The luminosity monitor of the HERMES experiment at DESY

A detector is described which measures the luminosity of the HERMES experiment at DESY. It is based on the coincident detection of electron-positron and photon pairs, or electron pairs, originating from the interaction of the beam positrons, or electrons, with the electrons of the atomic gas target. It consists of two calorimeters with radiation hard NaBi(WO(4))(2) crystals. Properties of the monitor, investigated in an electron test beam, and its performance in the experiment are presented. (C) 2001 Elsevier Science B.V. All rights reserved.