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In the recent years, coexisting shapes associated to different excitation energies have been clearly observed in Ni isotopes (Z=28) [1–3], and the results have been successfully reproduced from a microscopic point of view by Monte Carlo Shell Model calculations [4, 5]. Since a similar mechanism is expected at the N=50 neutron shell-closure, the current collaboration has started an experimental campaign at IFIN-HH aiming at investigating this phenomenon along this isotonic chain, such as $^{84}$Se [6] and $^{88}$Sr. In this work, we report on the results of $^{88}$Sr, populated through a $^{89}$Y($^{11}$B, $^{12}$C) one-proton pick-up reaction, which was performed in March 2025 at IFIN-HH with the ROSPHERE $\gamma$-ray spectrometer. The $0^+_2$ and $0^+_3$ states were successfully observed, and their lifetimes were measured for the first time by employing the Recoil Distance Doppler-Shift technique and the Doppler-Shift Attenuation Method, respectively, giving $T_{1/2} = 3.02(45)$ ps and $T_{1/2} = 417(39)$ fs for the two states. The corresponding $B(E2)$ strengths to the $2^+_1$ state were evaluated as $B(E2) = 2.01(30)$ W.u. and $B(E2) = 0.166(16)$ W.u., respectively. For the latter, the hindered $\gamma$ transition could point to a shape-isomer-like structure, as recently observed in $^{64,66}$Ni [1, 2] and $^{84}$Se [6] isotopes. Comparisons with theoretical model calculations will be discussed.
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[2] N. Mărginean, D. Little, Y. Tsunoda, S. Leoni, R.V.F. Janssens, B. Fornal, T. Otsuka, C. Michelagnoli, L. Stan et al., Phys. Rev. Lett. 125, 102502 (2020).
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