  Ortho-positronium (o-Ps) formation and decay can replace the annihilation
process, when positron interacts in liquid scintillator media. The delay
induced by the positronium decay represents either a potential signature for
anti-neutrino detection, via inverse beta 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 anti-neutrino experiments, while neodymium and tellurium
are double beta 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.
