1. G. Lütjens, internal report of Max-Planck-Institut für Physik und Astrophysik, München.
2. N. Schmitz, internal report of Max-Planck-Institut für Physik und Astrophysik, München.
3. A more complete discussion of track density and measuring technique is given by B.N. Fabian, R.L. Place, W.A. Riley, W.H. Sims and V.P. Kenney (to be published).