1. P. Hautoj�rvi (ed.), ?Positrons in Solids? (Springer Verlag, Heidelberg, 1979).
2. R. N. West,Adv. Phys. 22 (1973) 263.
3. C. F. Coleman, A. E. Hughes, F. A. Smith andM. J. Weaver,NDT International (1982) 87.
4. D. Segers, F. van Brabander, L. Dorikensvanpraet andM. Dorikens,Appl. Phys. A27 (1982) 129.
5. W. B. Gauster, W. R. Wampler, W. B. Jones andJ. A. van den Avyle, in Proceedings of the 5th International Conference on Positron Annihilation, Japan, April, 1979, edited by R. R. Hasiguti and K. Fujiware (The Japan Institute of Metals, Sendai 980, Japan, 1980) p. 125.