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

Abstract

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 m 2 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/ N pe , where N pe is the effective number of photoelectrons.