  Plastic scintillators that can discriminate between gamma rays, fast
neutrons, and thermal neutrons were synthesized and characterized while
considering the balance between processing and performance at the kilogram
scale. These trade-offs were necessitated by the inclusion of 0.1 wt. %
lithium-6 to enable detection of thermal neutrons. The synthesis and processing
of these plastic scintillators on the kilogram scale required consideration of
many factors. First, a comonomer (methacrylic acid) was used to solubilize
salts of lithium-6, which allow for a thermal-neutron capture reaction that
produces scintillation light following energy transfer. Second, scintillation
performance and processability were considered because the increasing content
of the comonomer resulted in a sharp decrease in the light output. The use of
small amounts of comonomer (less than or equal to 3 wt. %) resulted in better
performance but required high processing temperatures. At large scales, these
high temperatures could initiate an exothermic polymerization that results in
premature curing and/or defects. The deleterious effects of the comonomer may
be mitigated by using m-terphenyl as a primary dye rather than
2,5-diphenyloxazole (PPO), which has been traditionally used in organic
scintillators. Finally, the curing environment was controlled to avoid defects
like cracking and discoloration while maintaining solubility of dopants during
curing. For scintillators that were produced from kilogram-scale batches of
precursors, the effective attenuation of scintillation light was characterized.
