Calibration of An Aerogel Counter of Index 1.1 at the Bevalac

A Cerenkov counter consisting of a mosaic with 48 aerogel pieces having refractive indices n ≈ 1.1 has been exposed to sea level muons, and to a beam of relativistic ^(55)Mn at the Berkeley Bevalac. The counter is 6 cm thick and approximately 60 cm in diameter, and is viewed by twelve 12.7 cm diameter photomultipliers. A relativistic muon produces typically 23 ± 4 photoelectrons. The light yield has been mapped as a function of position and has an r.m.s. variation less than 1%/cm. Analysis of the light yield indicates that absorption is important at short wavelengths. The calibration shows that the light yield and uniformity of the counter suffice to determine particle velocity in the γ range from 2.4 to 3.1 precisely enough to achieve a mass resolution of 0.3 a.m.u. for the instrument described in paper OG2-7 of this conference.