Bai, JZ (reprint author), Georgia State Univ, Dept Phys & Astron, 20 Peachtree Ctr Ave, Atlanta, GA 30303 USA.
The Beijing Spectrometer (BES) detector is a general purpose solenoid detector at the Beijing Electron Positron Collider (BEPC) in Beijing, China, that has collected large numbers of J/psi, psi', D-s, D and tau events. In this paper, we describe the recent upgrade of the initial BES detector(BESI) to the improved BESII detector. (C) 2001 Elsevier Science B.V. AU rights reserved.