Radiation damage of NaI(Tl) by fast neutron irradiation:: Blocking of the energy transfer processes from Vk centers and electrons to the activator of Tl

The scintillation pulse heights and decay curves from fast-neutron irradiated NaI(Tl) crystals up to the integrated fast neutron flux of 2.4 x 10(15) cm(-2) were measured under excitation by electrons, alpha particles and fission fragments. For Tl concentrations of 0.01, 0.07 and 0.22 mole%, decreases by 20 similar to 40% in scintillation pulse heights are observed for the irradiated crystals compared with those of non-irradiated crystals. The decreases are enhanced for electron excitation compared with those fur alpha particle and fission fragment excitation. The decrease in scintillation pulse height is attributed to blocking of the energy transfer processes of V-k centers and electrons by the traps and lattice disorders which were produced by fast neutrons, The effect of this blocking of the energy transfer processes is clearly observed as a difference in decay curves for the crystals before and after fast neutron irradiation; a flat top which appeared before the irradiation is not present after irradiation.