Decay curves of NaI(Tl) scintillators with different Tl+ concentrations under excitation of electrons, alpha particles and fission fragments

Decay curves of NaI(Tl) crystals with Tl+ concentrations of 0.01, 0.07, 0.1 and 0.22 mole% have been measured under excitation by photo-electrons from a Cs-137 source, alpha particles from an Am-241 source and fission fragments from a Cf-252 source. A striking difference between the decay curve for electron excitation and those for excitation by alpha particles and fission fragments is observed. A flat top of the decay curve which is observed under electron excitation is analyzed on the bases of energy transfer processes of electrons and V-k centers to Tl+ ions reported by Dietrich et al. [Phys. Rev. B 8 (1973) 589]. A flat top of the decay curve from NaI(Tl) with a Tl concentration of 0.01 mole% under alpha particle excitation is also observed. Faster decay components of 34+/-5 ns (in addition to the main scintillation component of 150+/-10 ns) and 15+/-5 ns (in addition to the main component of 100+/-10 ns) are observed under alpha particle excitation and fission fragment excitation, respectively. This finding indicates the presence of a non-radiative process of (Tl+)* due to interaction with excited atoms under high specific energy loss dE/dx. This non-radiative process should be partly responsible to the decrease in scintillation efficiency for high dE/dx particles.