Delayed Coincidence Time Resolution: NaI(Tl)

For delayed coincidence experiments using NaI(Tl) scintillation counters, first photoelectron triggering (FPET) has been practicable since the advent of the RCA 8575. Using FPET, improved time resolution at low y ray energies was first demonstrated by Lynch, who also described his results by a model that fails to take into account the photomultiplier response. Using FPET, we have done systematic experiments from 6-511 keV for three different crystal sizes. We describe our results quantitatively by a simple model that takes into account the statistics of photoelectron production and the photomultiplier response, similar to El-Wahab and Kane. From the model calculations, we conclude that with present photomultipliers and NaI(Tl) scintillators, further improvements in resolution cannot be expected. The major difficulty is not the light output of NaI(Tl), but the single electron response of the photomultiplier.