Authors:	Hackspacher, Paul Christian
Title:	Determination of Liquid Scintillator Purity for the Jiangmen Underground Neutrino Observatory
Online publication date:	8-Apr-2021
Year of first publication:	2021
Language:	english
Abstract:	The Jiangmen Underground Neutrino Observatory (JUNO) is a neutrino experiment currently under construction in southern China. Its primary goal is the determination of the neutrino mass ordering by a spectral analysis of the reactor neutrino flux from two nearby nuclear power plants at a distance of 53 km using a 20 kt liquid scintillator (LS) detector. As the neutrinos are identified indirectly through measurement of scintillation light within the LS, it is imperative for the liquid to be as clean as possible and its properties sufficiently known. This thesis deals with both, optical and radioactive characteristics, and describes methods of determining scattering parameters and radioactive contamination of the LS. The first part of the thesis investigates Rayleigh scattering in the LS by means of a laboratory setup that illuminates samples with polarized light of selectable wavelengths while two photomultiplier tubes (PMTs) measure both, the throughgoing intensity and the emissions at 90◦ perpendicular to the beam path. Comparing the two allows to identify the so-called Rayleigh length of the respective sample. Rayleigh scattering is the dominant process determining the transparency of the LS to the scintillation light. The setup was calibrated using various samples with known scattering behaviour before conducting LS investigations. The resulting Rayleigh lengths of (26.1 ± 1.2stat ± 2.6sys) m for a pure LAB sample and (22.4 ± 0.7_stat ± 2.2_sys) m for a sample of SHiP scintillator at 430 nm are in agreement with expectations around 28 m and show the anticipated λ^4-dependence in the region above, but deviate for lower wavelengths. This discrepancy was verified with a spectrometer and suggests either contamination of the samples in question, divergent behaviour in proximity of the LS absorption/emission region or both and thus needs to be studied further. In the second part of the thesis, a radioactivity pre-detector for JUNO is envisioned and developed. The Online Scintillator Internal Radioactivity Investigation System (OSIRIS) will determine the radioactive contamination of the LS by a rate analysis of 214Bi-214Po-coincidence-signals compared to the accepted background rate. A Monte Carlo simulation was created in Geant4 to study the feasibility of such a device and investigate various designs with regards to their efficiency. Several concept stages are briefly shown and the final simulation is described, along with data production, processing and analysis methods. The simulation data shows that a nested arrangement of cylindrical tanks with a 3 m LS volume, a ∼ 5 m buffer housing 100 8" PMTs and an 8 m water shield is able to determine radioactive contaminations up to 2.22 · 10^−10 Bq/kg within 24 hours using a 50 cm fiducial volume cut. This is sufficient to verify the baseline purity requirements for solar neutrino investigation in JUNO (1.24 · 10^−9 Bq/kg) as well as the requirements for the mass ordering search (1.24 · 10^−8 Bq/kg).
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-5705
URN:	urn:nbn:de:hebis:77-openscience-1e0f3b6c-3fed-455c-9122-5ae704a762ee6
Version:	Original work
Publication type:	Dissertation
License:	CC BY
Information on rights of use:	https://creativecommons.org/licenses/by/4.0/
Extent:	V, 130 Blätter, Illustrationen, Diagramme
Appears in collections:	JGU-Publikationen