Speaker
Description
Until recently, many deformed nuclei have been considered to have excitations described by the collective model of Bohr and Mottelson [1]. However, the interpretation that some states have a collective vibrational character has been challenged to the point that it is necessary to ask whether or not two-phonon excitations exist among the low-energy states of atomic nuclei. In particular, experimental evidence for the characteristic Kπ = 0+, 4+ two-γ-phonon doublet
remains limited.
In the rare-earth region, a Kπ = 2+ band-head state is commonly observed and can be identified as the single-phonon γ excitation; however, only a small number of candidates for two-phonon γ-vibrational states have been identified [2–6]. This scarcity, together with the increasing prominence of alternative interpretations for low-lying Kπ = 0+ states such as shape coexistence [7] and rotations around a triaxial ground state [8], motivates a re-evaluation of proposed two-phonon assignments and the applicability of the vibrational model in deformed nuclei.
Dysprosium-162 (Z = 66) is one such nucleus, in which states at 1535 keV and 2181 keV have been interpreted as members of a split Kπ = 4+ two-phonon γ-vibration configuration [2, 9, 10]. Coulomb excitation provides a sensitive probe of these structures, selectively populating states via electromagnetic interactions and enabling the extraction of transition strengths. In this work, results from Coulomb-excitation of 162Dy using an 16O beam at the Heavy Ion Accelerator Facility (HIAF) at the Australian National University are presented [11, 12]. This study forms part of a broader experimental program aimed at probing 162Dy with higher-Z beams to enable multi-step Coulomb excitation
and direct population of states of interest.
In parallel, the development of a new particle-detector array for the CAESAR system at HIAF is underway. This upgrade is designed to improve particle identification and angular coverage, thereby enhancing the capabilities for future Coulomb-excitation measurements at ANU, with initial experiments planned for later this year.
References
[1] A. N. Bohr and B. R. Mottelson. “Collective and individual-particle aspects of nuclear structure”. Mat -fys Medd 27.16 (1953), pp. 1–174.
[2] A. Aprahamian, S. R. Lesher, et al. “Lifetime measurements in 162Dy”. Physical Review C 95.2 (2017), p. 024329.1
[3] F. Corminboeuf, J. Jolie, et al. “Kπ double-gamma vibration in 164Dy”. Physical Review C 56.3 (1997), R1201–R1205.
[4] C. Fahlander, A. Axelsson, et al. “Two-phonon γ-vibrational states in 166Er”. Physics Letters B 388.3 (1996),pp. 475–480.
[5] P. E. Garrett, M. Kadi, et al. “Kπ = 0+ and 4+ Two-Phonon γ-Vibrational States in 166Er”. Physical Review Letters 78.24 (1997), pp. 4545–4548.
[6] T. Härtlein, M. Heinebrodt, et al. “Collective excitations built on the 2+ γ state in 168Er”. The European Physical Journal A - Hadrons and Nuclei 2.3 (1998), pp. 253–261.
[7] K. Heyde and J. Wood. “Shape coexistence in atomic nuclei”. Reviews of Modern Physics 83.4 (2011), pp. 1467–1521.
[8] T. Otsuka, Y. Tsunoda, et al. Prevailing Triaxial Shapes in Heavy Nuclei Driven by Nuclear Tensor Force. 2023.
[9] C. Fahlander. “Double-Phonon γ-Vibration of Deformed Nuclei”. In: The Nucleus: New Physics for the New Millennium. Ed. by F. D. Smit, R. Lindsay, et al. Boston, MA: Springer US, 1999, pp. 185–189.
[10] C. Y. Wu, D. Cline, et al. “Electromagnetic properties of the rotationally aligned band in 162Dy”. Physical Review C 64.6 (2001), p. 064317.
[11] Tom Perissinotto. “Investigating Two-phonon gamma-vibrational States in 162Dy”. Undergraduate Honours thesis. Australian National University, 2024.
[12] Yet to be published.