End to the Cosmic-Ray Spectrum?

Kenneth Greisen
1966 Physical Review Letters  
tons. When the complete potential is deformed, the optical parameters (Table I) found so far to give the best fit to the elastic polarization also produce the best prediction of inelastic asymmetry. All the curves in Fig. 2 use a central-well deformation parameter of P, =0.39 for~si and P, = 0.22 for~Ni. The deformation parameter of the spin-orbit term is 1.5 times the centralwell value, which produces slightly better agreement with the asymmetry data for "Si. Both real and imaginary parts of
more » ... e spin-orbit interaction are included, but since i W~I «VS, the imaginary part makes little difference. The curves also include Coulomb-excitation amplitudes, ' which make little difference in either the asymmetry or the cross section. We find that for all of the calculations made, the predictions of inelastic asymmetry and inelastic polarization are very nearly identical. In summary we find that, provided the imaginary and spin-orbit terms are included, the collective-model generalization of the optical potential gives a good account of the present inelastic asymmetry data at all but the most forward angles. It is quite possible that a more comprehensive treatment of the spin-dependent interaction will improve matters in this region, and such calculations are in progress. It is a pleasure to acknowledge many useful conversations concerning this work with G. R. Satchler and N. M. Hi.ntz. We are much indebted to the indefatigable ORIC cyclotron operators, and to , Phys. Rev. 128, 2693 (1962). The present DW calculations include the spin-orbit term in the elag+ic distortion. M. P. Fricke and G. R. Satchler, Phys. Rev. 139, B567 (1965). 3T. Stovall and N. M. Hintz, Phys. Rev. 135, B330 (1964). The primary cosmic-ray spectrum has been measured up to an energy of 10' eV, ' and several groups have described projects under development or in mind' to investigate the spectrum further, into the energy range 10"-10" eV. This note predicts that above 10' eV the primary spectrum will steepen abruptly, and the experiments in preparation will at last observe it to have a cosmologically meaningful termi.nation. The cause of the catastrophic cutoff is the intense isotropic radiation first detected by Penzias and Wilson' at 4080 Mc/sec (7.35 cm) and now confirmed as thermal in character by measurements of Roll and Wilkinson4 at 3.2 cm wavelength. It is not essential to the present argument that the origin of this radiation conform exactly to the primeval-fireball model outlined by Dicke, Peebles, Roll, and Vfilkinson', what matters is only that the radiation exists and pervades the observable universe. The transparency of space at the pertinent wavelengths, and the consistency of intensity observations in numerous directions, 748
doi:10.1103/physrevlett.16.748 fatcat:2trqbejjofd2fgqcviyb7rbz4y