Development and simulations of neutron optics for an in-beam ultra-cold neutron (UCN) source

Abstract

The utilization of ultra-cold neutrons (UCNs) plays a crucial role in fundamental physics research aimed at gaining a deeper understanding of neutron properties. Within a spectrum of other experiments of interest, the search for a permanent neutron electric dipole moment (nEDM) and accurate measurements of the neutron β − decay lifetime are the driving forces behind the use of UCNs. Nevertheless, one of the main challenges faced in all these experiments is the usually limited amount of UCNs available. This master thesis has been carried out within the HighNESS project, which seeks to design a high intensity Cold Neutron (CN) source as a second moderator system at the European Spallation Source (ESS), currently under construction in Lund, Sweden. A use case for cold neutrons (CNs) is to utilize them to produce UCNs, by transporting them to a vessel filled with superfluid 4He, for a conversion into UCNs in a super thermal phonon scattering process. The purpose of this work was to study the design of a Neutron Optical Delivery System (NODS) for an in-beam Ultra Cold Neutron (UCN) source located at a distance larger than 15 meters from the HighNESS moderator. Among other neutron transport systems analysed, the particular focus was on the performance of nested mirror optics (NMO). The NODS have been studied with the Monte Carlo simulation software McStas, assessing their effectiveness in terms of the attained UCN production rates. Of the simulations performed using different geometries and layouts, a mono planar NMO implementation was the one that provided the highest CN flux to the con verter. The highest UCN production rate density obtained from the simulations was (336 ± 5) [1/s/cm3] for a converter diameter of 22 cm. After evaluating losses not considered in the simulations, a final UCN production rate density of 270 [1/s/cm3] was estimated.