Incoherent neutron-scattering determination of hydrogen content: Theory and modeling

Hydrogen concentrations of 0 up to 350?mg/kg in a titanium alloy have been determined at National Institute of Standards and Technology (NIST) with neutron incoherent scattering (NIS) and with cold neutron prompt gamma activation analysis. The latter is a well-established technique, while the former was demonstrated earlier at NIST by counting the neutrons scattered under 45° or 60° and establishing a linear relationship between hydrogen concentration and count rate. In this paper, it is shown that the NIS response is complicated by the dependence of the apparent hydrogen scattering cross section on the sample temperature and the hydrogen chemical binding. It is demonstrated that detection of scattered neutrons at two different angles as well as a more complex theoretical approach (i.e., including Monte Carlo modeling with the free-gas model) are required to correctly interpret NIS measurements. As a result, the process of matching standards and samples becomes less critical