Positronium signature in organic liquid scintillators for neutrino experiments

TL;DR

This study investigates the formation and lifetime of orthopositronium in common liquid scintillators used in neutrino experiments, proposing o-Ps signatures as a novel method for improved anti-neutrino detection.

Contribution

It provides the first measurements of o-Ps formation probability and lifetime in key scintillator solvents, and demonstrates their potential for enhancing anti-neutrino tagging.

Findings

o-Ps formation probabilities vary across solvents

o-Ps lifetime influences photon emission timing

o-Ps signature can improve neutrino detection efficiency

Abstract

Electron anti-neutrinos are commonly detected in liquid scintillator experiments via inverse beta decay, by looking at the coincidence between the reaction products, neutron and positron. Prior to positron annihilation, an electron-positron pair may form an orthopositronium (o-Ps) state, with a mean life of a few ns. Even if the o-Ps decay is speeded up by spin flip or pick off effects, it may introduce distortions in the photon emission time distribution, crucial for position reconstruction and pulse shape discrimination algorithms in anti-neutrino experiments. Reversing the problem, the o-Ps induced time distortion represents a new signature for tagging anti-neutrinos in liquid scintillator. In this paper, we report the results of measurements of the o-Ps formation probability and lifetime, for the most used solvents for organic liquid scintillators in neutrino physics (pseudocumene, linear alkyl benzene, phenylxylylethane, and dodecane). We characterize also a mixture of pseudocumene +1.5 g/l of 2,5-diphenyloxazole, a fluor acting as wavelength shifter. In the second part of the paper, we demonstrate that the o-Ps induced distortion of the scintillation photon emission time distributions represent an optimal signature for tagging positrons on an event by event basis, potentially enhancing the anti-neutrino detection.