1. F. Brooks, Nucl. Instrum. and Methods, 4, 151 (1959).
2. Yu. A. Kazanskii et al., At. Énerg.,10, 143 (1966).
3. V. K. Daruga, Dissertation [in Russian], OIYaI, Dubna (1971).
4. V. I. Daruga and N. N. Krasnov, At. Énerg.,30, 399 (1971).
5. L. Smith and P. Kruger, Phys. Rev.,83, 1137 (1951).