1. T. Stanev, in Proceedings of the 26th International Cosmic Ray Conference (ICRC), Salt Lake City, 1999 (American Inst. of Physics, Melville, 2000), p. 247.
2. N. L. Grigorov, G. P. Kakhidze, V. E. Nesterov, et al., Kosm. Issled. 5 (3), 383 (1967).
3. I. P. Ivanenko, V. Ja. Shestoperov, et al., in Proceedings of the 23rd International Cosmic Ray Conference (ICRC), Calgary, Canada, 1993 (World Sci., New York, 1994), Vol. 2, p. 17.
4. K. Asakimory, T. H. Burnett, M. Cherry, et al., Astrophys. J. 502, 278 (1998).
5. V. I. Zatsepin, T. V. Lazareva, G. P. Sazhina, and N. V. Sokol'skaya, Yad. Fiz. 57, 684 (1994) [Phys. At. Nucl. 57, 645 (1994)].