Optimized α/β pulse shape discrimination in Borexino

Borexino could efficiently distinguish between alpha and beta radiation in its liquid scintillator by the characteristic time profile of its scintillation pulse. This alpha l beta discrimination, first demonstrated on the ton scale in the counting test facility prototype, was used throughout the lifetime of the experiment between 2007 and 2021. With this method, the alpha events are identified and subtracted from the solar neutrino events similar to beta . This is particularly important in liquid scintillators, as the alpha scintillation is strongly quenched. In Borexino, the prominent 210 Po decay peak was a background in the energy range of electrons scattered from 7 Be solar neutrinos. Optimal alpha l beta discrimination was achieved with a multilayer perceptron neural network , with a 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 characterizing 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 key milestones in alpha l beta discrimination in Borexino as a term of comparison for current and future large liquid scintillator detectors.