Gamma Ray Detection with Long NaI(Tl) Scintillator Bars

Extensive position, energy and timing resolution measurements have been made for a l00×5×5 cm3 NaI(Tl) scintillator bar. The arrival times and the pulse heights of signals from two photomultiplier tubes, one at each end of the bar, are used to provide the energy loss, linear position and time-of-flight for gamma ray interactions in the bar over the energy range from 0.5 to 20 MeV. The scintillator bar has an approximately exponential roll-off with a mean attenuation coefficient of 0.015 cm-1. At 0.662 MeV, resolutions (FWHM) of 5 cm, 9.4% and 10 ns were obtained for position, energy and timing, respectively. The timing resolution is 6 ns at 1.25 MeV. At 6.13 MeV, a spatial resolution of 2.2 cm was obtained. This bar is a prototype element of a Compton scatter telescope that uses both plastic and NaI(TI) scintillator bars. The technique is being used for medium energy gamma ray astronomy from a balloon platform. The telescope will have 3.5° (FWHM) angle and 6% energy resolutions at 6 MeV.