Neutron Kikuchi Effect and Practical Problems Associated with Its Observation

Abstract

The theory of the neutron Kikuchi effect has been re-examined and an exact formulation in reciprocal space derived. The conditions necessary for its observation have been considered and appropriate data from a lead single crystal have been collected on the triple-axis spectrometer at the HIFAR reactor in the laboratories of the Australian Nuclear Science and Technology Organisation. A series of constant-Q phonon scans across the predicted positions of the Kikuchi lines show reductions in the integrated phonon intensity, which appear to move as expected with changes in the scattered neutron wavevector.

Observation of Kossel and Kikuchi Lines in Thermal Neutron Incoherent Scattering

“…An experiment relevant to the theory by Wilkins was carried out by Town, Smith, and Elcombe [9]. Their experimental results were consistent with the neutron Kikuchi effect.…”

“…An unpolarized, parallel, thermal neutron beam interacting with a single crystal containing hydrogen atoms should therefore give rise to both Bragg reflections, from the incident plane-wave neutron beam, and K lines, from the spherical waves produced by the incoherent scattering from the H atoms. This experimental approach of using incoherent elastic scattering to obtain K lines is different from previous methods using electrons, x rays, and a previous attempt with thermal neutrons [2,4,[9][10][11][12].…”

“…Despite these predictions and to the best of our knowledge, they have never been identified in experimental thermal neutron scattering data. Town, Smith, and Elcombe [9] carried out a series of constant-Q phonon scans, in a lead sample, across the predicted locations for Kikuchi lines and observed a reduction in integrated phonon intensity which they attributed to the neutron Kikuchi effect. Town et al speculated that the direct observation of neutron Kikuchi lines would be difficult, if not impossible [9].…”

Neutron Scattering at HIFAR—Glimpses of the Past

“…It was not always used in its intended fashion. Beam energy characterisation and the successful measurement of ν for plutonium [49], successful (and I believe first) observation of the neutron Kikuchi effect in lead [50] and diffuse scattering from cubic stabilised zirconia [51] were some of these.…”