1. E. Fermi:Phys. Rev.,57, 485 (1940).
2. G. C. Wick:Ric. Scient.,11, 273 (1940);12, 858 (1941),Nuovo Cimento,1, 302 (1943).
3. O. Halpern andH. Hall:Phys. Rev.,57, 459 (1940);73, 477 (1948).
4. A. Bohr:Det. Kgl. Dans. Vid. Sels.,24, n. 19 (1948).
5. M. Schönberg:Bull. Cent. Phys. Nucl. Brux., n. 20 (1950).