Measurements of the photon detection inefficiency of calorimeters between 185 and 505 MeV

The detection inefficiencies for photons of total-absorption-type electromagnetic calorimeters, an undoped CsI crystal counter and a lead-scintillator sandwich counter, have been measured at photon energies (Eγ) between 185 and 505 MeV with the tagged photon beam from the INS-ES. The effect of punch-through is estimated by an EGS calculation. The other source of inefficiency, the photonuclear interaction effect, is examined after the interaction is identified by neutron signals observed with liquid scintillation counters surrounding the sample calorimeter. The inefficiency due to the photonuclear interaction in the case of the CsI calorimeter at 1-MeV threshold is found to be 10−5–10−6 and shows a monotonic decrease with Eγ. The detection inefficiency of the sandwich calorimeter is found to be dominated by the sampling effect after photonuclear interactions.