Characterization of positronium properties in doped liquid scintillators

Abstract

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.Comment: 6 pages, 7 figure

Ortho-positronium observation in the Double Chooz experiment

“…Positronium can also be formed as a singlet state, which decays into two γ's, called para-positronium (p-Ps), however its lifetime of ∼ 125 ps is too short to exploit it for particle discrimination. The o-Ps lifetime has been measured in the most commonly used solvents for organic liquid scintillators in neutrino physics [7,9], and as a function of the dopants concentration [10]. Although the o-Ps lifetime is typically shortened down to a few nanoseconds, the distortion in the photon emission time distribution is still observable.…”

“…The apparatus, at IPHC Strasbourg, is very similar to the one described in detail in ref. [10]. The positron source is a 1 MBq 22 Na source, inserted between two 12.7 µm titanium layers, immersed in the liquid scintillator.…”

“…The details of the analysis can be found in ref. [10]. The obtained time distributions and the corresponding fits are shown in figure 1 for Target and Gamma Catcher scintillator.…”

“…The lifetime can be taken immediately from the fit parameter τ 3 whereas the o-Ps formation fraction has to be computed renormalizing correctly the number of events, neglecting the events in the titanium source support, and taking into account the different efficiencies for the three and two-γ decay modes. Once the systematics are considered (they account typically for about 1.7 % on the formation fraction and for about 1 % on the lifetime [10]) we obtain for the Gamma Catcher …”

Multi-site event discrimination in large liquid scintillation detectors

“…The multiple Compton scattering of these γ will give these events a multi-site component. Furthermore, in some LAB cocktails loaded with tellurium, for example, there is a 36% chance of producing a long-lived ortho-positronium (o-Ps) state that has a lifetime of 2.7ns [16]. If o-Ps is formed at the end of the positron track, the annihilation γ will be delayed by the decay time of o-Ps.…”