Lifetime measurements in 154,155Er nuclei and theoretical studies of collective properties in neutron-deficient erbium isotopes

AbstraktAbstract

Background: The collective properties of the neutron-deficient 154Er and 155Er nuclei remain poorly understood. To better characterize these properties, spectroscopic data, such as reduced quadrupole transition probabilities, are essential. These measurements help quantify the strength of collectivity in these isotopes, particularly near the N = 82 shell closure. With such data, the shape evolution of neutron-deficient Er isotopes can be investigated within the framework of theoretical nuclear structure models. Purpose: This paper aims to improve the understanding of the collective properties of neutron-deficient 154Er and 155Er isotopes by measuring reduced transition probabilities. These measurements provide key insights into the evolution of nuclear shapes near the N = 82 shell closure. By comparing experimental B(E2) values with theoretical predictions from the five-dimensional collective Hamiltonian (5DCH) and quasiparticle random-phase approximation (QRPA), this paper seeks to refine nuclear structure models and explore the role of quasiparticle excitations in shape transitions of rare-earth nuclei. Methods: We report on the lifetime measurements of the first 2+, 4+, 6+, 8+, and 10+ states in 154Er, as well as the 17/2+, 21/2+, and 25/2+ levels in 155Er. These measurements were performed using the recoil distance Doppler shift method at the IFIN-HH 9 MV Tandem accelerator. Excited states were populated via the 144Sm(13C, Xn) fusion-evaporation reaction, and data were collected using the ROSPHERE array. The experimental results for 154Er, along with previously published data for 156164Er isotopes, are analyzed within the framework of the 5DCH implemented with the D1S Gogny force. Additionally, QRPA calculations are performed for 162Er and 164Er to compare experimental and theoretical B(E2) values for interband transitions with those predicted by 5DCH. Results: Reduced transition probability B(E2) values have been obtained for the ground-state band transitions up to the 10+ state in 154Er, alongside strong evidence for the gamma vibrational band. For 155Er, B(E2) values are obtained for the 17/2+-* 13/2+, 21/2+-* 17/2+, and 25/2+-* 21/2+ transitions. The evolution of nuclear structure from a quasispherical to a strongly deformed shape in the 154164Er isotopes is analyzed based on the present 5DCH calculations. The B(E2) strengths calculated for interband transitions using QRPA are weaker than those predicted by 5DCH but show better agreement with experimental B(E2) values. Conclusions: The measured B(E2) values for intraband transitions in 154Er are significantly weaker than those predicted by 5DCH calculations. This suggests that more accurate B(E2) predictions may require incorporating quasiparticle degrees of freedom into the collective structure model. The structure of neutron-deficient erbium isotopes undergoes a transition from quasispherical to strongly deformed shapes that is reasonably well described by the 5DCH calculations. Additionally, extending systematic QRPA calculations to deformed nuclei in the rare-earth region is suggested.

Background: The collective properties of the neutron-deficient 154Er and 155Er nuclei remain poorly understood. To better characterize these properties, spectroscopic data, such as reduced quadrupole transition probabilities, are essential. These measurements help quantify the strength of collectivity in these isotopes, particularly near the N = 82 shell closure. With such data, the shape evolution of neutron-deficient Er isotopes can be investigated within the framework of theoretical nuclear structure models. Purpose: This paper aims to improve the understanding of the collective properties of neutron-deficient 154Er and 155Er isotopes by measuring reduced transition probabilities. These measurements provide key insights into the evolution of nuclear shapes near the N = 82 shell closure. By comparing experimental B(E2) values with theoretical predictions from the five-dimensional collective Hamiltonian (5DCH) and quasiparticle random-phase approximation (QRPA), this paper seeks to refine nuclear structure models and explore the role of quasiparticle excitations in shape transitions of rare-earth nuclei. Methods: We report on the lifetime measurements of the first 2+, 4+, 6+, 8+, and 10+ states in 154Er, as well as the 17/2+, 21/2+, and 25/2+ levels in 155Er. These measurements were performed using the recoil distance Doppler shift method at the IFIN-HH 9 MV Tandem accelerator. Excited states were populated via the 144Sm(13C, Xn) fusion-evaporation reaction, and data were collected using the ROSPHERE array. The experimental results for 154Er, along with previously published data for 156164Er isotopes, are analyzed within the framework of the 5DCH implemented with the D1S Gogny force. Additionally, QRPA calculations are performed for 162Er and 164Er to compare experimental and theoretical B(E2) values for interband transitions with those predicted by 5DCH. Results: Reduced transition probability B(E2) values have been obtained for the ground-state band transitions up to the 10+ state in 154Er, alongside strong evidence for the gamma vibrational band. For 155Er, B(E2) values are obtained for the 17/2+-* 13/2+, 21/2+-* 17/2+, and 25/2+-* 21/2+ transitions. The evolution of nuclear structure from a quasispherical to a strongly deformed shape in the 154164Er isotopes is analyzed based on the present 5DCH calculations. The B(E2) strengths calculated for interband transitions using QRPA are weaker than those predicted by 5DCH but show better agreement with experimental B(E2) values. Conclusions: The measured B(E2) values for intraband transitions in 154Er are significantly weaker than those predicted by 5DCH calculations. This suggests that more accurate B(E2) predictions may require incorporating quasiparticle degrees of freedom into the collective structure model. The structure of neutron-deficient erbium isotopes undergoes a transition from quasispherical to strongly deformed shapes that is reasonably well described by the 5DCH calculations. Additionally, extending systematic QRPA calculations to deformed nuclei in the rare-earth region is suggested.