Nanocomposite Scintillators for Neutron Capture Measurements.

Abstract

Nanocomposite scintillators, consisting of nanoscale (< 50 nm diameter) particles suspended in a matrix material, may extend both the number of scintillating materials that can be practically employed in radiation measurements and the range of scintillator applications. Current neutron capture cross-section experiments are unable to measure the cross-sections of isotopes with half-lives shorter than a few hundred days due to the slow signal decay times of the scintillators used. Cerium fluoride (CeF3) has been identified as a possible substitute, but CeF3 crystals are not currently available in large sizes or quantities. CeF3 nanoparticles with sizes < 20 nm were fabricated and suspended in a liquid scintillator as a nanocomposite detector. The structural properties of the nanoparticles and the optical properties and radiation response of the suspensions were characterized and used to optimize the mass loading of CeF3 in the nanocomposite detectors. An optimized detector was tested in a neutron beam line; compared with a BaF2 detector module, the CeF3 nanocomposite scintillator was able to detect neutron capture resonances from neutrons with energies one order of magnitude higher. Neutron flux measurements for beam line experiments are commonly performed using fission chambers, gas-filled detectors containing one or more foils that are thinly coated with a fissionable material. The design requirements of the fission chamber result in low efficiency and long signal rise times. A faster and more efficient neutron flux monitor could be created by loading fissionable nanoparticles into a scintillating matrix. As a proof of principle, a fissionable molecular complex was created and loaded into a liquid scintillator. Optical characterization techniques were used to verify that the scintillator luminescence was unaffected by the presence of the fissionable complex. Characterization in a neutron beam line determined that a liquid scintillator loaded with fissionable material showed a ~400% higher count rate due to fission, compared with an unloaded liquid scintillator, across the energy range of detected neutrons. This research was performed at and funded by Los Alamos National Laboratory. This document is available as Los Alamos Unlimited Release 10-07184.PhDNuclear ScienceUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/84605/1/stange_1.pd