The Tibet hybrid experiment composed of emulsion chambers, burst detectors and the Tibet-II air-shower array was done at Yangbajing (4,300m above sea level, 606 g/cm2) in Tibet. For 3-year operation, we have observed 177 γ-families (ΣEγ > 20 TeV) accompanying air showers with the shower size Ne > 2 × 105. Using this data set and a neural network method to select proton and helium induced events, the primary proton and helium energy spectra are deduced between 1015 and 1016eV. The proton spectrum, thus obtained, can be expressed by a single power-law function with a differential index of -3.01±0.11 and -3.05±0.12 based on the QGSJET+HD and SIBYLL+HD models, respectively, which are steeper than that extrapolated from the direct observations of -2.74±0.01 in the energy range below 1014 eV. Our result suggests that the main component responsible for making the knee structure of the all-particle spectrum should be composed of nuclei heavier than helium.
ASJC Scopus subject areas
- Physics and Astronomy(all)