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The experimental $B(E2; 2_1^+ \to 0_1^+)$ values in neutron-deficient, even-even Sn isotopes are found to be enhanced compared to calculations, a discrepancy which has eluded a satisfactory solution for over a decade. A Monte Carlo Shell Model (MCSM) [1] attributed this phenomenon to significant proton excitations across the $Z = 50$ shell in neutron-deficient Sn isotopes, and predicted a shape transition from a prolate to an oblate quadrupole deformation of the $2_1^+$ states from $^{106}$Sn to $^{110}$Sn.
A safe-energy Coulomb excitation campaign of $^{106,108,110}$Sn was conducted at HIE-ISOLDE, CERN. The radioactive Sn beams were accelerated to 4.4-4.5 MeV per nucleon and Coulomb excited on $^{206}$Pb targets. Gamma rays from the beam and the target nuclei were detected with the Miniball HPGe spectrometer [2]. In all three nuclei, record $\gamma$-ray counts were obtained from Coulomb excitation experiments [3].
Through excitation probability analysis in GOSIA [4,5], The $B(E2; 2_1^+ \to 0_1^+)$ value of $^{110}$Sn was determined with the best precision to date as 451(22) e$^2$fm$^4$, and the $B(E2; 4_1^+ \to 2_1^+)$ and $B(E2; 4_2^+ \to 2_1^+)$ values were also determined for the first time [6]. Furthermore, the spectroscopic quadrupole moment $(Q_s)$ of the $2_1^+$ state of $^{110}$Sn was newly determined as $+0.20(8)$ eb. Both the sign and the magnitude of $Q_s(2_1^+)$ are in agreement with the MCSM prediction of an oblate shape for the $2_1^+$ state in $^{110}$Sn [1]. Preliminary results suggest a negative $Q_s(2_1^+)$ for $^{106}$Sn and $Q_s(2_1^+) \sim 0$ for $^{108}$Sn, which are also consistent with MCSM. The shape transition in the light Sn isotopes will be discussed, as well as a more detailed view on the role of protons above the $Z = 50$ shell.
References:
[1] T. Togashi et al., Phys. Rev. Lett. 121, 052601 (2018).
[2] N. Warr et al., Eur. Phys. J. A 49, 40 (2013).
[3] J. Park et al., JPS Conf. Proc. 32, 010036 (2020).
[4] T. Czosnyka, D. Cline, and C. Y. Wu, Bull. Am. Phys. Soc. 28, 745 (1983).
[5] M. Zielinska et al., Eur. Phys. J. A 52, 99 (2016).
[6] J. Park et al., Phys. Rev. Lett. 135, 222502 (2025).