1. A.P. Mills, Jr.,Proc. Int. School of Physics “Enrico Fermi” Course LXXXIII (North-Holland, Amsterdam, 1983) p. 432.
2. P.J. Schultz and K.G. Lynn, Rev. Mod. Phys. 60(1988)701.
3. A. Vehanen,Positron Annihilation, eds. L. Dorikens-Vanpraet, M. Dorikens and D. Segers (World Scientific, Singapore, 1989) p. 38.
4. See, for example.Positron Annihilation, eds. Zs. Kajcsos and Cs. Szeles, Mater. Sci. Forum 105–110 (1992).
5. D. Schödlbauer, G. Kögel, P. Sperr and W. Triftshäuser, Phys. Stat. Sol. (a) 102(1987)549.