Average energies required per scintillation photon and energy resolutions in NaI(Tl) and CsI(Tl) crystals for gamma rays

The average energies required to produce one scintillation photon Ws were determined for 662 keV gamma rays in NaI(Tl) and CsI(Tl) crystals to be 15.0 ± 1.3 and 13.3 ± 1.1 eV, respectively, from the absolute numbers of photoelectrons measured for several combinations of a crystal and a photomultiplier tube (PMT) used as a vacuum photodiode. The numbers of scintillation photons were obtained by calculating the collection efficiency of scintillation photons at the photocathode using Monte Carlo simulations and by determining experimentally the photon-to-photoelectron conversion efficiency at PMT photocathode. The values of Ws determined in the present study are in good agreement with the theoretical values presented recently. The factors affecting energy resolutions were also examined. The calculated resolution agrees well with that obtained experimentally. (author)