Authors:	Kahlenberg, Jan
Title:	First full-magnetic storage of ultracold neutrons in the tSPECT experiment for measuring the neutron lifetime
Online publication date:	17-Dec-2020
Year of first publication:	2020
Language:	english
Abstract:	The lifetime of the free neutron, $\tau_n$, not only affects the helium-4 abundance shortly after the Big Bang. It also provides an experimental determination of the first element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, $V_{\text{ud}}$, free of nuclear structure effects. Probing unitarity of the CKM matrix is an attractive way to trace Beyond the Standard Model physics, as a violation of unitarity would call for additional fermion generations. The value of $\tau_n$ and therefore the corresponding conclusion for unitarity currently depends on the measurement method, with a $\sim 4 \sigma$ deviation found between beam- and bottle-type experiments. \par In beam-type experiments, $\tau_n$ is determined by detecting the decay products of a neutron beam, while bottle-type experiments are based on a direct measurement of neutrons. For bottle-type experiments, neutrons with especially low kinetic energies of $\lesssim \SI{200}{\nano\electronvolt}$ are required, which are termed `ultracold' neutrons (UCN). The UCN are kept inside a storage volume, in which their decay according to $\tau_n$ and additional loss terms $\tau_{\text{loss}}$ sets in. Instead of storing UCN in material bottles, modern bottle-type experiments rely on the magnetic confinement of UCN due to their magnetic moment. Magnetic confinement offers severely reduced loss mechanisms compared to material confinement and can therefore yield unprecedented accuracies of $\Delta \tau_n \le \SI{1}{\second}$. As a drawback, magnetic bottle-type experiments require more intense UCN sources due to lower storage depths $\lesssim \SI{60}{\nano\electronvolt}$ compared to material storage ($\lesssim \SI{200}{\nano\electronvolt}$). \par Being the first neutron lifetime experiment based on full-magnetic storage, the $\tau$SPECT experiment relies on storing UCN with a maximum energy of $\sim \SI{47}{\nano\electronvolt}$ inside a storage volume of $\sim \SI{6.8}{l}$. As part of this work, $\tau$SPECT has been successfully set up for phase I at the research reactor TRIGA Mainz, which aims at measuring $\tau_n$ with an uncertainty of $\Delta \tau_n = \SI{1}{\second}$. A Halbach octupole inserted into the former $a$SPECT cryostat provides the radial magnetic confinement, which along with the cryostat's longitudinal magnetic field forms a 3D magnetic trap. For filling UCN prepared to their storable spin state into the trap, a movable neutron guide was designed, which is retracted after the filling process to prevent material interaction. A detection of `surviving', \textit{i.e.,} non-decayed UCN after varying storage times is required for determining $\tau_n$. Moved into the magnetic trap after the storage time, a custom UCN detector counts these `surviving' UCN. It comprises a boron-10-coated silver-doped zinc sulfide scintillator for converting captured UCN to photons and a silicon photomultiplier array for photon detection and has been thoroughly characterised to allow for the reconstruction of UCN events from the pulse height spectra. \par Prior to the final commissioning of the $\tau$SPECT experiment, the pulsed UCN source at beamport D was upgraded to provide a 3.5 times higher UCN density, which corresponds to $\sim \SI{8.5}{\per\cubic\centi\metre}$ in a material bottle of $\sim \SI{32}{l}$. Additional to detailed characterisations of the source performance, the upgraded source was used to characterise central $\tau$SPECT components before the installation. Finally, a first full-magnetic UCN storage was demonstrated.
DDC:	530 Physik 530 Physics
Institution:	Johannes Gutenberg-Universität Mainz
Department:	FB 08 Physik, Mathematik u. Informatik
Place:	Mainz
ROR:	https://ror.org/023b0x485
DOI:	http://doi.org/10.25358/openscience-5455
URN:	urn:nbn:de:hebis:77-openscience-710d7f9e-17d4-4327-be09-cff47932fce21
Version:	Original work
Publication type:	Dissertation
License:	CC BY
Information on rights of use:	https://creativecommons.org/licenses/by/4.0/deed.en
Extent:	xiii, 216 Seiten
Appears in collections:	JGU-Publikationen