A concept for a 2 photon tagging device for e+e- interaction on the basis of a compact BGO calorimeter with photodiode readout

We present a design concept of a tagging device for 2 photon processes (i.e., e+e- → e+e-X) in high energy e+e- interactions. As an example we apply it to a proposed calorimeter detector at the future e+e- storage ring LEP but implementations in other storage ring detectors are possible. The device consists of fine sampled BGO calorimeters allowing a measurement of both the direction and energy of final state electrons without magnetic spectrometers. An energy resolution of approx. equal to 1% and a spatial resolution of approx. equal to 1 mm are conceivable. With large aperture superconducting interaction quadrupoles one might be able to tag as close as 8 mrad from the initial beam axis. Photodiode readout allows very compact constructions as well as installation in magnetic fields. Limitations, technical feasiblity and costs are discussed. (orig.)