A Deployable Fast-Neutron Coded-Aperture Imager for Quantifying Nuclear Material

Abstract

This brief report describes a newly constructed fast-neutron coded-aperture imager intended for performing quantitative measurements of plutonium holdup at fuel cycle facilities. The present report was prepared in fulfillment of milestone M4FT-13OR0402053, “Construct Imager,” of Fuel Cycle R&D Material Protection, Accounting, and Control Technology (MPACT) work package FT-13OR040205, “Fast-Neutron Imaging to Quantify Nuclear Materials - ORNL.” The first measurements taken with the imager will be reported later this year. For quantifying plutonium holdup, fast-neutron imaging is an appealing mode of measurement in scenarios where contact measurements with non-imaging detectors may be impractical or their results ambiguous because of surrounding material or nearby sources. The use of fast neutrons (as opposed to the more numerous gamma rays) is appealing because of their ability to penetrate equipment and nuclear material that is opaque to gamma radiation. However, the ability of fast neutrons to penetrate material also leads to challenges in constructing a fast-neutron imager sufficiently compact to be amenable for use in a facility while remaining sufficiently sensitive to neutron emanations from plutonium or other actinides. Furthermore, while fast-neutron detection is commonly achieved via organic liquid scintillators, use of the imager in a production environment favors mechanically robust detectors that do not contain flammable liquid. In response to these challenges, Oak Ridge National Laboratory (ORNL) has built a fast-neutron imager that has a compact footprint and whose segmented neutron detectors are built from the first cast plate of the recently developed plastic scintillator with pulse shape discrimination properties developed by Lawrence Livermore National Laboratory and commercially available from Eljen Technologies as experimental scintillator EJ-299-34.