General Formulae for the Optimized Design of Fermi Chopper Spectrometer

Abstract

The instrumental design of neutron scattering spectrometer is optimized in consideration of both requirements from scientific field and performance of state-of-the-art neutron devices. In this sense, we can say that there is no exactly the same design of neutron scattering spectrometers all over the world. In optimizing the design of spectrometer, it is necessary to firstly grasp the tendency by means of analytic formulation. Subsequently the Monte Carlo simulation plays a role to refine the optimization. In this study, we will show several analytical formulae to evaluate the performance of a Fermi chopper spectrometer which is a typical instrument for inelastic neutron scattering measurement at a pulsed neutron source. Fermi chopper rotates the stacked slits, consisting of materials that are absorbing and transmitting for neutrons, and monochromate the neutron beam, where these slit components are referred to as a slitpackage. A conventional Fermi chopper has a set of curved slits, where the curvature is adjusted to the passage of neutrons with a selected velocity [1]. On the other hand, the Fermi chopper spectrometer 4SEASONS in the Japan Proton Accelerator Research Complex (J-PARC) has a slitpackage with straight slits [2]. The slitpackage of 4SEASONS brought us to the success of multiple incident energy (Multi-Ei) measurement which realizes the simultaneous observation of multiple dynamical structure factors by one measurement [3]. The optimized procedures for the design of a slitpackage with straight slits will be discussed in this paper.