Primary cosmic γrays above 1012 eV

Abstract

An analysis is made of the interactions of very energetic cosmic ray primaries with the universal black body radiation in extragalactic space. The gamma rays from the initial pi 0 mesons produce electron pairs in collisions with further black body photons and a gamma -e cascade builds up through this interaction and the inverse Compton interaction. Results are given for the expected gamma -ray spectrum at the earth for a variety of assumptions about the magnitude of the primary spectrum and other parameters and comparison is made with upper limits to the gamma -ray intensity from experimental data. One conclusion that arises immediately is that if the higher of the two primary proton energy spectra is correct then the extragalactic infrared energy density must be less than about 2 eV cm-3. In general the predicted gamma to proton ratios are highest in the region of 3*1019 eV. They are mostly below present detection limits but should be detectable with the improved extensive air shower arrays at present being constructed.