Ageing tests and recovery procedures of silica aerogel

Silica aerogel has been extensively used in RICH detectors for the identification of charged particles over the momentum range between 1 and 10GeV/c. Tiles of hygroscopic aerogel with large transverse dimensions (20×20cm2) and refractive index n=1.03n=1.03 have recently been produced for use in the LHCb experiment, allowing pion–kaon identification up to 10GeV/c. The tiles have excellent optical properties (clarity factor better than 0.006μm4/cm and homogeneity σ(n-1)/(n-1)∼1%σ(n-1)/(n-1)∼1% within the tile). Extensive R&D tests on aerogel samples have been performed. Samples have been exposed to intense irradiation (proton, neutron and gamma), to humid air, to standard black varnish (used to paint the inner surface of RICH detectors), and to C4F10 and CO2 gases. The optical properties of the aerogel have been monitored during these tests and, when required, recovery procedures have been investigated and applied. In particular, regeneration of the tiles has been realized through exposure to dry atmosphere (gaseous N2) or through baking for several hours at 500∘C. The measurements demonstrate that the optical properties have been successfully restored to their values at the production stage, and in no case permanent degradation has been observed.