Passive neutron detection for interdiction of nuclear material at borders

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Abstract

Radiation portal monitor systems based upon polyvinyl toluene scintillator gamma-ray detectors and pressurized 3He-based neutron detector tubes have been deployed to detect illicit trafficking in radioactive materials at international border crossings. This paper reviews the neutron detection requirements and capabilities of passive, as opposed to active interrogation, detection systems used for screening of high-volume commerce for illicit sources of radiation at international border crossings. Computational results are given for the impact of cargo materials on neutron spectra, for the response of various detector geometries, the effects of backgrounds including “ship effect” neutrons, and for simulation of a large neutron detection array.

Introduction

Radiation portal monitor (RPM) systems have been deployed to detect illicit trafficking in radioactive materials at international border crossings over the last several years [1]. Such systems have also long been used for safeguards applications [2] and for screening of scrap metal [3]. These large, passive detection systems are currently based upon panels of polyvinyl toluene plastic scintillator for gamma-ray detection and pressurized 3He-based neutron detector tubes. Radiation alarm algorithms used in such systems are typically based upon the net-counts above background observed in the gamma-ray or neutron detectors. Much of the published information on radiation detection for interdiction, and the research being performed, has centered upon the gamma-ray detection aspects of these systems, since gamma rays are produced by all of the sources of concern for illicit trafficking [4], [5].Sources of most concern include: complete weapons of mass destruction (WMD); improvised nuclear devices (IND); special nuclear material (SNM) for weapons construction, including plutonium and highly enriched uranium (HEU); and material or assemblies for radiological dispersal devices (RDD), also known as dirty bombs. All of these radioactive materials produce a gamma radiation signature, while plutonium, unique in its role as part of a weapon of mass destruction, also emits significant neutron radiation. Of these threats, HEU is perhaps the most difficult to detect because the gamma rays are of low energy, and thus, easily shielded; HEU has a very low emission rate of neutrons.Detection of these threats through their gamma-ray signatures can be limited by a number of factors, including the high level and variability of natural background, the presence of naturally occurring radioactive material (NORM) in commerce [6], [7], the presence of individuals with radionuclide burdens from medical treatments [8], and the impact of cargo on the background environment observed by these detectors [9]. Neutron detection has the advantages of a low natural background, few neutron sources being carried in the normal flow of commerce, and different shielding characteristics compared to gamma rays as will be discussed in detail in subsequent sections.Given the need to interdict such threat sources, the issue addressed in this paper is how best to utilize neutron detection to complement gamma-ray detection. A number of authors have reported on research efforts on active interrogation methodologies where neutron or gamma-ray sources are used to stimulate neutron or gamma-ray responses from threat items hidden in cargo [11], [12], [13], [14], [15]. Such active interrogation systems are not considered here. This paper focuses only on passive detection of neutrons, such as that currently implemented in deployed RPM interdiction systems at US and foreign borders. More complex passive techniques such as coincidence counting are not considered.

The material presented below begins with a discussion of neutron sources and how neutrons are currently detected by RPMs. This is followed by a discussion of backgrounds and how these play into nuisance alarms for deployed systems. These sections serve as a backdrop for the subsequent discussion on how to detect radiological threats. To better understand how neutrons interact with instruments, results are discussed from simulations of neutron sources and detectors, ending with a proposed new detector array for which these insights are utilized.