  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.
