1. I. Ya. Emel'yanov et al., At. Énerg.,22, 367 (1967).
2. E. I. Grishanin et al., At. Énerg.,10, 565 (1961).
3. A. N. Grigor'yants et al., Paper No. 308 presented by the USSR at the Third International Conference on Peaceful Uses of Atomic Energy (Geneva, 1964).