Improving plastic scintillators for detection of neutrons and gamma rays

Abstract

Detection of special nuclear materials (SNMs) at borders and ports of entry is critical to ensuring peaceful use of fissile materials. Plastic scintillators are viable option for first line of detection for these materials due to their low cost and scalability. Based on common polymers such as poly(vinyl toluene) (PVT) or polystyrene, plastic scintillators can distinguish between neutron and gamma radiation via a technique called pulse shape discrimination (PSD). PSD capable plastic scintillators allow for more accurate detection of SNMs and would theoretically reduce the number of “nuisance alarms,” triggered by naturally occurring radioactive materials. Unfortunately, the high concentration (> 20 weight%) of fluorescent molecules required for PSD leads to unfavorable plasticizing effects like softening, low glass transition temperature, and dopant precipitation and leaching. Additionally, the best quality scintillators are achieved through a tedious oxygen-free, 5-7 day thermal polymerization. This dissertation describes the enhancement of the material properties and PSD capabilities of plastic scintillators using chemistry, polymer, and material science techniques. Initially the aim was to manipulate and improve the material properties through established polymer science strategies. Cross-linking the traditional PVT matrix with an aromatic dimethacrylate increased the hardness and reduced dopant aggregation without sacrificing radiation detection capabilities in PSD capable plastic scintillators. Building off the methacrylate motif, novel dopants with polymerizable methacrylate groups were synthesized. By co-polymerizing the dopant with the vinyl toluene matrix, dopant aggregation and leaching were eliminated and hardness and glass transition temperature increased without losing PSD capabilities. In order to broaden the application of PSD capable plastic scintillators, polysiloxanes were explored as an alternative to the traditional PVT matrix. The elastomeric polysiloxane scintillators required only 5 weight% or less of dopants to achieve neutron and gamma ray discrimination comparable to commercial PVT scintillators. Additionally, the polysiloxane scintillator could be fabricated in 3 hours in air, compared to the traditional 5-7 day process in oxygen free environments typically required for PVT scintillators. Both switching the polymer matrix and altering the traditional PVT matrix demonstrated that common polymer science techniques could be applied to plastic scintillators to balance material and radiation detection properties. The discovery of the dimethacrylate cross-linker also afforded the opportunity to explore alternative fabrication techniques. Unlike vinyl toluene or styrene, methacrylate groups can be easily photopolymerized. The second portion of this thesis applied established photopolymerization formulations to plastic scintillators in order to fabricate PSD capable plastics in 1 day via photoinitiation, compared to the traditional 5-7 day process. Plastic scintillators fabricated via photopolymerization had comparable radiation detection capabilities and material properties to traditional thermally polymerized analogues. The final part of the dissertation forays into the potential for gamma ray spectroscopy plastic scintillators. Incorporating high Z elements into plastic scintillators can result in the photoelectric effect, providing spectroscopic information on the gamma ray source and improving detection of SNMs. Three classes of bismuth dopants, triaryl, trialkoxy, and nanoparticles were explored as potential high Z dopants for plastic scintillators. Functionalized bismuth oxide nanoparticles were the most promising high Z dopants due to their combination of scalability and dispersibility in PVT matrices. To conclude, this dissertation highlights how an interdisciplinary approach to plastic scintillators can lead to a balance between material properties and radiation detection capabilities. Chemistry, materials, and polymer science techniques such as dopant synthesis, cross-linking, photopolymerization, and co-polymerization originally developed for other applications can be applied to PSD capable plastic scintillators to increase hardness, decrease dopant aggregation, and decrease fabrication time.