A photon tagging system for the GDH-experiment at ELSA

Abstract

This paper describes the set-up and calibration of the tagging system of the GDH-Experiment at the accelerator facility ELSA in Bonn. The system consists of a C-type dipole magnet and a hodoscope of 65 scintillation counters which are operated pairwise in coincidence to form 64 tagging channels. Photons can be tagged in a range of 68-97% of the primary electron energy E-0. The energy resolution ranges from 0.2% to 0.6% of E-0. The time resolution is better than 165 ps. The system was operated at rates up to 5 x 10(6) photons/s in the full tagged energy range. The energy dependence of the photon definition probability was determined using several different methods and was found to agree well with calculations. (C) 2003 Elsevier Science B.V. All rights reserved.