An experimental study of ${}^{258,260}\mathrm{Sg}$ produced in the complete fusion reactions ${}^{52,54}\mathrm{Cr}+{}^{207}\mathrm{Pb}$ was performed using the SHELS velocity filter. The prompt neutron multiplicities in the spontaneous fission of ${}^{258,260}\mathrm{Sg}$ were measured for the first time, yielding average values of $\overline{\nu }{(}^{258}\text{Sg})=4.9\pm 0.4$ and $\overline{\nu }{(}^{260}\text{Sg})=5.03\pm 0.14$. The $\alpha$-decay branching ratio of ${}^{260}\mathrm{Sg}$ was determined to be $b_{\alpha }=0.29\pm 0.03$. In addition, the half-lives were measured as $T_{1/2}{(}^{258}\text{Sg})=2.2_{-1.0}^{+2.4}\mathrm{ms}$ [95% C.L. (confidence limit)] and $T_{1/2}{(}^{260}\text{Sg})=7.0_{-1.7}^{+2.6}\mathrm{ms}$ (95% C.L.).

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• Fission
• Nuclear reactions
• Nuclear structure & decays