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The nucleation of persistent slip bands in stressed materials is described as a cooperative phenomenon for dislocation populations. It is the competition between their mobility and their nonlinear interactions (creation, annihilation, and pinning) which causes the instability of uniform dislocation distributions versus inhomogeneous ones and leads to the formation and persistence of dislocation patterns.

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167tracks with length >20/z to cosmic rays with Z>32. Using the new relation between Z and L, we find that their ratio 4550 should apply to the number of cosmic rays with Z=20 to 30 relative to those with Z>30. VIIL CONCLUSIONS1. Fe ions form tracks in hypersthene at energies between ^0.48 MeV/iV" and some as yet undetermined higher energy; Fe ions form tracks in diopside between ^0.29 MeV/iV and some undetermined higher energy. It will be very important experimentally to determine the high-energy threshold at which track formation ceases. We estimate that this energy is ^1.8 MeV/N for hypersthene and ^2.7 MeV/iV for diopside, energies which are now attainable in the newest tandem van de Graaff accelerators.2. Diopside is somewhat more sensitive than hypersthene. It should record tracks of ions as light as Cr. IRREVERSIBILITY IN PARAMAGNETIC SPIN SYSTEMS

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