Energy Transfer and the Role of Thallium Dimers in the Nal(Tl) Scintillation Process

The thallium dimer center, which is a pair of thallous ions occupying nearest neighbor cation sites in NaI, is found to have a decay time of less than 7 nanoseconds. Analysis of the emission spectrum of a typical NaI(Tl) scintillation crystal indicates that the dimer centers are responsible for approximately 25% of the scintillation light although only a small part of this is rapid. It is proposed that energy of excitation migrates among the monomers and between monomers and dimers as a result of thermal excitation.