The Geoneutrino Signal in the SNO+ experiment

Abstract

Since their first laboratory detection in 1956, neutrinos have played a unique role in our developing understanding of the universe around us. Because they are the only particle that interacts exclusively through the weak force, neutrinos interact very infrequently with matter, allowing most neutrinos to pass through many kilometers of matter without interacting. While this makes neutrinos hard to detect, it also allows for unique insights regarding processes occurring deep within the interior of the Sun and Earth. This thesis details the first SNO+ detection of antineutrinos coming from deep within the Earth’s crust, produced in radioactive decays of uranium and thorium. Calibration processes are undertaken to study the detector response to neutrons, which is needed to characterize the 13C(α,n)16O background events versus the antineutrino interactions. The antineutrino events are characterized by the prompt annihilation of a positron with an electron, followed by the delayed production of gamma rays produced when hydrogen or carbon captures the produced neutrons. Both the promptly annihilating positron and delayed neutron capture signals are produced by the antineutrino inverse β decay interaction with protons, ¯νe +p → n+e+. The geoneutrino flux observed at SNO+ will be particularly important for developing geophysical models, and in particular clarifying the Bulk Silicate Earth model, since this is only the third time this measurement has been possible, following prior measurements in Japan at Kamland and in Italy at Borexino. In this thesis, we discuss the observation of 55 candidate antineutrino events for 110.8 days of data, which were then fit using a maximum likelihood method based on the prompt energy distributions, resulting in 15.2 ± 4.9 identified geoneutrino events. This number corresponds to 109±35 TNU compared to the expected MidQ model prediction of 46.2+10.8 −7.5 TNU. We provide an introduction to geological models relevant to this thesis and discuss the implications of our findings for geophysical models, including suggestions for future analyses that can be undertaken at SNO+.