Numbers of scintillation photons produced in NaI(Tl) and plastic scintillator by gamma-rays

The W_S-value, which is defined as an average energy expended per scintillation photon, is determined to be 17.2±0.40 eV for a NaI(Tl) phosphor and 60.8±4.3 eV for a plastic scintillator (NE-102A). These are obtained from the numbers of photoelectrons measured with several combinations of a photomultiplier tube and a NaI(Tl) or a NE-102A scintillator. The number of photoelectrons, which are measured by the photomultiplier tube as a vacuum photodiode, are converted to the number of scintillation photons by using an averaged quantum efficiency of each photomultiplier photocathode and a calculated collection efficiency of the scintillation photons at the photocathode. The above values do not include the uncertainties due to the unknown exact emission spectra and the photomultiplier response curves