Trapping of positrons at grain boundaries in nanoparticle systems

The positron annihilation in nanoparticle materials has been discussed in terms of the thermal diffusion of positrons at grain surfaces and trapping at the grain interfaces and interfacial defects. The diffusion trapping model has been applied to obtain the mean lifetime of positrons as a function of grain size and temperature and the S-parameter of the annihilation radiation as a function of grain size in Ag, Nb, Zn–Al and Zn–Al–Mg nanocrystallites. The calculated mean positron lifetimes have been compared with the available experimental results. The value of has been found to decrease with an increase in the size of the grains, suggesting that the density of grain boundaries decreases gradually when the grain grows. A fast decrease in has been observed in the case of alloys but not in metals, suggesting that, in the case of alloys, grains of larger size provide trapping centres. The calculations show that the characteristics of the nanocrystalline materials undergo significant changes at lower grain size. At larger grain size the materials show behaviour similar to that in the bulk sample. The lifetime data also suggest that the grain boundaries may be composed of vacancy clusters. The diffusion coefficient has been found to be described by a relation .