Speaker
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The Coulomb excitation of $^{66}$Ge has been performed for the first time using ``safe'' bombarding energies at the HIE-ISOLDE facility at CERN. Motivation to study $^{66}$Ge arises from the anomalous rotational behaviour of the high-lying first 2$^+_1$ state observed in even-even isotopes in the $A \sim 70$ region [1]. Low-lying 0$^+$ excited states have been determined for even-even neutron-deficient Se[2] and Kr[3] isotopes, which are signatures of shape coexistence [4]. In particular, the Germanium and Selenium isotopes have received a considerable amount of interest because they lie between the doubly magic $^{58}$Ni and the strongly deformed neutron-deficient $^{76}$Sr isotopes. This region has shown a complicated interplay between non-collective and collective degrees of freedom due to large sub-shell gaps at both prolate and oblate deformation for proton and neutron numbers $N,Z = 34, 36$ [4,5]. In addition, macroscopic-microscopic models suggest gamma-softness for $^{64}$Ge through oblate-prolate shape coexistence in $^{68}$Se and $^{72}$Kr to some of the most deformed nuclei at $^{76}$Sr and $^{80}$Zr.
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 and the 2$^+_1$ states, or $B(E2;0^+_1 \rightarrow 2^+_1)$ 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 wavefunction calculated in the present work using beyond mean-field calculations and its magnitude agrees with the rotational model, assuming $B(E2) = 29.4(30)$ W.u.
[1] P.J. Davies et al., Phys. Rev. C 75 011302(R) (2007)
[2] J.H. Mamilton et al., Phys. Rev. Lett. 32, 239 (1974)
[3] E. Clement et al., Phys. Rev. C 75, 054313 (2007)
[4] J.L. Wood, K. Heyde, W. Nazarewics, M. Huyse and P. Vn Duppen, Phys Rep. 215, 101 (1992)
[5] M. Hasegawa et al., Phase transition in exotic nuclei along the N=Z line, Phys. Lett. B 656, 51 (2007).
[6] K. Nomura et al., Structural evolution in germanium and selenium nuclei within the mapped interacting boson model based on the Gogny energy density functional, Phys. Rev. C 95, 064310 (2017).
