Proton Shell Evolution below <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>Sn</mml:mi></mml:mrow><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>132</mml:mn></mml:mrow></mml:mmultiscripts></mml:mrow></mml:math>: First Measurement of Low-Lying <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>β</mml:mi></mml:math>-Emitting Isomers in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>Ag</mml:mi></mml:mrow><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>123</mml:mn><mml:mo>,</mml:mo><mml:mn>125</mml:mn></mml:mrow></mml:mmultiscripts></mml:mrow></mml:math>

Abstract

The $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$-ray spectroscopy of neutron-rich $^{123,125}\mathrm{Ag}$ isotopes is investigated at the Radioactive Isotope Beam Factory of RIKEN, and the long-predicted $1/{2}^{\ensuremath{-}}$ $\ensuremath{\beta}$-emitting isomers in $^{123,125}\mathrm{Ag}$ are identified for the first time. With the new experimental results, the systematic trend of energy spacing between the lowest $9/{2}^{+}$ and $1/{2}^{\ensuremath{-}}$ levels is extended in Ag isotopes up to $N=78$, providing a clear signal for the reduction of the $Z=40$ subshell gap in Ag towards $N=82$. Shell-model calculations with the state-of-the-art ${V}_{\mathrm{MU}}$ plus M3Y spin-orbit interaction give a satisfactory description of the low-lying states in $^{123,125}\mathrm{Ag}$. The tensor force is found to play a crucial role in the evolution of the size of the $Z=40$ subshell gap. The observed inversion of the single-particle levels around $^{123}\mathrm{Ag}$ can be well interpreted in terms of the monopole shift of the $\ensuremath{\pi}1{g}_{9/2}$ orbitals mainly caused by the increasing occupation of $\ensuremath{\nu}1{h}_{11/2}$ orbitals.