First observation of the decay of a 15<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow/><mml:mrow><mml:mo>−</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>seniority<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>v</mml:mi><mml:mo>=</mml:mo><mml:mn>4</mml:mn></mml:mrow></mml:math>isomer in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mmultiscripts><mml:mi mathvariant="normal">Sn</mml:mi><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>128</mml:mn></mml:mrow></mml:mmultiscripts></mml:math>

Isomeric states in the semimagic ${}^{128\ensuremath{-}130}$Sn isotopes were populated in the fragmentation of a $^{136}\mathrm{Xe}$ beam on a $^{9}\mathrm{Be}$ target at an energy of 750 $A\ifmmode\cdot\else\textperiodcentered\fi{}$MeV. The decay of an isomeric state in $^{128}\mathrm{Sn}$ at an excitation energy of 4098 keV has been observed. Its half live has been determined to be ${T}_{1/2}=220(30)$ ns from the time distributions of the delayed $\ensuremath{\gamma}$ rays emitted in its decay. $\ensuremath{\gamma}\ensuremath{\gamma}$ coincidence relations were analyzed in order to establish the decay pattern of the newly established state toward the known (7${}^{\ensuremath{-}}$) and (10${}^{+}$) isomers at excitation energies of 2092 and 2492 keV, respectively. Based on a comparison with results of state-of-the-art shell-model calculations the new isomeric state is proposed to have the $\ensuremath{\nu}{h}_{11/2}^{\ensuremath{-}3}{d}_{3/2}^{\ensuremath{-}1}$ configuration with the four neutron holes in $^{132}\mathrm{Sn}$ maximally aligned to a total spin of ${I}^{\ensuremath{\pi}}={15}^{\ensuremath{-}}$.

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