Long-term operational experience with the HERMES aerogel RICH detector

For nearly 10 years, from 1998 until the closure of the HERA ring last July, a focused RICH has been used in the HERMES experiment at DESY to measure the semi-inclusive deep-inelastic scattering of polarized electrons on polarized gaseous targets. The RICH utilized two radiators, a heavy fluorocarbon gas, and a wall of silica aerogel tiles. It has identified pions, kaons, and protons in the momentum range from 2 to 15 GeV/c with a K / π separation of nearly 3 σ at 4 GeV/c. The optical properties of the aerogel and the RICH performance related to this radiator are reviewed, and some parameters are sampled over the whole RICH operational period in order to test its long-term stability.