nuclear composition of UHECRs

Today Michael Unger (Karlsruhe) told us over lunch about ultra-high energy cosmic rays from Auger. There are many mysteries, but it does look like the composition moves to higher-Z nuclei as you go to higher energies, or at least that's my read. He told us also about a very intriguing extension to Auger which would make it possible to distinguish protons from iron in the ground detectors; if that became possible, it might be possible to do cosmic-ray imaging: It is thought that the cosmic magnetic fields are small enough that protons near the GZK cutoff should point back to their sources. So far this hasn't been possible, presumably because the iron (and other heavy elements) have charge-to-momentum ratios too large; they get heavily deflected by the magnetic fields they encounter.

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