An active collimator system for a high-energy photon beam

Active collimators are designed to both collimate the tagged photon beam an d suppress the background of secondary photons. These secondary photons are generated by interaction processes in the collimator medium. They may reac h the hadron target with a lower energy compared to the energy determined b y the tagging system. Accordingly, those events are tagged with a wrong ene rgy and have to be rejected. By means of a Monte Carlo simulation we invest igated the effect and the performance of an active collimator system and de veloped an optimal design of the collimators to prepare a clean high-energy photon beam. The resulting set-up ensures a precise determination of the t agging efficiency and of the photon flux. The active collimators are a nece ssary tool to correctly determine photoproduction cross-sections. Results o f test measurements at ELSA in Bonn, and MAMI B in Mainz are discussed. Spe cial interest is given to the use of the active collimators in the experime nt to test the GDH sum rule at ELSA up to 3.2 GeV. (C) 2001 Elsevier Scienc e B.V. All rights reserved.