Time resolution of a 1.8-m long BC-420 plastic scintillator bar with metal-packaged H11934 photomultiplier tubes

We tested a plastic-scintillator (PS) bar for the forward time-of-flight (TOF) detector of the J-PARC E50 spectrometer using a 740-MeV/c positron beam. The bar is composed of Saint-Gobain Crystals (BC-420), with a 30mm×40mm cross section and 1800-mm length. Each side of the bar is connected to a metal-packaged photomultiplier tube (PMT), Hamamatsu H11934-200 with an ultra-bialkali (UBA) photoelectric surface. The PMTs have an advanced feature of a small spread of the electron transit time (0.27ns) and high quantum efficiency (≈43%). The intrinsic resolutions for the timing measurement using the two PMTs connected to both the ends are found to be 0.053 and 0.059 ns in the standard deviation for the 40- and 30-mm-thick penetrations at the center, respectively. The results show that metal-packaged PMTs enable TOF measurements using an ordinary hodoscope consisting of 1.8-m-long PS bars with an excellent time resolution (∼0.06ns in the standard deviation).