DOE OSTI2013
This brief report describes commissioning measurements performed using 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 M3FT-13OR0402054, “Imager commissioning and 3D source configuration measurements” of Fuel Cycle R&D Material Protection, Accounting, and Control Technology (MPACT) work package FT-13OR040205, “Fast Neutron Imaging to Quantify Nuclear Materials - ORNL.” In addition to representing the first measurements made with the new device, these measurements also demonstrate for the first time the ability to localize one or more neutron sources in three dimensions by combining imaging data from multiple measurements in which the imager was positioned at different locations. The development and use of fast-neutron imaging technology are motivated by a desire to locate and quantify plutonium holdup in scenarios where contact measurements with non-imaging detectors may be impractical or their results ambiguous because of surrounding material or nearby sources. Considering the goal of quantitative measurements, fast neutrons (as opposed to the more numerous gamma rays) are preferred in many situations because of their ability to penetrate equipment and nuclear material that are essentially opaque to gamma radiation. However, there are many challenges associated with constructing a practical imager for fast neutrons. For example, the ability of fast neutrons to penetrate material also leads to challenges in constructing a fast-neutron imager that is sufficiently compact to be suitable for use in a facility while remaining sufficiently sensitive to neutron emanations from plutonium or other actinides. Moreover, use of the imager in a production environment favors mechanically robust detectors that do not contain flammable liquid, unlike the organic liquid scintillators that are typically used to achieve fast-neutron detection. Last of all, a single-sided measurement geometry, as is typically used in imaging applications, necessitates accurate measurement of the location of a deposit of plutonium holdup in three dimensions in order to correctly infer the source strength. 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. This report presents a description and preliminary analysis of mock holdup measurements intended to determine the three-dimensional location of one or more neutron sources placed in prototypical equipment from a set of imaging measurements that scanned the extent of the equipment.