A novel aerogel Cherenkov detector for DIRAC-II

We present a new threshold Cherenkov detector using silica aerogel radiators with wavelength shifter. A duplex design was chosen to discriminate between kaons and protons in the momentum range 4–8 GeV/c for the DIRAC-II experiment at CERN. The counter consists of overlapping modules with two different refractive indices, 1.015 and 1.008, covering the low and high momentum regions, respectively. The detector performance was optimized with cosmic rays and Monte-Carlo simulations. Thereby we developed a new design of alternating layers of aerogel tiles and diffusive reflector foils coated with wavelength shifter. The best results were obtained with p-terphenyl giving 50% increase in light yield. A pyramidal shape of the aerogel radiator was also adopted for the best signal uniformity over a large sensitive area.