Proton-nucleus reactions are discussed in the framework of the evolution model proposed by Hwa. The longitudinal-momentum distributions of baryons are calculated and compared with the data. A momentum-degradation length of 5.7 to 6.9 fm is found. These results are contrasted with other recent theoretical analyses. The implication is that the central cores of heavy nuclei such as U and Pb may stop each other at beam energies up to 25 GeV per nucleon.
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