Broken seniority symmetry in the semimagic proton mid-shell nucleus <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>Rh</mml:mi><mml:mprescripts/><mml:none/><mml:mn>95</mml:mn></mml:mmultiscripts></mml:math>

Lifetime measurements of low-lying excited states in the semimagic (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mi>N</a:mi><a:mo>=</a:mo><a:mn>50</a:mn></a:mrow></a:math>) nucleus <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mmultiscripts><b:mi>Rh</b:mi><b:mprescripts/><b:none/><b:mn>95</b:mn></b:mmultiscripts></b:math> have been performed by means of the fast-timing technique. The experiment was carried out using <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mi>γ</c:mi></c:math>-ray detector arrays consisting of <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:msub><d:mi>LaBr</d:mi><d:mn>3</d:mn></d:msub></d:math>(Ce) scintillators and germanium detectors integrated into the DESPEC experimental setup commissioned for the Facility for Antiproton and Ion Research () Phase-0, Darmstadt, Germany. The excited states in <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mmultiscripts><e:mi>Rh</e:mi><e:mprescripts/><e:none/><e:mn>95</e:mn></e:mmultiscripts></e:math> were populated primarily via the <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mi>β</f:mi></f:math> decays of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mmultiscripts><g:mi>Pd</g:mi><g:mprescripts/><g:none/><g:mn>95</g:mn></g:mmultiscripts></g:math> nuclei, produced in the projectile fragmentation of a 850 MeV/nucleon <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:mmultiscripts><h:mi>Xe</h:mi><h:mprescripts/><h:none/><h:mn>124</h:mn></h:mmultiscripts></h:math> beam impinging on a <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mrow><i:mn>4</i:mn><i:mspace width="0.16em"/><i:mi mathvariant="normal">g</i:mi><i:mo>/</i:mo><i:msup><i:mrow><i:mi>cm</i:mi></i:mrow><i:mn>2</i:mn></i:msup><i:mspace width="4pt"/><i:mmultiscripts><i:mi>Be</i:mi><i:mprescripts/><i:none/><i:mn>9</i:mn></i:mmultiscripts></i:mrow></i:math> target. The deduced electromagnetic E2 transition strengths for the <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"><m:mi>γ</m:mi></m:math>-ray cascade within the multiplet structure depopulating from the isomeric <n:math xmlns:n="http://www.w3.org/1998/Math/MathML"><n:mrow><n:msup><n:mi>I</n:mi><n:mi>π</n:mi></n:msup><n:mo>=</n:mo><n:mn>21</n:mn><n:mo>/</n:mo><n:msup><n:mn>2</n:mn><n:mo>+</n:mo></n:msup></n:mrow></n:math> state are found to exhibit strong deviations from predictions of standard shell model calculations which feature approximately conserved seniority symmetry. In particular, the observation of a strongly suppressed E2 strength for the <o:math xmlns:o="http://www.w3.org/1998/Math/MathML"><o:mrow><o:mn>13</o:mn><o:mo>/</o:mo><o:msup><o:mn>2</o:mn><o:mo>+</o:mo></o:msup><o:mo>→</o:mo><o:mn>9</o:mn><o:mo>/</o:mo><o:msup><o:mn>2</o:mn><o:mo>+</o:mo></o:msup></o:mrow></o:math> ground state transition cannot be explained by calculations employing standard interactions. This remarkable result may require revision of the nucleon-nucleon interactions employed in state-of-the-art theoretical model calculations, and might also point to the need for including three-body forces in the Hamiltonian. Published by the American Physical Society 2024