Development of a Large-Area Aerogel Cherenkov Counter Onboard BESS

TL;DR

This paper reports the development of a large-area aerogel Cherenkov counter for cosmic-ray detection onboard BESS, achieving high muon rejection and proton detection efficiency in a magnetic environment.

Contribution

It introduces a novel large-area aerogel Cherenkov counter with optimized design for space-based cosmic-ray measurements, including performance under magnetic fields.

Findings

Achieved a muon rejection factor of 10^4

Maintained 97% proton detection efficiency

Successfully operated in a magnetic field environment

Abstract

This paper describes the development of a threshold type aerogel Cherenkov counter with a large sensitive area of 0.6 m$^2$ 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 10$^4$ was obtained against muons with $\beta \approx 1$, while keeping 97% efficiency for protons below the threshold.