Comment on "High energy neutron scattering from hydrogen using a direct geometry spectrometer". C. Stock, R A Cowley, J W Taylor and S M Bennington [arXiv:0907.1945]

TL;DR

This paper critiques a neutron scattering study, arguing that its energy resolution claims are inaccurate and that the conclusions about experimental issues with inverse geometry spectrometers are unfounded, emphasizing the superior resolution of inverse geometry methods.

Contribution

The paper provides evidence that direct geometry spectrometers have inferior energy resolution compared to inverse geometry spectrometers, challenging recent claims about neutron cross section measurements.

Findings

Direct geometry spectrometers have coarser energy resolution than inverse geometry spectrometers.

The conclusions of the criticized study are unfounded due to resolution inaccuracies.

Inverse geometry spectrometers are more suitable for high-energy neutron scattering measurements.

Abstract

The paper in the title [1] reports measurements of neutron scattering from hydrogen in the 1-100 eV range of energy transfers, using the direct geometry MARI spectrometer at ISIS. Stock et al claim that their measurements have better or comparable energy resolution to those on the inverse geometry VESUVIO spectrometer at ISIS. Most importantly the main conclusions of ref [1] are not valid unless this claim is true: in particular the conclusion that anomalous neutron cross sections measured on VESUVIO [2] are "the result of experimental issues using indirect geometry spectrometers". We present here overwhelming evidence that the energy resolution of the measurements in ref [1] is much coarser than on VESUVIO. It follows that the conclusions of Stock et al are unfounded. In reality the measurements of reference [1] serve mainly to demonstrate that at eV neutron energies, direct geometry chopper spectrometers have greatly inferior energy resolution to inverse geometry spectrometers based on resonance foil methods.