An active collimator system for a high-energy photon beam

Active collimators are designed to both collimate the tagged photon beam and suppress the background of secondary photons. These secondary photons are generated by interaction processes in the collimator medium. They may reach the hadron target with a lower energy compared to the energy determined by the tagging system. Accordingly, those events are tagged with a wrong energy and have to be rejected. By means of a Monte Carlo simulation we investigated the effect and the performance of an active collimator system and developed an optimal design of the collimators to prepare a clean high-energy photon beam. The resulting set-up ensures a precise determination of the tagging efficiency and of the photon flux. The active collimators are a necessary tool to correctly determine photoproduction cross-sections. Results of test measurements at ELSA in Bonn, and MAMI B in Mainz are discussed. Special interest is given to the use of the active collimators in the experiment to test the GDH sum rule at ELSA up to 3.2 GeV.