V. I. Yakovlev, M. I. Vil'danova, N. G. Vil'danov, “Detection and investigation of delayed extensive air showers with $N_e>5\cdot10^6$”, Pis'ma v Zh. Èksper. Teoret. Fiz., 2007, Volume 85, Issue 2,Pages 111–113
Abstract:
Delayed extensive air showers with Ne > 5 × 106 are detected. The detection of such events indicates the existence of new processes for energies above 1016 eV.