Characterization of Positronium Properties in Doped Liquid Scintillators

Orthopositronium (o-Ps) formation and decay can replace the annihilation process, when a positron interacts in liquid scintillator media. The delay induced by the positronium decay represents either a potential signature for antineutrino detection, via inverse β decay, or to identify and suppress positron background, as recently demonstrated by the Borexino experiment. The formation probability and decay time of o-Ps depend strongly on the surrounding material. In this paper, we characterize the o-Ps properties in liquid scintillators as function of concentrations of gadolinium, lithium, neodymium, and tellurium dopers used by present and future neutrino experiments. In particular, gadolinium and lithium are high neutron cross section isotopes, widely used in reactor antineutrino experiments, while neodymium and tellurium are double β decay emitters, employed to investigates the Majorana neutrino nature. Future neutrino experiments may profit from the performed measurements to tune the preparation of the scintillator in order to maximize the o-Ps signature, and therefore the discrimination power.