A fiber detector to monitor ortho-Ps formation and decay

We describe a novel method to use a scintillating fiber detector similar to the Fast Annihilation Cryogenic Tracking (FACT) used at the antimatter experiment AEgIS to monitor the presence of ortho-positronium. A single scintillating fiber was coupled to a photomultiplier tube and irradiated by flashes of about $6\cdot10^6$ $511\,\text{keV}$ $\gamma$-rays produced by $\approx10\,\text{ns}$ long positron pulses. The results were used to demonstrate the ability to track the creation and annihilation of ortho-positronium atoms over time in cryogenic and highly magnetic environments by using the FACT detector as a "digital calorimeter".