Advanced designs for a next-generation cold neutron source at ESS

The insatiable desire for higher intensity neutron sources has always been a driving force in fundamental and applied neutron research. In spite of this, very few neutron sources are available worldwide, especially when compared to the development of X-rays sources, which for materials research complements neutron scattering experiments. The construction of the European Spallation Source (ESS) is meant to meet the increasing demand for neutrons with a design that both outperforms and complements the current world landscape of neutron sources. The ESS will be a long-pulsed spallation source with a 5-MW proton beam impinging on a rotating tungsten target. The choice of using a single bi-spectral advanced moderator above the target, opened the possibility of installing a second neutron source below the target wheel. The High Intensity Neutron Source at the European Spallation Source (HighNESS) project had the goal to design a next-generation, colder and more intense source at ESS, meaning a larger number of neutrons emitted from the moderator surface. The second source aims to give researchers access to the spectral regions of cold (CNs, 2-20Å), Very Cold (VCNs, 20–120 Å), and Ultracold (UCNs, >500 Å) neutrons. As part of HighNESS, this thesis contributed to the design of the cold neutron source and studied in depth the options for in-pile VCN and UCN sources. The CN source comprising a liquid ortho-deuterium (LD2) moderator at 20K and the initial geometry was optimized with respect to the intensity of cold neutrons. For the VCN source, a design providing unprecedented intensity below 40 Å is identified. Taking advantage of recent developments in simulation capabilities, concepts based on solid ortho-deuterium (SD2) crystals at 5K are presented. The effect of using diamond nanoparticles (NDs) as advanced reflectors in these designs is studied in simulations with empirical and theoretical modeling implemented in NCrystal. These new simulation tools coupled with McStas are validated with existing experimental data and a dedicated experimental campaign aiming at observing the reflective properties in the relevant configuration is designed. Both NDs and SD2 are used within a detailed MCNP® model of the cold neutron source. Depending on the design, the VCN source could either replace or supplement the cold LD2 moderator. These advanced designs that combine SD2 and NDs are found to yield, for the first time, order-of-magnitude gains in the long-wavelength range of the neutron spectrum. Finally, different scenarios for a complementary UCN source at ESS are explored. Currently, the ESS lacks the experimental infrastructure for a UCN program, so the preliminary designs based on an in-pile SD2 converter are optimized only for the UCN production inside the converter.
Despite the challenges lying ahead, these advanced designs have the potential to open the doors to a completely new future for ESS, in which VCNs and UCNs are produced with an intensity never seen before and allowing scientists to explore new instrument concepts and, ultimately, leading to new breakthroughs in science.