The aerogel threshold Cherenkov detector for the High Momentum Spectrometer in Hall C at Jefferson Lab

We describe a new aerogel threshold Cherenkov detector installed in the HMS spectrometer in Hall C at Jefferson Lab. The Hall C experimental program in 2003 required an improved particle identification system for better identification of pi/K/P, which was achieved by installing an additional threshold Cherenkov counter. Two types of aerogel with n=1.03 and n=1.015 allow one to reach 10^{-3} proton and 10^{-2} kaon rejection in the 1-5 GeV/c momentum range with pion detection efficiency better than 99% (97%). The detector response shows no significant position dependence due to a diffuse light collection technique. The diffusion box was equipped with 16 Photonis XP4572 PMT's. The mean number of photoelectrons in saturation was ~16 and ~8, respectively. Moderate particle identification is feasible near threshold.