Wavelength frame multiplication chopper system for the multi-purpose neutron-imaging instrument ODIN at the European Spallation Source

We have designed the chopper configuration for the future multi-purpose neutron-imaging instrument ODIN at the European Spallation Source (ESS). The configuration is based on the concept of wavelength frame multiplication (WFM) using a pair of optically blind choppers, which results in a constant wavelength resolution over the utilized spectrum. When using the WFM mode, the wavelength resolution can be adjusted in the range 0.19%≤Δλ∕λ≤0.84% and the instrument will feature a spectrum in the range 1.0 Å ≤λ≤8.7 Å where either the full bandwidth or half of it can be accessed. Additionally, a mode utilizing the natural resolution in the range 2%≤Δλ∕λ≤10% given by the instrument length can be realized by parking the WFM choppers in an open position. After an initial analytical specification of the chopper system, simulations have been performed using the Monte Carlo software package McStas in order to optimize the parameters. In this paper we discuss the proposed WFM chopper concept and present an analytical description of the chopper discs involved. Furthermore, we present simulation results demonstrating the performance of the chopper system, implemented in the neutron guide system that is being realized for the imaging beam line ODIN at the ESS.