The highest-energy cosmic rays may be iron nuclei

Up against the Andes in the high plains of Argentina, the 3000-km 2 Pierre Auger Observatory has, since 2004, been recording the particle and fluorescence showers initiated by ultrahigh-energy cosmic-ray (UHECR) particles - mostly protons and fully ionized nuclei - hitting the top of the atmosphere. Such an enormous expanse, studded with 1600 surface detectors watched over by four fluorescence-telescope complexes, is appropriate because UHECRs become ever more scarce with increasing energy. Above 1019eV, one can expect to find only a few dozen CRs per square kilometer per century. © 2010 American Institute of Physics.