30-ps time resolution with segmented scintillation counter for MEG II

A new timing detector has been developed to measure ∼50 MeV/c positrons with a time resolution of σt≃30ps in the MEG II experiment. The detector are segmented into 512 scintillation counters, each of which consists of 120×(40or50)×5mm3 size BC-422 and two arrays of six AdvanSiD silicon photomultipliers. The single-counter resolutions are measured to be 70–80 ps. The counter layout is optimized to get the maximum number of hit counters (on average 9 for signal positrons). This multiple-counters measurement leads to a significant improvement in the time resolution down to 30 ps. Using the first one-fourth (128) counters, a pilot run was carried out using the MEG II beam of 7×107μ+/s and the basic functionality was tested.