Valentin Czamlera, Richard Wagnerb, Olivier Zimmerc
aLaboratory of Subatomic Physics & Cosmology (LPSC), CNRS, Grenoble INP, LPSC-IN2P3, 38000 Grenoble, France
bLaboratoire Léon Brillouin, Institut Rayonnement Matière de Saclay (DRF), Université Paris-Saclay, CNRS, 91191 Gif-sur-Yvette, France
cInstitut Laue Langevin, 71 avenue des Martyrs, CS 20156, CEDEX 9, 38042 Grenoble, France
Very cold neutrons (VCN) are low-energy neutrons that fall in the long-wavelength range of what’s typically produced by cold neutron sources. Their energies are below 1 meV and can go down to just a few hundred neV, approaching the range of ultra-cold neutrons. High-intensity VCN sources could significantly enhance existing neutron scattering techniques. They would, for example, extend the accessible q-range in small-angle neutron scattering and broaden the energy range in time-of-flight or neutron spin-echo spectroscopy. In particle physics experiments employing beams of neutrons, they would also increase the sensitivity of precision experiments, such as searches for neutron–antineutron oscillations or a permanent neutron electric dipole moment [1].
The research conducted over the course of Valentin Czamler’s thesis explores clathrate hydrates – water-based solids that are able to trap small guest molecules – as novel materials for VCN production. Their effectiveness as a moderator stems from localized low-energy excitations of the guest species, which allow neutrons to lose energy without being restricted by the dispersion relation of phonons. Two guest molecules are of particular interest: hydrates containing deuterated tetrahydrofuran (THF-d), which provide broad localized Einstein modes, and dioxygen (O₂), whose magnetic triplet ground state introduces an additional energy-loss mechanism via zero-field splitting at 0.4 meV. This enables a stepwise cooling cascade, where neutrons are progressively slowed down through successive inelastic scattering events (see Figure) [2].
Figure. Maxwellian spectra of thermal, sub-thermal, and cold neutron beams. Arrows illustrate the cooling cascade enabled by clathrate hydrate moderators. Black arrows represent magnetic down-scattering from O₂, while purple and orange arrows indicate inelastic scattering from localized vibrational modes of the THF-d molecule.
To characterize the neutronic properties of these materials, we conducted a comprehensive series of neutron scattering experiments. The structural properties of the hydrates were investigated using neutron diffraction, their dynamical behavior was studied with inelastic neutron scattering, and their total neutron cross-sections were measured through transmission experiments on both cold and very cold neutron beamlines. As a result we established a synthesis method for binary hydrates containing both deuterated tetrahydrofuran (THF-d) and dioxygen (O₂), achieving high structural purity (98.4 ± 1.6%) and O₂ cage occupancy (80.1 ± 1.1%) [3]. Inelastic neutron scattering experiments mapped the dynamic structure factor S(q, ω) over a wide phase space and in absolute units. Notably, they captured for the first time magnetic down-scattering from confined dioxygen at 0.4 meV in a clathrate environment. Additionally, vibrational excitations of THF-d at 2.9 and 4.7 meV were observed, contributing further to efficient neutron thermalization within the moderator. These measurements were complemented by the development of thermal scattering libraries for clathrate hydrates, based on ab initio molecular dynamics and density functional theory. In parallel, a magnetic scattering model was developed specifically for dioxygen. This work was carried out as part of the HighNESS project, which aims to enhance the scientific reach of the European Spallation Source (ESS) through novel moderator concepts [4].
[1] V. Czamler, PhD Thesis, Univ. Grenoble Alpes (2024)
[2] O. Zimmer, Phys Rev. C93, 035503 (2016)
[3] V. Czamler et al., Materials 18(2), 298 (2025)
[4] V. Santoro et al., arXiv:2309.17333 (2023
