Development of a large-area aerogel Čherenkov counter onboard BESS

This paper describes the development of a threshold-type aerogel Čherenkov counter with a large sensitive area of 0.6 m2 to be carried onboard the BESS rigidity spectrometer to detect cosmic-ray antiprotons. The design incorporates a large diffusion box containing 46 finemesh photomultipliers, with special attention being paid to achieving good performance under a magnetic field and providing sufficient endurance while minimizing material usage. The refractive index of the aerogel was chosen to be 1.03. By utilizing the muons and protons accumulated during the cosmic-ray measurements at sea level, a rejection factor of 104 was obtained against muons with β≈1, while keeping 97% efficiency for protons below the threshold.