Neutron Scintillation Detectors Based on Polymers Containing Lithium-6 for Radiation Portal Monitor Applications

The work presented herein describes an investigation of four main types of thin film polymer scintillators containing ⁶Li [lithium-6] for neutron detection: polystyrene containing ⁶LiF [lithium-6 fluoride] and a preblended fluor mixture comprising 2,5-diphenyloxazole and 1,4-bis(5-phenyloxazol-2-yl)benzene; poly(styrene-co-lithium maleate) containing salicylic acid; poly[styrene-co-lithium maleate-co-2-phenyl-5-(4-vinylphenyl)oxazole]; and poly(styrene-co-lithium 4-vinylbenzoate). A variety of chemical and physical characterizations as well as optical and scintillation characterizations were performed to guide the development of optimized compositions of each type of polymer film. The scintillation performances of optimized compositions of each type of polymer film were calibrated using GS20 lithium glass and evaluated against neutron detection and neutron/gamma-ray discrimination criteria established for radiation portal monitors.

Thin films were fabricated 2 inches in diameter over a variety of thicknesses using solution-casting methods. Investigation of polystyrene-based films by photoluminescence and scintillation indicated that the optimum concentration of fluor was 5.00%. Optimum neutron/gamma-ray discrimination was achieved for 50 micrometer thick films containing 10% ⁶LiF [lithium-6 fluoride]. Two transparent lithium-containing polymers were successfully synthesized: poly(styrene-co-lithium maleate) containing salicylic acid and poly[styrene-co-lithium maleate-co-2-phenyl-5-(4-vinylphenyl)oxazole]. To the author’s knowledge, the latter polymer represents the first polymer comprising the matrix, the thermal neutron capture nuclide ⁶Li [lithium-6], and the fluor that has been synthesized for the purpose of thermal neutron detection. The polymer poly(styrene-co-lithium 4-vinylbenzoate) could not be solvated and decomposed below its melting temperature and was thus considered not useful for this application. The polystyrene-based materials had the greatest light yields whereas the poly[styrene-co-lithium maleate-co-2-phenyl-5-(4-vinylphenyl)oxazole] material had the best neutron/gamma-ray discrimination properties. All three classes of materials can be used to satisfy the detection criteria in the current radiation portal monitor footprint by implementing a multilayer format.