Gamma ray source using internal targets in the TRISTAN Accumulation Ring

A gamma ray source using internal targets in an electron storage ring, TRISTAN Accumulation Ring (AR), has been operated since May 1984. Two detectors for high energy physics experiments called TOPAZ and VENUS are to be installed in the TRISTAN main ring. They have several thousands of lead glass counters as calorimeters. Each counter needs calibration using electron beams of several GeV before the installation. Such high energy electron beams could be obtained in the KEK 12 GeV proton synchrotron, which was, however, not operational due to the construction of the TRISTAN project in 1984. As an alternative electron source TRISTAN AR is now operational, and is able to accelerate and store an electron beam of more than 5 GeV. We planned to extract high energy gamma rays by inserting a internal target into the AR, because a direct beam extraction is not easy. Two gamma ray lines are prepared by the two detector groups. Each detector group has its own target and gamma ray line. It is also required that the gamma rays should be simultaneously produced at the two targets with the least interference between them. The circulating electron beams gradually collide with the target and produce gamma rays, which are extracted from the AR through a Be-foil window. By a converter the gamma ray is changed into high energy electrons and positrons, which are finally used for the calibration of the lead glass counter. The momentum of the electron beam is defined by an analyzer magnet. At present two gamma ray lines, IT1 and IT4, are available and are able to produce the electron beams for the two detector groups simultaneously