Novel small molecules for detection of sensitive nuclear materials

Abstract

Plastic scintillation is a simple detection process for sensitive nuclear materials such as uranium and plutonium isotopes. The material consists of a polymer matrix, a fluorescent primary dopant and a wavelength shifter and is directly connected to a photomultiplier tube. The matrix interacts with the incident radiation and transfers the absorbed energy to the primary dopant that then transfers some of the energy to the wavelength shifter. The cascade of energy transfers determines the efficiency of detection that currently is not on par with other radiation detectors e.g. crystal scintillators or gas-filled proportional counters in terms of energy resolution and neutron sensitivity. However, plastic scintillators are inexpensive, mass producible in different shapes and forms, and safe for transportation and field deployment. The efficient detection of sensitive nuclear materials that emit gamma and fast neutrons via plastic scintillator would provide an affordable and safe alternative to the existing state-of-the-art radiation detectors. To achieve a comparable level of detection, the understanding of the fundamental radiation detection process in plastic scintillator needs to be investigated further and more thoroughly. In this thesis, two methods were explored to achieve enhanced radiation detection versus commercial products: 1. detection of mixed radiation fields (neutron and gamma) via over-doping of fluorescent aromatic small molecules and 2. detection of thermal neutrons via incorporation of boron containing aromatic small molecules. The over-doping method led us to explore several classes of fluorescent small molecules synthetically and computationally. These molecules were p-terphenyl (PTP), fluorene and 2,5-diphenyloxazole (PPO) derivatives. The solubility limit of para-alkylated and meta-alkylated PTP derivatives in polyvinyltoluene (PVT) were inversely correlated with the respective melting points of the derivatives. The para-alkylated PTP derivatives precipitated from the PVT matrix around 7 wt% despite showing promising radiation detection. On the other hand, meta-alkylated PTP had solubility limits up to 20 wt% in PVT with surprisingly poor mixed radiation discrimination. The reason for their poor performance was attributed due to the softening of the plastic matrix at higher dopants concentrations. This reason was confirmed by ground state geometry calculations that showed PTP derivatives as non-planar dopants prone to vibrationally dissipate triplet state energies. Experimentally, this was proved by the near identical discrimination of PPO and one of the meta-alkylated PTP mixed radiation discrimination in poly(methyl methacrylate). Several fluorene derivatives were synthesized via simple chemistry with good to excellent yields. Half of the derivatives were planar and the other half were non-planar. The derivatives all had different optical and physical properties that determined their radiation detection performance. In these derivatives, material impurity (< 0.2%) drastically impacted their performance. Also, the isomeric effect on the detection efficiency was explored. Similar to PTP derivatives, the inverse correlation between melting point of derivatives and their solubility limit in PVT were observed. One of the fluorene derivatives demonstrated the second best mixed radiation discrimination reported in literature. In addition, two of the fluorene derivatives showed superior mechanical hardness when overdoped compared to corresponding PPO concentrations. Encouraged by this result, high melting point PPO derivatives were synthesized, one of which showed superior mechanical hardness compared to PPO while demonstrating comparable radiation discrimination. The detection of thermal neutrons was explored by both commercially available, and synthesized boron containing dopants. The synthesized boron dopants were produced via both microwave and conventional heating modified Miyaura borylation conditions from multi-halo functional benzene and pyrene starting materials. Non-symmetrical boron dopants showed high solubility and better overall thermal neutron detection. 10B enriched small molecule showed similar result as commercial scintillator but it needs to be explored more. Boron containing pyrene molecules developed a deep yellow color and demonstrated poor detection efficiency.