A MODEL FOR INTERPRETING THE POSITRON-ANNIHILATION CHARACTERISTICS OFDEFORMED METALS - APPLICATION TO VANADIUM

The positron characteristics in the temperature range 10-300 K have be en investigated for single crystals of vanadium deformed under differe nt conditions. Positive temperature dependences for the annihilation p arameters S and <(tau)over bar> are found. The average rate of increas e Delta S/Delta T appears to be controlled by the dislocation density and concentration of vacancies induced by deformation. A positron trap ping model, assuming trapping at dislocations decreasing exponentially with temperature, thermally activated detrapping from dislocations an d temperature-independent trapping at deep traps, predicts a temperatu re-dependent competing trapping that determines the temperature depend ences of the annihilation parameters. The validity and predictions of the model are discussed in terms of the parameters of the functions S( T) or <(tau)over bar>(T). The positron binding energy for dislocation lines, their positron trapping coefficient, the dislocation density an d the rate of trapping at deep traps can be determined by fitting the observed temperature dependences to this competing-trapping model. The method permits us to investigate quantitatively the evolution of the defect structure of metals during their recovery.