Measurement of the photon detection inefficiency of electromagnetic calorimeters at energies below 1Â GeV

The photon detection inefficiency of electromagnetic calorimeters due to photonuclear reactions has been studied at photon energies below 1 GeV using a tagged-photon beam at the KEK-Tanashi 1.3-GeV electron synchrotron. Photonuclear reactions are identified by detecting low-energy neutrons with liquid scintillation counters surrounding the sample calorimeter. For a Cesium Iodide (CsI) calorimeter with a detection threshold of 10 MeV, the inefficiency due to photonuclear reactions is 10-4 at Eγ=100MeV, and decreases to 2×10-7 at Eγ=1GeV. For a lead-scintillator sampling calorimeter, the inefficiency is larger than the above values by a factor of 2-3, reflecting the sampling effect after photonuclear reactions. By decreasing the detection threshold down to 1 MeV, the inefficiencies are reduced by a factor of 10 for both types of calorimeters.