Measurement of $pp$ and CNO cycle solar neutrinos with Borexino

Abstract

This work mainly deals with the study and measurement of solar neutrinos with the liquid scintillator detector Borexino, which is located at the Laboratori Nazionali del Gran Sasso (LNGS) in L'Aquila, Italy. Solar neutrinos are particles, which are created in the core of the Sun as a result of fusion reactions. This involves the proton-proton or $pp$ fusion as the main mechanism and the carbon-nitrogen-oxygen or CNO cycle as a secondary one. Both processes explain how energy is produced in the Sun or stars in general, i.e. how hydrogen is burned into helium. In the $pp$ fusion, two hydrogen nuclei or protons fuse together and initiate the so-called $pp$ chain. The CNO cycle on the other hand is initiated by the fusion of protons with carbon-12 isotopes. As a result of these reactions, solar neutrinos are produced, which need about eight minutes to reach the earth and thus represent a direct source of information to observe the solar interior. The energy and flux distribution, i.e. the number of emitted neutrinos per square centimeter and per second, of these neutrinos can be predicted precisely with the help of nuclear physics and the Standard Solar Model. This information can be used to determine the energy distribution of scattered electrons induced by solar neutrinos. These spectra can be used as a model to describe the data measured with the Borexino detector, keeping the scintillator background to a minimum and knowing its components precisely. This work is mainly concerned with the measurement of $pp$-, $pep$-, $^{7}\textrm{Be}$- and $^{8}\textrm{B}$-neutrinos from the $pp$ chain, and CNO neutrinos from the CNO cycle. \newline The second topic of this work is a study about the measurement of the shape factor of the $\beta^{-}$ radioactive source $^{144}\textrm{Pr}$ using a plastic scintillator setup located at the research center CEA in Paris-Saclay, France. The investigation of the $^{144}\textrm{Pr}$ source was developed for the SOX project (SOX = short distance neutrino oscillations with Borexino) and represented a sample source with lower activity. The main source, which was in production and ultimately could not be made available, was to be placed under the Borexino detector to study neutrino oscillations at short distances with respect to the existence of sterile neutrinos. However, the project was officially cancelled at the beginning of $2018$ as it was not possible to prepare the main source. As a consequence, the focus on this topic had to be significantly reduced within this work