Positron lifetime and Doppler-broadening studies of vacancy formation and phase transformation in uranium

The positron lifetime and the Doppler-broadening of the annihilation radiation have been investigated in high-purity uranium at temperatures between 290 K and 1400 K. We find a significant dependence of the bulk lifetime and the annihilation lineshape on crystal structure and on temperature. Trapping at vacancies is observed only in γ-uranium. The total trapping rate is surprisingly low and increases by only a factor of two from the β -> γ transition temperature to the melting point. In γ-uranium the vacancy formation parameters as well as the specific trapping rate for positrons at vacancies most probably depend on temperature. The average formation enthalpy in the γ-phase is of the order of 0.3 eV. (orig.)