A structured organic scintillator for neutron imaging

Modern day research in a variety of disciplines requires sensitive neutron detectors having a high spatial resolution that are able to discriminate against unwanted gamma ray events. A novel detector concept comprising cylindrical plastic scintillators embedded in a fiber optic array was designed and developed to determine if the desired performance can be attained. Monte-Carlo design studies were performed to optimize detector geometry and calculate the anticipated neutron detection efficiency. Prototype detectors were characterized by measuring detection efficiency and pulse height spectra using isotopic neutron and gamma-ray sources. Images of neutron absorbing phantoms were acquired with a lens-coupled EMCCD viewing the sample during neutron irradiation via a surface coated 45° mirror. Specimens with areas up to 6 × 6 cm² were fabricated by submerging fiberoptic arrays with a capillary diameter of 100 μm and a relative open area of 83% in a polymer of the desired mixture and curing slowly. Increasing the concentration of boron was observed to reduce light output by as much as 28% for the highest boron concentrations tested. Pulse height spectra from the samples exhibited a peak characteristic of neutron capture in boron at an energy (electron equivalent) of approximately 90 keV and the detection efficiency for a 5.7 mm thick scintillator containing 5% o-carborane was 40%, which agrees with calculations. The spatial resolution of these detectors is 250 μm as determined from the modulation transfer function at 10% modulation.