1. For earlier work, see the review articles inPositron Annihilation. ed. by A. T. Stewart and L. O. Roellig (Academic Press, New York 1967)
2. For up-to-date reviews, see. R. N. West: Advances Phys.22, 263 (1973); and
3. I. Y. Dekhtyar: Phys. Reports9C, 243 (1974)
4. V. I. Goldanskii: Atom. Energy Rev.6, 3 (1968) see also the up-to-date reviews by V. I. Goldanskii, V. P. Shantarovich: Appl. Phys.3, 335 (1974); and S. J. Tao: Appl. Phys.3, 1 (1974)
5. See, for example, L. F. Chollet, I. M. Templeton: Phys. Rev.170, 656 (1968);