identify isotopes, and estimating source energy distributions from measured spectra. GADRAS-DRF can compute and provide detector responses quickly and accurately, giving users the ability to obtain usable results in a timely manner (a matter of seconds or minutes).

GADRAS-DRF user's manual.

The Gamma Detector Response and Analysis Software-Detector Response Function (GADRAS-DRF) application computes the response of gamma-ray detectors to incoming radiation. This manual provides step-by-step procedures to acquaint new users with the use of the application. The capabilities include characterization of detector response parameters, plotting and viewing measured and computed spectra, and analyzing spectra to identify isotopes or to estimate flux profiles. GADRAS-DRF can compute and provide detector responses quickly and accurately, giving researchers and other users the ability to obtain usable results in a timely manner (a matter of seconds or minutes).

Simulating higher-dimensional geometries in GADRAS using approximate one-dimensional solutions.

The Gamma Detector Response and Analysis Software (GADRAS) software package is capable of simulating the radiation transport physics for one-dimensional models. Spherical shells are naturally one-dimensional, and have been the focus of development and benchmarking. However, some objects are not spherical in shape, such as cylinders and boxes. These are not one-dimensional. Simulating the radiation transport in two or three dimensions is unattractive because of the extra computation time required. To maintain computational efficiency, higher-dimensional geometries require approximations to simulate them in one-dimension. This report summarizes the theory behind these approximations, tests the theory against other simulations, and compares the results to experimental data. Based on the results, it is recommended that GADRAS users always attempt to approximate reality using spherical shells. However, if fissile material is present, it is imperative that the shape of the one-dimensional model matches the fissile material, including the use of slab and cylinder geometry.