The positron from the radioactive source crossing the fiber deposits some energy. If there is a  signal in coincidence in the two photomultipliers placed outside the calorimeter the trigger for the data acquisition is opened. The positron after the fiber can enter in the target (aerogel) and form positronium. The annihilation photons are detected in the calorimeter. In the off-line analysis the 1.27 MeV which is always emitted 3ps after the positron is asked to be in the trigger BGO counter, this is done to have a very high confidence level to have a positron in the positronium formation region.