Large-area scintillator hodoscope with 50 ps timing resolution onboard BESS

We describe the design and performance of a large-area scintillator hodoscope onboard the BESS rigidity spectrometer; an instrument with an acceptance of 0.3 m2 sr. The hodoscope is configured such that 10 and 12 counters are, respectively, situated in upper and lower layers. Each counter is viewed from its ends by 2.5 in. fine-mesh photomultiplier tubes placed in a stray magnetic field of 0.2 T. Various beam-test data are presented. Use of cosmic-ray muons at ground-level confirmed 50 ps timing resolution for each layer, giving an overall time-of-flight resolution of 70 ps rms using a pure Gaussian-resolution function. Comparison with previous measurements on a similar scintillator hodoscope indicates good agreement with the scaling law that timing resolution is proportional to 1/Npe, where Npe is the effective number of photoelectrons.