1. Andersson, B., Otterlund, I., and Kristiansson, K.: 1966,Arkiv. Fysik 31, 527.
2. Badhwar, G. D., Durgaprasad, N., and Vijayalakshmi, B.: 1965,Proc. Ind. Acad. Sci. 61, 374.
3. Cleghorn, T.F., Freier, P. S., and Waddington, C. J.: 1968,Can. J. Phys. 46, 5572.
4. Dixon, H. E., Turver, K. E., and Waddington, C. J.: 1974,Proc. Roy. Soc. London,A339, 157.
5. Dixon, H. E., Earnshaw, J. C., Hook, J. R., Smith, G. J., and Turver, K. E.: 1973,Proc. Denver Cosmic Rays Conf. 4, p. 2473.