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Surface α-particle emissivity testing and spectral characterization of two leaded tin spheres (Sn_10%Pb_90%, Sn_63%Pb_37%) and one lead-free tin sphere (Sn_96.5%Ag_3.0%Cu_0.5%, SAC305) were carried out. The results show that Sn_10%Pb_90% Sn spheres have the highest α-particle emissivity; Sn_63%Pb_37% Sn spheres are the next highest, which is an order of magnitude lower than the α-particle emissivity of Sn_10%Pb_90% Sn spheres; and SAC305 Sn spheres have the lowest emissivity, which is reduced by about 55.6% compared to the emissivity of Sn_63%Pb_37% Sn spheres. All three types of tin spheres, after purification treatment, achieved the grade of ultra-low alpha particle emissivity (<0.002 α/(cm²·h)). The internal radionuclide traceability of the tin sphere, combined with the energy spectrum, reveals that the emission spectrum of the tin sphere exhibits an obvious “single peak” characteristic, with the peak energy in the interval of 5 MeV~5.5 MeV. Comparative analyses revealed that ²¹⁰Po is the main nuclide that produces alpha particles, and ²¹⁰Po originates from the decay of ²¹⁰Pb. Further Monte Carlo simulations show that α-particles with energies greater than 4.1 MeV in the measured energy spectrum all come from the contribution of radionuclides within 5 μm of the surface layer of the tin sphere, which accounts for 60% of the total radioactivity. Combining the experimental and simulation results, it is found that the internal radionuclides of the tin sphere are characterized by more surface layer and less internal layer. The above results are of great significance for the establishment of α-particle mitigation methods for tin spheres.