γ-ray Compton profile of polycrystalline lithium

The Compton profile of polycrystalline lithium has been measured using a ${}_{}{}^{241}{}_{}{}^{}\mathrm{Am}$ source of 59.54-keV $\gamma$ rays. In this work the high-momentum region up to 3 a.u. was investigated and a non-negligible tail was observed in the Compton profile. Previously reported x-ray measurements did not extend into the momentum region beyond 1.6 a.u. The contribution of the two core electrons was substracted from the data using free-atom wave functions. The resultant conduction-electron profile was first compared with calculations in the cell approximation in which the electron-ion interactions are represented by a model using $s$-like wave functions. In particular such functions were computed in the renormalized free-atom model, previously employed by Berggren for calculation of the Compton profile of vanadium. Next, electron-electron interactions were taken into account in the former calculation by introducing occupation numbers obtained for an homogeneous interacting electron gas using a method due to Lam. The results indicate that the existence of the high-momentum tail in the profile is mainly due to the electron-ion interactions. This is in accord with recent work of Eisenberger et al., but in contradiction with Phillips and Weiss who attributed this tail to electron-electron correlations.