Study of a nonhomogeneous aerogel radiator in a proximity focusing RICH   detector
  The use of a nonhomogeneous aerogel radiator, i.e. one consisting of layers with different refractive indices, has been shown to improve the resolution of the Cherenkov angle measured with a proximity focusing RICH detector. In order to obtain further information on the performance of such a detector, a simple model has been used to calculate the resolution and search for optimal radiator parameters. 
Peter Krizan, Samo Korpar, Toru Iijima