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 137 Cs source, alpha particles from an 241 Am source and fission fragments from a 252 Cf 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 d E /d x . This non-radiative process should be partly responsible to the decrease in scintillation efficiency for high d E /d x particles.