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Abstract
A neutron optical filter is a device that is designed to have high transmission for low energy neutrons and very low transmission for high energy neutrons and gamma-rays. This is achieved using three sections of reflecting neutron guide, typically several meters in length, with small angular offsets between sections such that there is no direct path through the device. High energy radiation is lost in and through the walls, but low energy (thermal and sub-thermal) neutrons are retained, to the extent that they are totally reflected within the guide. We describe an analytical method to calculate the transmission properties of neutron optical filters, with either parallel-sided or tapered central sections. We present the results of calculations for a number of filters, including a tapered filter which is to be installed at one of the guides of the Cold Neutron Research Facility at the National Institute of Standards and Technology.