The HERMES RICH aerogel radiator

The HERMES RICH detector was designed to separate pions, kaons and protons in the momentum range from 2 to 15 GeV/c, using two different types of radiators, the gaseous C4F10 and silica aerogel. For the aerogel, a refractive index in the range from 1.01 to 1.10 was required for the momentum range of interest. A total of 1680 of the aerogel tiles with dimensions about 11×11×1 cm3 were produced by the Matsushita Electric Works. We measured the refractive index and dimensions for all tiles and 1020 tiles were accepted as they satisfy the required parameters. The optical properties of the aerogel (light transflectance, reflectance and transmittance) were measured in the wavelength range from 200 to 900 nm. Eight hundred and fifty of the selected tiles were installed in the RICH detector. Since 1998 the HERMES RICH detector has been operated and it has provided the particle identification with high precision. Using the experimental data accumulated, the long-term stability of the aerogel has been studied. The specifications and measured optical parameters of the aerogel radiators will be presented.