Speaker
Description
The Coulomb excitation of $^{66}$Ge has been performed for the first time using ``safe'' bombarding energies at the HIE-ISOLDE facility at CERN in July 2017. A particle-$\gamma$ coincidence experiment using the MINIBALL array and double-sided silicon detectors has allowed the determination of transitional and diagonal matrix elements in $^{66}$Ge, yielding new measurements of the reduced transition probability connecting the ground state, 0$^+_1$, and the first excited state, 2$^+_1$, or $B(E2;2^+_1 \rightarrow 0^+_1)$ value, and the spectroscopic quadrupole moment of the 2$^+_1$ state, $Q_{_S}(2^+_1)$. A relatively large $B(E2) = 29.4(30)$~W.u. has been extracted using beam-gated data at forward angles -- less sensitive to second-order effects -- as compared with the adopted value of $16.9(7)$ W.u., but in closer agreement with modern large-scale shell-model calculations using a variety of effective interactions and beyond-mean field calculations. A spectroscopic quadrupole moment of $Q_{_S}(2^+_1) = +0.41(12)$ eb has been determined using the reorientation effect from the target-gated data at projectile backward angles -- more sensitive to the reorientation effect. Such an oblate shape is in agreement with the corresponding collective wave-function calculated using beyond mean-field calculations and its magnitude agrees with the rotational model, assuming $B(E2) = 29.4(30)$ W.u. This work solves a long-standing puzzle regarding the loss of quadrupole collectivity in $^{66}$Ge and provides a deeper insight into how oblate nuclei rotate.
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