Simulation studies and implementation of Linsley's EAS time structure method for the primary cosmic ray spectrum

A compact extensive air shower (EAS) array of eight plastic scintillators viewed by HAMAMATSU H7195 photomultiplier tubes covering a total area of 2m2 is built in the rooftop of the Faculty of Technology building, Okayama University of Science, and operated since April 2006. We have installed a shift register system in our EAS array to record EAS particle arrival time within 5μs. We have also performed detector simulations based on the database obtained from the AIRES simulator and developed the procedures to estimate the primary cosmic ray energy from Linsley's method. Applying this method to our EAS data and the simulation result, we derived the energy spectrum from 1016 to 1019.5eV. Consequently, we obtained the power-law index of −3.2(+0.46−0.8) in the primary energy range of 1016 to 1018.5eV, and obtained that a change around 1018eV appeared if not taking account of the zenith angle distribution of primary cosmic rays. We also showed the improvement of energy resolution by applying the restriction of zenith angle of primary cosmic rays in our simulation, as well as the potential of Linsley's method with a mini array.