Fragmentation of M1 Transition Strengths along the N = 52 Isotones: Lifetime Measurements in ⁹⁰Sr

Lifetimes of low-lying excited states in ⁹⁰Sr were measured using the Doppler-shift attenuation method following a (¹⁸O, ¹⁶O) transfer reaction. The measured transition strengths reveal fragmentation of the M1 strength primarily between the transitions 2⁺₂ → 2⁺₁ and 2⁺₃ → 2⁺₁, with the transition 2⁺₃ → 2⁺₁ exhibiting a higher strength. These results were analyzed using large-scale shell-model calculations tailored to the N = 52 isotones, successfully reproducing the newly measured M1 transition strengths. Extending these calculations to existing data on M1 strengths across the N = 52 isotones demonstrates good overall agreement and accurate predictions for the main M1 components. The observed fragmentation does not correlate with subshell closures, suggesting a more intricate underlying mechanism affecting the evolution of M1 strengths in this region. Additionally, a previously unobserved state was identified, which is strongly populated in the (¹⁸O, ¹⁶O) transfer reaction.