1. K. Griesen:Ann. Rev. of Nucl. Sci., (1960).
2. Private discussion with Prof.B. Rossi and Prof.S. Olbert.
3. V. V. Guzhavin andG. T. Zatsepin:Žurn. Ėksp. Teor. Fiz.,32, 365 (1957).
4. B. Peters:Proc. of the Intern. Conf. on Cosmic Rays and the Earth Storm (Kyoto, 1961), III Cosmic Rays, p. 522.
5. B. Peters:Moscow Cosmic Ray Conference,3, 157 (1960).