Magicity of the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>N</mml:mi></mml:mrow><mml:mprescripts/><mml:mrow/><mml:mrow><mml:mn>68</mml:mn></mml:mrow><mml:mrow/><mml:mrow/></mml:mmultiscripts></mml:mrow><mml:mi>i</mml:mi></mml:math>Semidouble-Closed-Shell Nucleus Probed by Gamow-Teller Decay of the Odd-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi mathvariant="italic">A</mml:mi></mml:math>Neighbors

The particle-hole excitations through the $N\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}40$ subshell around ${}^{68}\mathrm{Ni}$ have been studied by the $\ensuremath{\beta}$ decay of ${}^{69}\mathrm{Co}$ and ${}^{69}\mathrm{Ni}$. The half-life of ${}^{69}\mathrm{Co}$ was measured to be 0.22(2) s, and a new $\ensuremath{\beta}$-decaying isomer with a half-life of 3.5(5) s was identified in ${}^{69}\mathrm{Ni}$. From the decay of the ${}^{69}\mathrm{Ni}$ isomer a 9(4)% mixing of the $\ensuremath{\pi}{p}_{3/2}^{+1}\ensuremath{\nu}{p}_{1/2}^{\ensuremath{-}2}\ensuremath{\nu}{g}_{9/2}^{+2}$ configuration into the ground state of ${}^{69}\mathrm{Cu}$ can be deduced. Significant polarization of the ${}^{68}\mathrm{Ni}$ core nucleus is observed with the coupling of a single nucleon, which implies a rapid decrease in the stabilizing effect of the $N\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}40$ semimagic shell gap.

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