Fast neutron scattering and multiple-neutrion detection in MoNA

With the new FRIB facility coming online soon, studies of neutron-unbound systems near the neutron dripline will have 

access to heavier systems, including many new unbound states decaying via multiple-neutron (MN) emission.  The MoNA/LISA 

neutron detector array measures the energy and momentum of fast neutrons resulting from in-flight decays, and the 

challenge for studying MN decays has been to filter out contributions from 1N scattering two or more times in the array. 

We have traditionally employed a simple ``causality cut'' to filter out 1N events by requiring a minimum distance 

between hits (typically ~30-50 cm) and a minimum effective velocity between hits (neutron beam velocity).  This method 

provides a clean removal of 1N events, but reduces the number of MN events that survive the filter and reduces the 

efficiency for detecting lower decay energies. We have developed a new sorting algorithm that increases the MN/1N signal 

for MN datasets.  All hits in an event above a pre-determined light threshold are sorted according to cluster type with 

the neutron scatter site assigned to the earliest hit in the cluster.  Events are then sorted by pair according to their 

causal relationship to determine the number of independent neutrons involved in the event, and their first-scatter 

sites.  This sorting is performed in a two-dimensional parameter space consisting of scattering angle and neutron 

spacetime interval for the pair ($NSI =(v_{beam}^2-v_{12}^2)t_{12}^2$). Successful tests of the MN filter using previous 

MoNA 1N and 2N datasets and Monte Carlo simulations will be presented, as well as additional diagnostics for tracking 

scattering behavior.