A hybrid experiment consisting of emulsion chambers , burst detectors and the Tibet II air-shower array was carried out at Yangbajing ( 4,300 m a.s.l. , 606 g/cm ^ { 2 } ) in Tibet to obtain the energy spectra of primary protons and heliums .
From three-year operation , these energy spectra are deduced between 10 ^ { 15 } and 10 ^ { 16 } eV by triggering the air showers associated with a high energy core and using a neural network method in the primary mass separation .
The proton spectrum can be expressed by a single power-law function with a differential index of -3.01 \pm 0.11 and -3.05 \pm 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 \pm 0.01 in the energy range below 10 ^ { 14 } eV .
The absolute fluxes of protons and heliums are derived within 30 % systematic errors depending on the hadronic interaction models used in Monte Carlo simulation .
The result of our experiment suggests that the main component responsible for the change of the power index of the all-particle spectrum around 3 \times 10 ^ { 15 } eV , so-called “ knee ” , is composed of nuclei heavier than helium .
This is the first measurement of the differential energy spectra of primary protons and heliums by selecting them event by event at the knee energy region .