  Electron antineutrinos are detected in organic liquid scintillator based
neutrino experiments by means of the inverse beta decay, producing both a
positron and a neutron. The positron may form a bound state together with an
electron, called positronium (Ps). The longer-lived spin state of Ps,
orthopositronium (o-Ps) has a lifetime of about $3\,\mathrm{ns}$ in organic
liquid scintillators (LS). Its formation changes the time distribution of
photon emission, which affects positron reconstruction algorithms and allows
the application of pulse shape discrimination (PSD) to distinguish electron
from positron events. In this work, we measured the lifetime $\tau_2$ of o-Ps
in the linear alkylbenzene (LAB) based LS of the JUNO (Jiangmen Underground
Neutrino Observatory) experiment including wavelength shifters, obtaining
$\tau_2 = 2.97\,\mathrm{ns} \pm 0.04\,\mathrm{ns}$. Due to systematics, which
are not yet completely understood, we are not able to give a final result for
the o-Ps formation probability $I_2$. We use a novel type of setup, which
allows a better background suppression as compared to commonly used PALS
(positron annihilation lifetime spectroscopy) measurements.
