The time dependence of scintillation intensity in aromatic materials

A description is given of the kinetics of the prompt and delayed luminescence components from aromatic materials excited by ionizing radiation. A theoretical model has been developed to describe the origin of the delayed scintillation component. From initial assumptions of molecular triplet state diffusion and triplet-triplet annihilation to form delayed singlet states, equations are obtained defining the intensity of the delayed component with time. These equations are shown to agree well with existing experimental results. From the comparison the following parameters for triplet excitations in anthracene and stilbene crystals have been calculated: diffusion constant, 6 x 10-6 cm2 s-1 (anthracene) and 3 x 10-6 cm2 s-1 (stilbene); triplet-triplet interaction rate constant, 1·3 x 10-11 cm3 s-1 (anthracene) and 7·5 x 10-12 cm3 s-1 (stilbene); and diffusion length, 3·5 x 10-4 cm (anthracene).