MONTE CARLO SIMULATION OF A LIQUID SCINTILLATOR MULTIPLICITY COUNTER

Year
2006
Author(s)
Morag Smith - Los Alamos National Laboratory
E. McKigney - Los Alamos National Laboratory
Pete Karpius - Los Alamos National Laboratory
Jonathan L. Thron - Los Alamos National Laboratory
Kate Frame - Los Alamos National Laboratory
William Clay - Department of Physics, Stanford University
Duncan W. MacArthur - Los Alamos National Laboratory
Abstract
Neutron multiplicity counters (NMCs) are widely used for nondestructive assay (NDA) of plutonium-bearing items and are the leading contenders for use in agreement verification. Such detectors use coincidences between neutrons produced in fissions to determine the mass of special nuclear material (SNM). The presence of light element impurities leads to a large neutron rate from (a,n) reactions. These single neutrons register as accidental coincidences in proportion to the coincidence gate width. To address this background we are developing a new neutron multiplicity counter liquid scintillator detector. In contrast to conventional NMC methods that employ 3He tubes embedded in polyethylene, the liquid scintillator multiplicity counter (LSMC) will have a more narrow coincidence gate made possible by a detector response that is faster by several orders of magnitude. This narrow gate will reduce the number of accidental coincidences counted and, therefore, increase the accuracy of the measurement. In addition to plutonium NDA, the LSMC can be applied to the case of a uranium item in the presence of a relatively large interrogating neutron source. By reducing the number of coincident interrogating neutrons, the induced fission signal will be easier to detect and quantify. We present here Monte Carlo simulation studies using the GEANT toolkit to estimate the performance of an LSMC.