In this work, we investigate the sequential $\alpha$-decay chains of ${}^{219}\mathrm{Np}, {}^{220}\mathrm{Np}, {}^{223}\mathrm{Np}$, and ${}^{224}\mathrm{Np}$ nuclei produced in fusion-evaporation reactions, within the framework of the Dynamical Cluster-decay Model (DCM) using compact configurations. For comparison, the ground-state $\alpha$ decay of the ${}^{227}\mathrm{Np}$ nucleus is analyzed using the Preformed Cluster Model (PCM) with elongated configurations. The sensitivity of $\alpha$-decay half-lives to the nuclear surface diffuseness parameter is examined, revealing contrasting trends in compact and elongated orientations, which impart significant modifications in the corresponding barrier characteristics. Motivated by these observations, an empirical prescription for the diffuseness parameter is proposed, explicitly incorporating isospin asymmetry, mass number, and decay energy. This relation reproduces experimental half-lives at near-touching configurations, yielding a root-mean-square deviation of approximately 0.04 across the studied Np chains. Its predictive strength is further validated in reference to the $\alpha$-decay chain of ${}^{214}\mathrm{U}$, confirming its wider applicability. Overall, the results demonstrate that the proposed diffuseness formulation provides a physically grounded and quantitatively reliable framework for describing sequential $\alpha$-decay dynamics in the heavy and actinide nuclei.

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• Alpha decay
• Low & intermediate energy heavy-ion reactions
• Nuclear fragmentation
• Nuclear reactions
• Nucleon distribution
• A ≥ 220