Temperature dependence of the Doppler-broadened positron annihilation spectra in single and polycrystalline Cd

Abstract

The Doppler shifts of annihilation photons have been measured in well-characterized single and polycrystalline samples of Cd as a function of sample temperature between 10 and 540 K by a two-detector Doppler-broadening method which permits separation of core-state and valence-state annihilations. The temperature dependence of the broadening associated with those annihilations involving predominantly low-momentum electrons was found to be dependent on the orientation of the single-crystal sample with respect to the axis of the two detectors. This detected anisotropy at various sample temperatures (220< or =T< or = 325 K) is thought to be associated with the asymmetric nature of the positron and electron wave functions, possibly localization of the positron. The variation with crystallographic direction in the low-momentum region occurs both at low temperatures in the so-called prevacancy region and at temperatures where vacancy trapping is observable. The components of the Doppler-broadened spectra that correspond to annihilations with very-high-momentum electrons as might be expected do not show this orientation dependence although they agree with the low-momentum components in implying the existence of a well-defined prevacancy region.