Time resolution of time-of-flight detector based on multiple scintillation counters readout by SiPMs

A new timing detector measuring ∼50MeV/c positrons is under development for the MEG II experiment, aiming at a time resolution σt∼30ps. The resolution is expected to be achieved by measuring each positron time with multiple counters made of plastic scintillator readout by silicon photomultipliers (SiPMs). The purpose of this work is to demonstrate the time resolution for ∼50MeV/c positrons using prototype counters. Counters with dimensions of 90×40×5mm3 readout by six SiPMs (three on each 40×5mm2 plane) were built with SiPMs from Hamamatsu Photonics and AdvanSiD and tested in a positron beam at the DAΦ  NE Beam Test Facility. The time resolution was found to improve nearly as the square root of the number of counter hits. A time resolution σt=26.2±1.3ps was obtained with eight counters with Hamamatsu SiPMs. These results suggest that the design resolution is achievable in the MEG II experiment.