Temperature dependence of positron trapping at small voids in neutron-irradiated Mo

The temperature dependence of the positron lifetime spectra in Mo single crystals irradiated with fast neutrons at 570K and 370K has been measured. In the specimen irradiated at 570K the intensity of the long lifetime component, attributed to trapping at voids about 6-10 AA in radius, increases linearly from 40K to 460K. This result can be understood if it is supposed that the trapping is diffusion limited and that the diffusion constant varies linearly with temperature as it might if scattering is primarily from extended defects. The specimen irradiated at 370K shows very little temperature dependence of positron trapping into small voids-clusters of perhaps a dozen vacancies. It is presumed that the trapping is transition-rate limited in this specimen.