Fast neutron multiplicity counter based on stilbene and EJ-309 detectors

Year
2015
Author(s)
D.L. Chichester - Idaho National Laboratory
S. D. Clarke - University of Michigan
M. M. Bourne - Department of Nuclear Engineering & Radiological Sciences, University of Michigan
S. A. Pozzi - Department of Nuclear Engineering & Radiological Sciences, University of Michigan
C.S. Sosa - University of Michigan
T. H. Shin - Department of Nuclear Engineering & Radiological Sciences
A. Di Fulvo - Department of Nuclear Engineering and Radiological Science, University of Michigan
Abstract
?Technology is needed to prevent diversion of fissile material and further special nuclear material accounting, control, and safeguards efforts under the Treaty on the Nonproliferation of Nuclear Weapons. Most measurement systems used in these applications still rely on He-3, whose shortage drives the need of a replacement technology for neutron detection in the near future. In He-3 counters, fission neutrons need to be moderated. Complex electronic circuitry and unfolding procedures are also needed to derive fission correlated counts from detected neutrons in order to quantify fissile mass. Fast neutron counting systems are capable of shortening the coincidence window by several orders of magnitude and thereby provide a direct estimate of fissile mass. We present the design and experimental validation of a fast-neutron multiplicity counter, using a matrix of liquid and stilbene scintillators. The system is based on eight 7.62 cm x 7.62 cm EJ-309 liquid and eight 5.08 cm x 5.08 cm stilbene detectors, arranged in a checkerboard assembly. Given the superior gamma-neutron discrimination capabilities of stilbene, assay of higher-order multiplicity and of samples with high (a,n) source terms are made possible, suitable for the analysis of newer fuels containing high concentrations of plutonium and other fissionable actinides. The system measured separate Cf-252 and PuBe sources to evaluate its capability to detect and discriminate fission neutrons. Neutron, photon, and joint neutron and photon multiplicities were measured, acquiring detected pulses by means of fast electronic digitizers (sampling rate up to 500 MHz). Measured results have been also used to validate a Monte Carlo model of the system, developed in MCNPX-PoliMi code, and to fully characterize signal crosstalk component due to false positive coincidence counts following scattering in nearest-neighbor detectors. The detailed Monte Carlo model also allowed optimizing crosstalk rejection during system design stage. In the full paper, we show that the fast neutron multiplicity counter presented in this work achieves good efficiency for the measurement of neutron, photon, and joint neutron and photon multiplicities, and is a promising alternative to He-3 based systems.