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 betewen 10 and 540 K by a two-detector Doppler-broadening method which permits separation of core-state and valencestate 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 ($220<~T<~325$ 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.