Optimisation of the design of a neutron diffractometer for strain measurement

In this paper we describe a method for optimising the design of a time-of flight neutron diffractometer designed to measure lattice parameters. Such diffractometers are now used extensively by engineers and metallurgists for measuring strain within metallic and ceramic components. In the past neutron diffractometers have generally been built as 'all-purpose' instruments, with designs that are compromises, balancing competing requirements to measure the intensities, positions and widths of diffraction peaks simultaneously. In contrast ENGIN-X is designed with the single aim of making engineering strain measurements; essentially the accurate measurement of polycrystalline lattice parameters, at a precisely determined position. The method presented relies on the identification of a Figure of Merit (FOM) which accurately describes the performance of such an instrument. Although the instrument described is based on the time-of-flight technique, the FOM derived may equally well be used to optimise a reactor based instrument. While lattice parameter measurement is a particularly straightforward example, it is shown that similar Figures of Merit may be found for other types of instrumentation. (author)