Development of a Supermirror Coated Fermi Chopper Slit Package for Inelastic Neutron Scattering

Thermal neutron scattering is an extremely powerful technique for investigating the structure and properties of materials at the atomic level. As a consequence, many countries are currently making large investments in new neutron sources and the associated beamline instruments to carry out neutron scattering experiments. This project will develop a supermirror coated slit package that could be used on neutron scattering beamlines. The proposed slit package would be constructed from either silicon (Si) or sapphire (Al2O3) single crystal wafers coated with m=3 Ni/Ti supermirror layers and interspaced with gadolinium (Gd) neutron absorbing foils. While theoretical calculations suggest that such a slit package would provide a 35% boost in intensity over the currently used aluminum and gadolinium foil slit packages, a number of materials science and engineering must be resolved. The primary issue is whether the silicon and/or sapphire can withstand the forces generated within the slit package and its frame holder when it is rotated at 600Hz (the high-end frequency at which Fermi choppers operate). The design of the frame holder must be optimized to minimize undesirable forces being exerted on the slit package by the frame holder during rotation. In Phase I, mechanical tests will be conducted on single crystal pieces of silicon and sapphire wafers, in order to determine their anisotropic mechanical properties and frictional characteristics. Then, iterative design and finite element analysis calculations will be performed to determine if the slit package can survive at 600Hz. Commercial Applications and other Benefits as described by the awardee: The US has recently completed the construction of what will be the world¿s most powerful spallation neutron source, known as the SNS, at Oak Ridge National Laboratory. The new slit package should be immediately applicable to the HYSPEC instrument located at the SNS, boosting the intensity of low energy neutrons at the sample position by up to 35%.