Basic research into performance improvements in plastic scintillator materials for homeland security

Abstract

Plastic scintillators are currently deployed around the world in first line radiation detectors for international borders, sensitive nuclear sites, national and academic lab settings. These scintillators are simple in their composition and their function, being composed of a common polymer matrix which has been doped with a small percentage of fluorescent molecules. This product fluoresces when radiation is present and incident on the plastic. This fluorescence is then detected by photodetectors which are coupled to the plastic. Currently, these detector systems are unable to provide any spectroscopic or particle identification information, and therefore can only be used for initial screening purposes. Further information about the radiation after a positive response is gleaned by using additional detector systems (e.g. sodium iodide crystals and then followed by HPGE detectors). In this dissertation, two broad basic research approaches were explored to achieve a better understanding of these systems with the goal of enhancing them for better radiation detection capabilities. The first approach involved enhancing the plastic scintillators' sensitivity to both fast and thermal neutrons, allowing for particle identification and a reduction in false positive detections of naturally occurring radioactive material (NORM). This was achieved via admixture of several different boron containing materials into the plastic scintillator's basic formulation, allowing for both thermalization of a fast neutron spectrum via the (n,p) scattering reaction in the hydrogenous bulk matrix, and then a coincident signal of thermal neutron capture on the highly neutron sensitive 10B isotope. This effect was further enhanced by incorporating a recently identified method of inducing PSD capabilities into plastic scintillators, an analysis that has traditionally only been able to be performed with liquid organic scintillators or certain crystalline scintillators. Synthesized enriched 10B molecules compatible with common polymer matrices and liquid scintillator solvents were developed, a well studied, a commonly available and cheap boron containing chemical precursor was identified which can be quickly and easily admixed into basic scintillator formulations, and finally, a family of aromatic, boron containing molecules which can be synthesized in both 10B enriched or natural boron variants has been identified and studied for effective use in plastic scintillators. The second broad approach of research was aimed at furthering the understanding of the scintillation process and specifically testing the current theory of why pulse shape discrimination (PSD) capabilities occur. This was examined by altering several different families of fluorescent dopants, extensively cataloging both the dopant properties and the properties of the final scintillator plastic they produced. The results from these experiments will be useful to guide future research towards the ability of designing specific scintillator properties.