Simulation study on neutron detection properties of MAGRO: A large-area neutron detector based on BC-408.

In nuclear physics experiments involving neutron measurements, spectroscopic information such as neutron energy, width, and intensity can be measured using the time-of-flight (TOF) method. Analyzing TOF spectra is often challenging because neutrons suffer severe scattering from various materials in the surrounding experimental apparatus. Therefore, it is inevitable to evaluate scattered neutrons with the experimental setup and decompose TOF lines into direct and out-scattered components. We demonstrate the utility of the large-area neutron detector MAGRO for counting low-energy neutrons down to 300 keV in the β-delayed neutron-decay study of rare isotopes. We performed Monte Carlo simulations using GEANT4 to investigate the detector's responses to incident radiation. In the simulations, physics models for optical physics and high-precision neutron physics were invoked. The optical physics parameters were adjusted to reproduce experimental data for light attenuation and propagation time, which were measured with the 90Sr β source. The simulation codes were further validated with in-beam tests at HIMAC (Heavy Ion Medical Accelerator in Chiba) of QST (National Institutes for Quantum Science and Technology) and RCNP (Research Center for Nuclear Physics) of Osaka University for intrinsic efficiency and TOF measurements using 16C and 17N beams. The neutron efficiency was calculated for a single MAGRO detector from 200 keV to 7 MeV, and it is shown that the calculated efficiency curve follows the measured data below 2 MeV. TOF spectra were reproduced in the simulations with the details of the experimental setup for β-n-γ coincidence, providing a unique way to identify out-scattered neutron events in the measurements.