Observation of neutron scattering from MoNA detectors at LANSCE and comparison with simulation*

To better understand fast neutron scattering from carbon and hydrogen in plastic scintillator detectors in the Modular Neutron Array (MoNA), we've conducted two experiments at the Los Alamos Neutron Science Center. An array of 16 MoNA detectors, each 200x10x10 cm<span style="font-size:10.8333px">$^3$ with PMTs attached to each end, was placed in the path of a 3 mm collimated neutron beam with energy range 20-200 MeV. In the first experiment the detectors were stacked two high and eight deep, so that neutrons scattered into a continous scintillator volume ~0.8 m deep. Elastic scattering of fast neutrons from carbon is large compared with all other H and C scattering channels, and produces "dark scattering," where neutron trajectory is altered without detection, since light from the recoiling carbon is well below detector threshold. Dark scattering worsens neutron position resolution in MoNA, and its negative effect increases with scintillator depth. A followup experiment was conducted with a large air gap between the first and second scattering sites located in the active target detector and in a 45<span style="font-size:10.8333px">$^o$ ramp of 15 MoNA detectors placed approximately 1.5 - 2 m away, respectively. New angular distributions for neutron scattering using this array geometry, and comparison with Monte Carlo simulation will be presented.