Nicola Rizzi

Nanodiamonds have attracted attention in recent years for their exceptional albedo of slow neutrons. Several theoretical models have been proposed to compute the total elastic cross section. However, these models neglect the relatively complex internal structure and chemical composition of the diamond nanoparticles, relying often on the monodisperse structureless spheres approximation. In this work, we explore the possibility of adding the small-angle neutron scattering (SANS) process for nanodiamonds to NCrystal, a library that enables calculations for Monte Carlo simulations of neutrons in polycrystalline materials and powders. This approach aims to describe the scattering process also at neutron wavelengths below the diamond Bragg cutoff where simple models usually struggle. The extension relies on modeling the SANS process through the fitting of experimental data as well as theoretical inputs. The code can then be coupled with an existing simulation framework, such as McStas, and benchmarked in different setups. We tested the plugin for two properties of diamond powder nanoparticles: the backward reflection of very cold neutrons and the quasispecular reflections of cold neutrons. The validation of this simulation tool is intended to pave the way for the design of the beam extraction system for a future high-intensity cold neutron moderator at the European Spallation Source.

The European Spallation Source, which is under construction in Lund, Sweden, aims to be the world’s brightest source of neutrons. During the first phase of construction, users will have access to a suite of 15 instruments, which will produce neutrons through a butterfly-shaped moderator system located above the spallation target. The HighNESS project, funded by the European Union Horizon 2020 program, has the goal of providing a new design for the second moderator, which will be situated below the spallation target and will be the most intense neutron source in the world. The new moderator will provide a higher intensity of neutrons in the cold, very cold, and ultracold energy regions, which will advance research in several areas of condensed matter and offer unique opportunities in fundamental physics. Work Package 2 of the HighNESS project is focused on supporting the design of the second moderator by providing nuclear data support for neutronics calculations and improving the accompanying physics software. This paper showcases recent advancements in NJOY + NCrystal, a tool for creating thermal neutron scattering–evaluated nuclear data files and continued work on nanodiamonds as advanced neutron reflectors.