The thermalization time of positrons in metals has been computed as a function of the electron density parameter rs, in the complete metallic electron density range (2 s 2/r exp (-λTFr) where λTF is the Thomas-Fermi momentum given by √4π∝ pF. Here ∝ is related to rS, the usual density parameter, by ∝ = rS/1.919π2 Further, we find that, in general, the time taken by the positron to drop to an energy of .025 e.v. is not as short as is generally believed, although it can be said beyond doubt that complete thermalization has taken place before annihilation at room temperature. However, for aluminium at 100°k, the thermalization time is longer than the annihilation time. On the basis of this result, we suggest that this lack of thermalization in aluminium might be detectable in an experiment similar to that recently reported by Stewart and Shand (1966), concerning the positron effective mass in sodium, although without more extensive calculations it is not possible to say precisely how a small amount of non-thermalization would affect the angular correlation curves.

Master of Science (MSc)