"Angular Anisotropy of Correlated Neutrons in Lab Frame of Reference and Application to Detection and Verification,"
M.S. Thesis, Nuclear Engineering, Texas A&M University, College Station, TX (2012).
It has been shown that neutrons emitted from the same 252Cf fission
event are preferentially detected within small angles of each other
and at angles around 180 degrees. The distribution of this angular
anisotropy is dependent upon the nuclide emitting the neutrons.
Coincident neutrons can be detected from a shielded source, so a
study of the angular anisotropy between coincident neutrons is
useful for this context. This could allow for the dynamic
determination of the ratio of the rate of (alpha,n) neutron
production to the spontaneous fission neutron production
(designated alpha) used in neutron coincidence counting for
safeguards. This could also be used to identify neutron emitting
isotopes in a homeland security application. An angular frequency
distribution for coincident neutrons was produced via experiments
using an array of cylindrical liquid scintillators and a 252Cf
source. It was found, in accordance with previous experiments, that
the angular frequency distribution peaks at small angles and at
angles around 180 degrees. A Monte Carlo, physics-based simulation
program was created to simulate the distribution of angles between
neutrons from the same fission event from 252Cf and 240Pu sources.
The resulting distributions were clearly distinguishable from each
other. The code was benchmarked to measured results from a 252Cf
source at Lawrence Livermore National Laboratory. Knowledge of the
unique angular distributions of coincident neutrons from various
fissioning sources is useful for identification and verification
purposes. Another practical application of angular anisotropy
information for coincident neutrons from a given source is
determining the ratio of the (alpha,n) to spontaneous fission rates
for a source undergoing neutron coincidence counting. The utility
of this was verified by using measurements made by faculty and
students of the University of Michigan Nuclear Engineering
Department for a MOX fuel pin at the Joint Research Center in
Ispra, Italy. Good agreement between the predicted and declared
values for alpha was found.
Associated Project(s):SHIELD (Smuggled HEU Interdiction through Enhanced anaLysis and Detection): A Framework for Developing Novel Detection Systems Focused on Interdicting Shielded HEU