1. E. O. Lawrence and N. E. Edlefsen, Science 72, 376 (1930).
2. E. O. Lawrence and D. Cooksey, Phys. Rev. 50, 1131 (1936).
3. E. O. Lawrence and D. Cooksey, Phys. Rev. 86, 411 (1937).
4. V. N. Rukavishnikov, Phys. Rev. 52, 1077 (1937).
5. J. R. Dunning and H. L. Andersen, Phys. Rev. 53, 334 (1938).