An intense source of very cold neutrons using solid deuterium and nanodiamonds for the European Spallation Source

The European Spallation Source (ESS), currently under construction, is based on a high-brightness, bi-spectral, low-dimensional moderator placed above a spallation target, intended to initially serve fifteen neutron scattering instruments. Within the upgrade path of ESS, the HighNESS project aims at designing a source complementary to this upper moderator, focusing on delivering a higher intensity and a colder spectrum of neutrons. We have investigated the use of solid ortho-deuterium at 5 K as a source of very cold neutrons (VCNs). This source performs competitively as a high-intensity cold-neutron moderator, while also showing an order-of-magnitude flux increase in the very cold range above 40 ̊ A compared to a liquid deuterium moderator of similar volume and shape, also designed within HighNESS. The long-wavelength performance of the source can be improved further by encasing it in a thin layer of nanodiamonds. The cooling of a solid deuterium moderator placed so close to the spallation target of a high-power neutron source like ESS is very challenging, but may be feasible by augmenting the heat conductivity with the addition of low-density metallic foam structures within the moderator vessel. Such a source could provide unprecedented opportunities in fundamental physics research and neutron scattering using VCNs.