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The highest energy cosmic rays

1991; Elsevier BV; Volume: 22; Issue: 2 Linguagem: Inglês

10.1016/0920-5632(91)90011-3

1873-3832

Alan Watson,

Neutrino Physics Research

The experimental work on the energy spectrum, mass composition and arrival direction distribution of cosmic rays above 1018eV is reviewed. Particular attention is given to the various measurements which have been made of the cosmic ray energy spectrum, including a detailed discussion of cross-calibration procedures. It is concluded that the evidence favours a spectral slope which is steeper (γ>3) above 1018eV than below, but beyond 1019eV the slope flattens again with γ = 2.5 ± 0.2. Shower events produced by cosmic rays well in excess of 5 × 1019eV certainly exist and there is no evidence of a cut-off in the energy spectrum at least to 1020eV. The rudimentary evidence on mass composition, interpreted with a simple two mass component model, suggssts that the composition is mixed at least to 1018eV with evidence against significant further iron enrichment beyond. Measurements of arrival directions are also limited although recent analysis lends support to a picture in which there is increasing anisotropy associated with the galactic plane up to 1019eV. Above this energy the limited data support the view that there is an extra-galactic anisotropy, perhaps associated with the Virgo Supercluster. These overall conclusions are not particularly new; rather the recent evidence and new analyses have served to strengthen similar views reached some years ago: it is very clear that exposures of at least 1000km2 per year are needed if the field is to advance substantially over the next decade.