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

TL;DR

This paper discusses how Borexino's low polonium environment was achieved and utilized to improve the detection of CNO neutrinos, providing crucial insights into solar neutrino measurements.

Contribution

It introduces the methods used to establish the Low Polonium Field in Borexino, enhancing background discrimination for CNO neutrino detection.

Findings

Successful reduction of $^{210}$Po in the Low Polonium Field

Improved upper limit measurement of $^{210}$Bi

Enhanced sensitivity for CNO neutrino detection

Abstract

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 5$\sigma$ at 99\%C.L. The intrinsic $^{210}$Bi is an important background for this analysis due to its similar spectral shape to that of CNO neutrinos. $^{210}$Bi can be measured through its daughter $^{210}$Po 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 additional $^{210}$Po 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 the $^{210}$Bi upper limit in Phase-III (Jul 2016- Feb 2020) of the experiment through the analysis of $^{210}$Po in the cleanest region of the detector called the Low Polonium Field.