The HERMES RICH aerogel radiator

Y. Miyachi 
on behalf of the HERMES collaboration

Tokyo Institute of Technology 
Oookayama 2-12-1, Megoro, Tokyo, 152-8551, Japan

The HERMES RICH detector was designed to separate pions, kaons and protons in the momentum range form 2 to 15 GeV, using two different types of radiators, the gaseous C4F10 and silica aerogel. For the aerogel, a refractive index in a range from 1.01 to 1.10 was required for use in the momentum range of interest.
   A total of 1680 of the aerogel tiles with dimensions about 11x11x1 cm3 were produced by the Matsusita Electric Works. From measurements of the variation of the refractive index and the tile dimensions [1], 1020 tiles were accepted as possessing the required qualities. The optical properties of the aerogel like light transflectance, reflectance and transmittance, were measured in the wavelength range from 200 to 900 nm [2].
   850 of the selected tiles were installed in the RICH detector. They were stacked in 5 layers with 5 horizontal rows and 17 vertical columns for both top and bottom detectors. The containers which enclose the tiles are sealed gas tight and dry nitrogen is continuously circulated through the enclosure.
   Since 1998 the HERMES RICH detector has been operated and provided the particle identification of particles 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 as well as the obtained performance during the data taking.


[1] N. Akopov et al., " The HERMES Dual-Radiator Ring Imaging Cerenkov Detector," Nucl. Instrum. Meth. A 479, 2-3, 551 (2002).
[2] E. Aschenauer et al., "Optical Characterization Of  N = 1.03 Silica Aerogel Used As Radiator In The Rich Of Hermes," Nucl. Instrum. Meth. A 440, 338(2000).