  Silica aerogels are important to be used as photon radiators in Cherenkov
counters for high-energy-physics experiments because of their optical
transparency and intermediate refractive indices between those of gases and
liquids or solids. Cherenkov counters that employ silica aerogels as radiators
and photodetectors are often used to identify subatomic charged particles
(e.g., electrons, protons, and pions) with momenta on the order of sub-GeV/$c$
to GeV/$c$; they are also used to measure particle velocities in
accelerator-based particle- and nuclear-physics experiments and in space- and
balloon-borne experiments in the field of cosmic-ray physics. Recent studies
have demonstrated that it is important for the design of Cherenkov counters
that the transparent silica-aerogel tiles comprise solid material with recently
improved transparency and a refractive index that can be controlled between
1.003 and 1.26 by varying the bulk density in the range of 0.01$-$1.0 g/cm$^3$.
Additionally, a technique for fabricating large-area silica-aerogel tiles
without cracking has been developed. In this chapter, we describe advances in
the technologies for producing silica aerogels with high optical performances
to be used in scientific instruments. We further discuss the principles
underlying the operation of detectors based on the Cherenkov effect. We also
review applications of silica aerogels in specific high-energy-physics
experiments.
