Investigating two-γ-phonon vibrational states in ¹⁶²Dy through Coulomb excitation

¹ Department of Nuclear Physics and Accelerator Applications, Research School of Physics, The Australian National University, Canberra, ACT 2601, Australia.
² Department of Physics, Lund University, 22100 Lund, Sweden
³ Department of Physics, College of Science, Northern Border University, Arar, Saudi Arabia.

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Published online: 13 May 2026

Abstract

The existence of two-y-phonon excited states in rare-earth nuclei remains a contentious issue in nuclear structure. While examples of single-phonon γ-vibrational states are prevalent in even-even deformed nuclei, identifying two-phonon excitations is challenging due to strength fragmentation and competing non-collective states. Although states with a strong two-phonon contribution are predicted by the collective model of Bohr and Mottelson, experimental evidence remains scarce, with only six cases reported in rare-earth nuclei. While pproperties of these states are largely in agreement with predictions from the model, their rarity challenges its validity. This work presents an overview of a Coulomb-excitation investigation into the low-lying states of ¹⁶²Dy, which exhibits two K^π = 4⁺ states suspected to be a splitting of the two-y-phonon contribution. Analysis methods for determining reduced transition matrix elements (I_f∥Eλ∥I_i), a key indication of collectivity, are presented. A new particle-detector array under development to enhance the Coulomb-excitation capabilities at the Australian Heavy Ion Accelerator Facility is also discussed.