Background considerations for the magnetic recoil spectrometer for Pacific Fusion's pulsed power device

The MAgnetic Recoil Spectrometer (MARS) will diagnose neutron spectra from MagLIF experiments at Pacific Fusion's pulsed power facility, enabling inference of fusion yield, ion temperature, fuel density, and alpha heating. The diagnostic exploits scattering in a CH₂ foil, where incident neutrons produce recoil protons that are forward-selected by a collimating aperture, energy-dispersed in space by a dipole bending magnet, and detected using a position-sensitive detector. The neutron spectrum is reconstructed from the measured proton energy distribution using known scattering cross-sections and an ab-initio instrument response function. Background radiation from photons and scattered neutrons poses a significant challenge for measurement accuracy. Comprehensive neutron transport simulations using OpenMC have been conducted to characterize this background and optimize collimator and shielding designs. Through iterative design, the signal-to-background ratio for an electronic detector system has been reduced to acceptable levels. The simulation framework additionally serves as a synthetic diagnostic for validating plasma parameter measurements. These studies provide essential guidance for MARS design, construction and operation.