Linsley's EAS time structure method for the primary cosmic ray spectrum at LAAS

We have installed a shift register system of ex- tensive air show (EAS) particles in a compact EAS array built on the rooftop of the Faculty of Engineering building in the campus of Okayama University of Science and be- ing operated since April 2006 as part of Large Area Air Shower (LAAS) experiments , in order to register each ar- rival time of EAS particles within 5µs. Detector simulations based on the database obtained from one of the standard cos- mic ray propagation simulator in the atmosphere (AIRES) have also been carried out and the procedures to estimate the primary cosmic ray energy from the Linsley's method have been developed and examined. Applying Linsley's method to the EAS data obtained by our EAS arrays and the simula- tion results, we derived the energy spectrum from 10 16 eV to 10 19.5 eV. Consequently, we obtained the power-law index of 3.2(+0.46 0.8) in the primary energy range of 10 16 eV to 10 18.5 eV, and obtained that a change around 10 18 eV ap- peared if not taking account of the zenith angle distribution of primary cosmic rays. We compared the obtained energy spectrum with other experimental data above 10 16 eV ener- gies, and showed the two components of power-law energy spectra well described our data. We also showed the im- provement of energy resolution by applying the restriction of zenith angle of primary cosmic rays in our simulation re-