1. C.B.A. McCusker, Physics Reports (in the press). This is a review article giving references to the original work.
2. A.M. Bakich, D. Melley, C.B.A. McCusker, D. Nelson, L.S. Peak, M.H. Rathgeber & M.M. Winn, Canad. J. Phys. 46 No. 10 Part 2 30 (1968).
3. The Cosmic Ray Research Group of the Yunnan Institute of Atomic Energy, Scientia Sinica XVI No. 1 123 (1972).
4. M.R. Krishnaswany, M.G.K. Menon, V.S. Narasimhan, N. Ito, S. Kawakami, & S. Miyake, University of Tokyo, C.R.L. Report-21 (1975).