Due to increasing terrorist threats and illegal proliferation of nuclear material and technology, there is a need for increased research in the area of detection of smuggled fissile material, some of which is designated by the International Atomic Energy Agency as special fissile material. This thesis focuses on a hypothetical scenario in which a terrorist organization has managed to smuggle an amount of special fissile material onto a personal recreational watercraft and sail it into a marina. If the boat could be forced to go through a detector system, then the contents could be interrogated and a determination made of whether any special fissile material was aboard. This thesis examines the hypothesis that active interrogation may be used successfully in the detection of special fissile material in such an environment. It shows that it is feasible to use an active neutron system to detect a significant quantity of special fissile material onboard a small boat via the differential dieaway technique. The MCNP Monte Carlo transport code was used to simulate the use of a pulsed neutron generator to induce fission in the fissile material and then estimate the detector response. The detector modeled was based on elastic scattering-induced recoil protons using pure hydrogen gas. There was a significant difference between the system with and without the presence of fissile material, and the estimated detector response for the system with fissile material present was shown to be sufficiently greater than the response due to background radiation only. Additionally, dose was estimated and found to be small enough that the system would not likely pose a significant radiological health risk to passengers on the boat.

La détection de rayonnements ionisants - notamment les neutrons de basse énergie qui sont la signature de matériaux fissibles - est un enjeu majeur pour des applications duales civiles et militaires. La raréfaction de l’hélium 3 indispensable à la production de détecteur nécessite la découverte et la mise au point d’alternative fiable. Le développement de prototypes de détecteurs pour la caractérisation de neutrons permet de répondre aux risques d’accidents technologiques et de terrorisme. Mais ils permettent également l’amélioration de l’analyse de radiations ionisantes en recherche fondamentale. Une approche convergente de la synthèse des matériaux salins luminescents permet d’adresser à la fois la détection et la discrimination de rayonnements ionisants mais également de développer de nouveaux matériaux pour la détection de petites molécules en phase gazeuse. Les matériaux ioniques à base de sels d’imidazolium présentent des propriétés physiques remarquable tout en ayant une structure chimique malléable à façon. Ce projet vise à explorer de nouvelles méthodes pour la synthèse de sels d’imidazolium luminescents et leurs propriétés en détection. / Detector of ionizing radiation is a key challenge for both civil and military applications. New prototype of ionizing radiation discrimination helps us against technological accident and terrorism act. For instance, neutron of low energy is sign of fissile material. However current detectors require helium 3 which is scarce resource. Thus, a sustainable alternative has to be found. A convergent approach to synthesis active luminescent salt materials cope both. It identities and discriminate ionizing radiation. Especially for small cell in gas phase, this approach allows to develop new material to detect them. Based on imidazolium salt, ionic material has remarkable property with the benefit of being scalable.This project aims to explore new synthesis methods of luminescent imidazolium and their properties to identify ionizing radiation characteristic.

Neutron radiation detectors are an integral part of the Department of Homeland Security (DHS) efforts to detect the illicit trafficking of radioactive or special nuclear materials into the U.S. In the past decade, the DHS has deployed a vast network of radiation detection systems at various key positions to prevent or to minimize the risk associated with the malevolent use of these materials. The greatest portion of this detection burden has been borne by systems equipped with 3He because of its highly desirable physical and nuclear properties. However, a dramatic increase in demand and dwindling supply, combined with a lack of oversight for the existing 3He stockpile has produced a critical shortage of this gas which has virtually eliminated its viability for detector applications. A number of research efforts have been undertaken to develop suitable 3He replacements; however, these studies have been solely targeted toward simple detection cases where the overall detection efficiency is the only concern. For these cases, an insertion of additional detectors or materials can produce reaction rates that are sufficient, because the neutron spectral response is essentially irrelevant. However, in applications such as safeguards, non-proliferation efforts, and material control and accountability programs (MC&A), a failure to use detectors that are spectrally matched to 3He can potentially produce dire consequences. This is because these more difficult detection scenarios are associated with fissile material assessments for 239Pu and other actinides and these analyses have almost universally been calibrated to an equivalent 3He response. In these instances, a “simple” detector or material addition approach is neither appropriate nor possible, due to influences resulting from the complex nature of neutron scattering in moderators, cross sections, gas pressure variations, geometries, and surrounding structural interference. These more challenging detection cases require a detailed computational transport analysis be performed for each specific application. A leveraged approach using adjoint transport computations that are validated by forward transport and Monte Carlo computations and laboratory measurements can address these more complex detection cases and this methodology was utilized in the execution of the research. The initial task was to establish the fidelity of a computational approach by executing radiation transport models for existing BF3 and 3He tubes and then comparing the modeling results to laboratory measurements made using these identical devices. Both tubes were 19.6 cm in height, 1-inch in diameter, and operated at 1 and 4 atm pressure respectively. The models were processed using a combination of forward Monte Carlo and forward and adjoint 3-D discrete ordinates (SN) transport methods. The computer codes MCNP5 and PENTRAN were used for all calculations of a nickel-shielded plutonium-beryllium (PuBe) source term that provided a neutron output spectra equivalent to that of weapons-grade plutonium (WGPu). Once the computational design approach was validated, the adjoint SN method was used to iteratively identify six distinct plug-in models that matched the neutron spectral response and reaction rate of a 1-inch diameter 3He tube with a length of 10 cm and operating at 4 atm pressure. The equivalent designs consist of large singular tubes and dual tubes containing BF3 gas, 10B linings, and/or 10B-loaded polyvinyl toluene (PVT). The reaction rate for each plug-in design was also verified using forward PENTRAN and MCNP5 calculations. In addition to the equivalent designs, the adjoint method also yielded various insights into neutron detector design that can lead to additional designs using a combination of different detector materials such as BF3/10B-loaded PVT, 10B-lined tubes/10B-loaded PVT, etc.