The Low Polonium Field of Borexino and its significance for the CNO neutrino detection

Borexino is a liquid scintillator detector located at the Laboratori Nazionale del Gran Sasso, Italy with the main goal to measure solar neutrinos. The experiment recently provided the first direct experimental evidence of CNO-cycle neutrinos in the Sun, rejecting the no-CNO signal hypothesis with a significance greater than 5σ at 99%C.L. The intrinsic²¹⁰Bi is an important background for this analysis due to its similar spectral shape to that of CNO neutrinos.²¹⁰Bi can be measured through its daughter²¹⁰Po which can be distinguished through an event-by-event basis via pulse shape discrimination. However, this required reducing the convective motions in the scintillator that brought additional²¹⁰Po from peripheral sources. This was made possible through the thermal insulation and stabilization campaign performed between 2015 and 2016. This article will explain the strategy and the different methods performed to extract the²¹⁰Bi upper limit in Phase-III (Jul 2016- Feb 2020) of the experiment through the analysis of²¹⁰Po in the cleanest region of the detector called the Low Polonium Field.