{We present a systematic calculation of α-decay half-lives of even-even heavy and superheavy nuclei in the framework of the preformed α model. The microscopic α-daughter nuclear interaction potential is calculated by double-folding the density distributions of both α and daughter nuclei with a realistic effective Michigan three-Yukawa nucleon-nucleon interaction, and the microscopic Coulomb potential is calculated by folding the charge density distributions of the two interacting nuclei. The half-lives are found to be sensitive to the density dependence of the nucleon-nucleon interaction and the implementation of the Bohr-Sommerfeld quantization condition inherent in the Wentzel-Kramers-Brillouin approach. The α-decay half-lives obtained agree reasonably well with the available experimental data. Moreover, the study has been extended to the newly observed superheavy nuclei. The interplay of closed-shell effects in α-decay calculations is investigated. The α-decay calculations give the closed-shell effects of known spherical magicities

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