Speaker
Description
The major goal of a conceptual exploratory study at the Second Target Station (STS) of the Oak Ridge National Laboratory’s (ORNL) Spallation Neutron Source (SNS) was to develop an alternative moderator design with a significantly improved neutronics performance at longer wavelengths compared to the current final design, which comprises supercritical parahydrogen moderator, light water premoderator, and a beryllium reflector.
A wide range of MCNP radiation transport (neutronics) calculations was performed with various geometry configurations and four candidate materials: solid and liquid methane and supercritical ortho- and para hydrogen. None of the alternative designs with a single material and geometry modifications outperformed the current design. However, a significant neutronics gain was achieved with a design that combined a central region made of solid methane at 20K and an outer region filled with supercritical parahydrogen at the same temperature. In this configuration, a relatively transparent flowing hydrogen can efficiently cool the submerged methane core with a higher proton density, which increases both neutron production and extraction.
In this alternative design, the peak of the neutron energy distribution is shifted towards longer wavelengths (from 3 Å towards 5 Å). At 5 Å, the peak neutron brightness improves considerably by 60%, while also providing a steeper leading edge of a neutron pulse with a sharper summit and better time resolution (FWHM) by 20%. Such a remarkable improvement is achieved when using scattering kernels for solid methane “smeth.40t” from the ENDF/B-VIII.0 library. However, this improvement is reduced only to 20% for peak brightness and 10% for resolution when employing newer scattering kernels available at the European Spallation Source (ESS)’s GitHub repository of the Spallation Physics Group [1]. Although both data files have similar cross sections for inelastic neutron scattering, the latter file shows a significantly higher cross section for elastic scattering at low energies (a factor of 5 higher at 5 Å).
Clearly, the selection of neutron data has a dramatic effect on the predicted performance of this alternative design with solid methane. Additional investigation of the difference in the files is necessary, potentially requiring experimental verification at one of the current or planned moderator test stations.
As another final note, although this design might not be suitable for high-power sources such as the STS due to radiolysis and spontaneous energy release from solid methane, it can provide an attractive alternative for designing low-power neutron sources where this issue is limited.
[1] Jose Ignacio Marquez Damian, https://git.esss.dk/spallation-physics-group/methane-tsl-libraries/-/blob/main/ace_files/smeth-0210.ace as of 03/24/2026
| Other | Alternative track: Target-Moderator Design |
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