α-decay rates of180,182,184Pband theZ=82shell closure

The proton-rich isotopes ${}^{180}\mathrm{Pb},$ ${}^{182}\mathrm{Pb}$, and ${}^{184}\mathrm{Pb}$ were produced in ${}^{92}\mathrm{Mo}$ bombardments of ${}^{90}\mathrm{Zr},$ ${}^{92}\mathrm{Zr}$, and ${}^{94}\mathrm{Zr}$, respectively, and their $\alpha$-decay properties were investigated with the use of a fragment mass analyzer and a double-sided Si strip detector. These data yielded unambiguous identifications of ${}^{180}\mathrm{Pb}$ and ${}^{182}\mathrm{Pb}$ and half-lives for all three Pb nuclei that are more precise than previously available values. An examination of $\alpha$ reduced widths in the Pb mass region leads to the conclusion that the $Z=82\mathrm{}$ shell gap is greatly reduced for neutron numbers midway between 82 and 126. Available mass data, manifested in $\Delta Q_{\alpha }$ values and two-proton separation energies of even-even nuclei with the same neutron numbers, support this conclusion.