1. See the following review articles. G. H. Kinchin and R. S. Pease: Rep. Progr. Phys, 18, 1 (1955);
2. J. W. Glen: Adv. Phys, 4, 381 (1955);
3. K. Lintner und E. Schmid: Ergebn. exakt. Naturw., 28, 302 (1955);
4. F. Seitz and J. S. Koehler: Solid State Phys, 2, 305 (1956);
5. 2) A. K. Seeger: Second U. N. Int. Conf. Peacef. Uses Atomic Energy, P/998 (1958).