  Borexino could efficiently distinguish between alpha and beta radiation in
its liquid scintillator by the characteristic time profile of their
scintillation pulse. This alpha/beta discrimination, first demonstrated at the
tonne scale in the Counting Test Facility prototype, was used throughout the
lifetime of the experiment between 2007 and 2021. With this method, alpha
events are identified and subtracted from the beta-like solar neutrino events.
This is particularly important in liquid scintillator as alpha scintillation is
quenched many-fold. In Borexino, the prominent Po-210 decay peak was a
background in the energy range of electrons scattered from Be-7 solar
neutrinos. Optimal alpha-beta discrimination was achieved with a "multi-layer
perceptron neural network", which its higher ability to leverage the timing
information of the scintillation photons detected by the photomultiplier tubes.
An event-by-event, high efficiency, stable, and uniform pulse shape
discrimination was essential in characterising the spatial distribution of
background in the detector. This benefited most Borexino measurements,
including solar neutrinos in the \pp chain and the first direct observation of
the CNO cycle in the Sun. This paper presents the key milestones in alpha/beta
discrimination in Borexino as a term of comparison for current and future large
liquid scintillator detectors
