Optimum distance is defined as  a distance from the air -shower core in which  the density of a number of particles calculated by the lateral distribution function at that distance  has the least uncertainty. Furthermore, with a good approximation, this distance is independent  of  the characteristics of primitive ray  and it only depends on the geometric shape of array . In this paper, by simulating 1000 vertical EAS of protons with the energy 300 TeV , using the CORSIKA Monte Carlo code and trigger condition, the calculated distance for the array triggering ALBORZ-1 was found to be equal to 9±1 m. Also, it was found that this distance as well as the density of the number of calculated particles was independent of the lateral distribution function used in the calculations


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