1. Crooks, R. N., et al., A.E.R.E.–R 2952 (1959).
2. Burton, W. M., and Stewart, N. G., Nature, 186, 584 (1960).
3. Cambray, R. S., et al., A.E.R.E.–R 4392 (1963).
4. Patterson, R. L., and Lockhart, L. B., in The Natural Radiation Environment, edit. by Adams, J. A. S., and Lowder, W. M., 383 (Univ. Press. Chicago, 1964).
5. I.C.R.P.—Report of Committee II (Pergamon Press, 1959).