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The Cosmic Ray Composition Towards The Knee: Experimental Status And Implications

INTRODUCTION Sources of the cosmic particle radiation cannot directly be observed. Rather, their properties must be inferred from measurements of the ambient cosmic ray flux near Earth. This requires an assessment of the changes in composition and energy spectra that occur while the cosmic rays propagate from the source regions to the observer. Among the relevant processes are ionization energy losses, nuclear spallation reactions, diffusive escape from the galaxy, and, locally, solar modulation. At high energies, above -10 GeV/nucleon, solar modulation and ionization energy losses are insignificant, and nuclear spallation cross sections become independent of energy. Therefore, interstellar propagation effects are much easier to quantify at high energies than in the low energy region. Unfortunately however, because of the decreasing flux the quality of available data on cosmic ray composition decreases rapidly with increasing energy: much detail is known in the GeVregion, but only the spectra of some of the major components are measured up to TeV energies, and virtually no information on cosmic ray composition exists beyond N 1014 eV per particle. At present, all detailed information on the cosmic ray composition is derived from direct measurements with detectors above the atmosphere, flown either on balloons or spacecraft. Indirect measurements with air shower observations have determined the “all-particle” spectrum from 1014 eV to lo2’ eV and have detected a conspicuous steepening of the spectrum between 1015 and 10l6 eV (usually referred to as “the knee”) but have encountered great difficulty in providing more detail about the primary flux than an average classification such as “light” (proton-like), “heavy” (ironlike), or “mixed”. Even if new installations improve this situation, it would be highly desirable if the energy ranges between future direct and indirect measurements overlap to permit a cross calibration of the techniques. OVERVIEW OF EXPERIMENTAL DATA Direct measurements on the cosmic ray composition at high energy have been performed with detectors on board the HEAO-3 satellite (l), the Spacelab-2 mission (2), the Russian Sokol mission (3), and with several balloon payloads (4-9) including the JACEE program (10). In figures l and 2 we show a compilation of data for the more abundant primary nuclei. The elements C, N, 0 and also Ne to S are grouped together although individual spectra have been reported up to N 10’’ evlnucleon. For higher energies, data with individual element resolution are not available for the heavier nuclei, and the statistical accuracy of the results deteriorates with increasing energy. Except for the most abundant elements, H and He, no significant data exist above 2 x 1013 evlnucleon. A most conspicuous feature in figure 1 is the apparent steepening of the proton spectrum above eV. The dashed lines in figure 2 refer to a model fit to the data which will be