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

Cordoba, Aylen Betania (author), Žerovnik, Gašper (mentor), Wagner, Richard (comentor), Suikkanen, Heikki (comentor)

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 $\beta^{-}$ 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 $^4$He, 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 converter. The highest UCN production rate density obtained from the simulations was (336 ± 5) [1/s/cm$^3$] 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/cm$^3$] was estimated.

• Language: English
• Keywords: UCNs, UCN production rate, McStas, NMO, ESS
• Work type: Master's thesis/paper
• Typology: 2.09 - Master's Thesis
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2023
• PID: 20.500.12556/RUL-151487
• COBISS.SI-ID: 16732467
• Publication date in RUL: 07.10.2023

Secondary language

• Language: Slovenian
• Title: Razvoj in simulacije nevtronske optike za ultra-hladen izvor nevtronov (UCN) v snopu

Abstract

Raba ultra-hladnih nevtronov (angl. UCN) ima ključno vlogo v temeljnih raziskavah fizike, usmerjenih v pridobivanje globljega razumevanja lastnosti nevtronov. Med številnimi drugimi zanimivimi poskusi je iskanje stalnega nevtronskega električnega dipolnega momenta (angl. nEDM) in natančne meritve življenjske dobe nevtronske beta-minus razpada ena pomembnejših aplikacij UCN-jev. Kljub temu je eden glavnih izzivov, s katerimi se soočajo vsi ti poskusi, omejena količina virov UCN-jev. Vir nevtonov European Spallation Source (ESS), trenutno v izgradnji v Lundu na Švedskem, bo predvidoma postal najmočnejši pulzni nevtronski vir na svetu. Magistrsko delo se izvaja v okviru projekta HighNESS, katerega cilj je z uporabo moderatorja zasnovati vir hladnih nevtronov (angl. CN) visoke intenzivnosti na ESS. Hladni nevtroni (HN) so osnova za proizvodnjo UCN-jev. Za pridobitev slednjih lahko visoko intenzivne HN prenesemo v konverter, napolnjen s superfluidnim $^4$He, za pretvorbo v UCN-je v postopku super-termalnega sipanja fononov. Namen tega dela je zasnovati Nevtronske Optične Dostavne Sisteme (NODS) za vir ultra-hladnih nevtronov v snopu, ki se nahaja na oddaljenosti več kot 15 metrov od moderatorja. Med drugimi analiziranimi sistemi za prenos nevtronov se osredotočamo na vgnezdeno optično ogledalo (angl. ''nested mirror optics'' - NMO), niz eliptčnih ogledal, postavljenih na srednjo razdaljo med virom HN in virom UCN. Učinkovitost NMO-jev je proučena z Monte Carlo programskim orodjem McS-tas, pri čemer se oceni njihova učinkovitost glede doseganja proizvodnih stopenj UCN-jev. Med izvedenimi simulacijami, v katerih upoštevamo različne geometrije in postavitve, je največji fluks CN-jev v konverter zagotovila uporaba monoplanarnega NMO-ja. Največja gostota hitrosti proizvodnje UCN-jev, ki jo pridobimo iz simulacij, je (336 ± 5) [1/s/cm$^3$] za konverter s premerom 22 cm. Po ovrednotenju izgub, ki jih simulacije ne upoštevajo, končno gostoto hitrosti proizvodnje UCN-jev ocenimo na 270 [1/s/cm$^3$].

• Keywords: ultra-hladni nevtroni, proizvodna stopnja UCN-jev, McStas, vgnezdena optična ogledala, ESS