Optimization of a Fermi Chopper for an Inelastic Neutron Spectrometer with Focusing Guide

In order to evaluate an optimum condition of a Fermi chopper, especially for the multiple-incident-energy (multi-Ei) method, we demonstrated a numerical simulation by Monte Carlo method and analytical calculation. In a calculation using the actual optical design of the Fermi chopper spectrometer 4SEASONS installed at Japan Proton Accelerator Research Complex (J-PARC), a peak width of a beam monochromated by the chopper becomes wider as the energy decreases. The energy dependence is found to be caused by the critical-reflection-angle of the neutron supermirror utilized for the ellipsoidal guide tubes. We argue that the energy dependence should be advantageous to realize the optimum condition for every monochromated beam in the multi-Ei measurement by making it follow the energy dependence of the moderator pulse width.