E F M El-Zaidia, A A A Darwish
The fast-neutron and gamma-ray attenuation characteristics of metal-centered and pristine naphthalocyanines (Nc, VONc, CoNc, CuNc, and SnNc) were studied. In this theoretical screening study, radiation shielding parameters were evaluated computationally using standard photon-interaction formalisms and database-driven attenuation calculations. Photon interaction parameters, including mass attenuation coefficients (μ/ρ), effective atomic numbers Zeff, effective electron densities Neff, half-value and tenth value layers HVL and TVL, and mean free paths MFP, were evaluated over a broad photon energy interval to resolve the relative contributions of photoelectric absorption, incoherent Compton scattering, and pair production. Exposure buildup factors EBF were calculated up to 40 mean free paths using the geometric-progression (G-P) fitting formalism to characterize multiple-scattering and secondary photon accumulation at large penetration depths. Fast-neutron attenuation performance was assessed via effective neutron removal cross sections ΣR. The results indicate a strong compositional dependence of photon attenuation, particularly at low energies, where Zeff-driven photoelectric interactions dominate. Conversely, pristine Nc exhibits comparatively higher fast-neutron removal cross sections due to its larger hydrogen contribution, indicating a trade-off between photon attenuation and neutron moderation performance. At photon energies relevant to low-energy radiation shielding (e.g., around 0.1 MeV), SnNc exhibits approximately 30-40% lower HVL values than pristine Nc, reflecting substantially enhanced attenuation efficiency associated with the higher-Z Sn center. In the intermediate energy range governed by Compton scattering, SnNc also demonstrates lower EBF values at ≥20-40 mfp, indicating reduced photon buildup under deep-shielding conditions. Conversely, pristine Nc shows comparatively higher ΣR due to its hydrogen content, enhancing fast-neutron moderation. The findings demonstrate that central metal substitution provides a useful computational strategy for tuning the coupled photon-neutron attenuation behavior in organic macrocyclic systems and may help guide future experimental development of lightweight shielding materials.