Abstract
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 137Cs source, alpha particles from an 241Am source and fission fragments from a 252Cf 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 Vk 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&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.
