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"A very high-energy cosmic ray iron nucleus shoots into some photographic emulsion & collides with a silver or bromine nucleus in the emulsion to produce a tremendous \jet\"" of about 850 mesons. From the divergence of the jet, the total energy of the incoming iron nucleus is calculated to be more than 15,000 GeV - an example of the enormous energy that can be carried by some cosmic rays. (The central bright core of the jet is about 0.04 mm across).""" (KEYSTONE/SCIENCE PHOTO LIBRARY/POWELL, FOWLER & PERKINS)