Neutron Response Vectorization and Optimization of Moderating Neutron Spectrometers for Active Interrogation

Abstract

The detection of special nuclear material (SNM) is no trivial task; the low activity of isotopes such as 235U and the ease of shielding make their passive detection virtually impossible in the maritime environment. Active interrogation, while increasing the radiative signature, also increases the radiative background such that measuring SNM signatures becomes difficult, if not impossible, especially given an unknown and complex environment. Over the last two decades, numerous active interrogation methods and technologies have been proposed, many of which reduce or filter the relevant background by interrogating one type of radiation and measuring another type or property domain. However, by focusing on single properties of the SNM signature (e.g. time or energy) in laboratory conditions, the effectiveness of these methods in dynamic and unknown environments remains inadequate. In order for a method—or combination of methods—to be effective, it must be able to incorporate as much of the SNM signature as possible into the measurement, including the signature’s energy, time, and directional (or spatial) properties. The first part of this thesis provides a review of these methods and their limitations with an emphasis on outlining all of the SNM signature properties available to active interrogation applications. The second part discusses two improvements to multi-detector moderating neutron spectrometers in order to exploit all of these SNM signature parameters: time, energy, and space. The two new methods are: 1) an energy-specific optimization method for application driven spectrometer design via virtual detector simulations and genetic algorithms, and 2) a neutron response vectorization method for determining neutron source location by vectorizing the moderating neutron response functions. Although these new approaches by no means solve the problem of SNM detection, they provide a crucial step in tailoring moderating spectrometers to detecting the SNM signature, upon which future works can expand.