The cosmic-ray energy spectrum between 10^14.5 and 10^16.3 eV covering the "knee" region

The Tibet array is the first air shower detector to search for 10 TeV gamma-ray point sources. This array also has a high sensitivity to air showers initiated by cosmic rays around the "knee" of the all-particle spectrum at 10¹⁵-10¹⁶ eV, with quite small ambiguity for the energy estimation, since the development of air showers in this high energy region is close to maximum at Yangbajing altitude (4300 m above sea level). Using the data set taken with the Tibet array in the period from 1990 October through 1993 July, we obtained the differential energy spectrum of primary cosmic rays between 3 × 10¹⁴ and 2 × 10¹⁶ eV covering the knee region. The overall spectrum obtained does not resemble a single power law and shows a gradual steepening of the slope at energies around 10^15.25 eV. The spectrum is expressed as J(E₀) = 1.5 × 10^-20(E₀/10^14.75 eV)_-2.60±0.04(m^-2 s^-1 sr^-1 eV^-1) at E₀ < 10^14.75 eV and J(E₀) = 1.2 × 10^-23(E₀/10^15.85 eV)^-3.00±0.05(m^-2 s^-1 sr^-1 eV^-1) at E₀ > 10^15.85 eV, where this changes gradually between 10^14.75 and 10^15.85 eV and takes the value J(E₀) = 6.7 × 10^-22 (m^-2 s^-1 sr^-1 eV^-1) at E₀ = 10^15.25 eV.