New measurement of the scattering cross section of slow neutrons on liquid parahydrogen from neutron transmission

TL;DR

This study provides a new, precise measurement of the slow neutron scattering cross section on liquid parahydrogen, resolving discrepancies in previous data and highlighting the importance of equilibrium conditions for accurate neutron physics applications.

Contribution

The paper presents the first unambiguous neutron transmission measurement of the total and scattering cross sections of slow neutrons on liquid parahydrogen at 15.6 K, clarifying previous inconsistent results.

Findings

Our measurement at 1 meV is three times lower than previous data.

Previous measurements were contaminated by non-equilibrium orthohydrogen.

Accurate cross section data are crucial for neutron source design.

Abstract

Liquid hydrogen is a dense Bose fluid whose equilibrium properties are both calculable from first principles using various theoretical approaches and of interest for the understanding of a wide range of questions in many body physics. Unfortunately, the pair correlation function $g(r)$ inferred from neutron scattering measurements of the differential cross section $d\sigma \over d\Omega$ from different measurements reported in the literature are inconsistent. We have measured the energy dependence of the total cross section and the scattering cross section for slow neutrons with energies between 0.43~meV and 16.1~meV on liquid hydrogen at 15.6~K (which is dominated by the parahydrogen component) using neutron transmission measurements on the hydrogen target of the NPDGamma collaboration at the Spallation Neutron Source at Oak Ridge National Laboratory. The relationship between the neutron transmission measurement we perform and the total cross section is unambiguous, and the energy range accesses length scales where the pair correlation function is rapidly varying. At 1~meV our measurement is a factor of 3 below the data from previous work. We present evidence that these previous measurements of the hydrogen cross section, which assumed that the equilibrium value for the ratio of orthohydrogen and parahydrogen has been reached in the target liquid, were in fact contaminated with an extra non-equilibrium component of orthohydrogen. Liquid parahydrogen is also a widely-used neutron moderator medium, and an accurate knowledge of its slow neutron cross section is essential for the design and optimization of intense slow neutron sources. We describe our measurements and compare them with previous work.