Cerenkov Emission from Aerogel

Particle identification is a vital part of the PHENIX detector. In present, hadron identification capability of the PHENIX has holes in the momentum range; 2.5 - 5 GeV/$c$ for $\pi$/K separation, $>$ 5 GeV/$c$ for K/p separation. For further physics research, extension of the momentum range is desired. For particle identification in high momentum region, Cerenkov radiation can be used. Aerogel Cerenkov counters (AEROGEL) are proposed to extend the momentum range. The ACC together with the existing TOF and gas-RICH can provide $\pi$/K separation upto 3.7 GeV/$c$ and K/p separation upto 7 GeV/$c$.

The refractive index of aerogel should be $\sim$1.01 from the desired momentum range. Because of the low index, Cerenkov photons emitted by passing of charged particle is intrinsically small. Therefore, efficient light collection system should be considered to obtain enough resolving power when identifying different particles.

The intensity of Cerenkov light is inversely propotional to the square of the wavelength. We should collect the Cerenkov light of shorter wavelength. The Cerenkov photons are emitted in a narrow cone of about $8^{\circ}$ with respect to the particle trajectory in a aerogel of n = 1.01. In the aerogel, Rayleigh scattering is known to be the dominant process. Rate of the scattering decreases as the fourth power of the wave length.

Thus, these two optical properties are important when designing the counter: the directionaliy of Cerenkov lights with respect to the particle trajectory and diffusive optical property of the aerogel.