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Description
Nuclei in the rare-earth region, especially the samarium (Sm) isotopes, exhibits a variety of shapes. The aim of the present work it to test the theoretical calculations predicting the $^{140}$Sm isotope to be a transitional nuclei in between a spherical and a deformed shape [1].
Relativistic Hartree-Fock-Bogoliubov (HFB) calculations find a smooth transition from spherical $^{144}$Sm to well-deformed prolate $^{134}$Sm with a $\gamma$-soft potential energy surface for the transitional nucleus $^{140}$Sm [2]. The observation and tentative assignment of a $2_{2}^{+}$ state at 990 keV and a $3_{1}^{+}$ state at 1599 keV in $^{140}$Sm following the $\beta$-decay of $^{140}$Eu was interpreted as evidence for a low-lying $\gamma$-band [3].
To investigate this transitional nucleus a CoulEx experiment was performed at REX-ISOLDE in 2012 by M. Klintefjord et al. [4]. The results showed no indication of low-lying shape coexistence, rather it seems $^{140}$Sm shows weak quadrupole deformation with maximum triaxility of $\gamma = 30^{o}$ and significant $\gamma$-softness.
To investigate the degree of $\gamma$-softness a new CoulEx experiment was performed using the newly upgraded HIE-ISOLDE in 2016. With the higher beam energies provided by the new HIE-ISOLDE post-accelerator and the resulting higher cross-section many higher-lying states were populated. The goal of the analysis is to measure transition probabilities connected to the states in the $\gamma$-vibrational band and to the excited $0^{+}$ states. This will be a large GOSIA analysis, and the results from this experiment will improve existing B(E2)-values, create new B(E2)-values and quadrupole moments.
References
[1] R. F. Casten and N. V. Zamfir. Empirical realization of a critical point description in atomic nuclei. Phys. Rev. L, 87 (2001).
[2] T. Niksic, P. Ring, D. Vretenar, Y. Tian and Z. Ma, Phys. Rev. C 81, (2010).
[3] B. D. Kern, R. L. Mlekodaj, G. A. Leander, M. O. Kortelahti, E. F. Zganjar, R. A. Braga, R. W. Fink, C. P. Perez, W. Nazarewicz, and P. B. Semmes, Phys. Rev. C 36, (1987).
[4] M. Klintefjord et al., “Structure of low-lying states In Sm-140 studied by Coulomb excitation”, Phys. Rev. C, vol. 93, (2016).