The bremsstrahlung tagged photon beam in Hall B at JLab

We describe the design and commissioning of the photon tagging beamline ins talled in experimental Hall B at the Thomas Jefferson National Accelerator Facility (JLab). This system can tag photon energies over a range from 20% to 95% of the incident electron energy, and is capable of operation with be am energies up to 6.1 GeV. A single dipole magnet is combined with a hodosc ope containing two planar arrays of plastic scintillators to detect energy- degraded electrons from a thin bremsstrahlung radiator. The first layer of 384 partially overlapping small scintillators provides photon energy resolu tion, while the second layer of 61 larger scintillators provides the timing resolution necessary to form a coincidence with the corresponding nuclear interaction triggered by the tagged photon. The definitions of overlap chan nels in the first counter plane and of geometric correlation between the tw o planes are determined using digitized time information from the individua l counters. Auxiliary beamline devices are briefly described, and performan ce results to date under real operating conditions are presented. The entir e photon-tagging system has met or exceeded its design goals. (C) 2000 Else vier Science B.V. All rights reserved.