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The region around N≈60 with Z≤40 has generated considerable interest as it features the most abrupt shape transition known to date in the nuclear chart, when crossing from N=58 to N=60 [1]. This transition is closely linked to shape coexistence [2], a phenomenon where two or more states with different intrinsic shapes coexist within the same nucleus at low excitation energy and within a narrow energy range. Specifically, the abrupt change arises from the inversion of two distinct quantum configurations of nucleons, each corresponding to different nuclear shapes. These shifts are interpreted as quantum phase transitions [3], indicating a fundamental transformation in nuclear properties. This phase transition emphasizes the importance of nuclear deformations and the variety of shapes present in neutron-rich nuclei such as strontium, yttrium and zirconium.
To investigate the evolution of nuclear structure across this region, a fast-timing experiment (IS622) was performed at the ISOLDE Decay Station [4]. The experimental setup combined high-purity germanium (HPGe) detectors for precise γ-ray spectroscopy with fast LaBr$_{3}$(Ce) scintillators optimized for high-resolution timing measurements. Excited states were populated through the β decay of neutron-rich $^{98–102}$Rb isotopes, populating nuclei along the decay chain including Sr, Y, and Zr isotopes. The fast-timing method [5], based on both β–γ and γ–γ coincidences, enables the measurement of lifetimes from the nanosecond to the picosecond range.
The results provide updated and newly determined lifetimes for several excited states of interest in nuclei around N≈60. These lifetimes allow the extraction of transition strengths sensitive to nuclear deformation and collectivity, offering a more systematic view of structural evolution across the Sr–Y–Zr region and contributing to a better understanding of shape coexistence in neutron-rich nuclei.
[1] R. Rodriguez-Guzman, P. Sarriguren, and L. M. Robledo. Shape evolution in yttrium and niobium neutron-rich isotopes. Phys. Rev. C 83, 044307 (2011).
[2] A. Poves. Shape coexistence in nuclei. J. Phys. G: Nucl. Part. Phys. 43, 020401 (2016).
[3] T. Togashi, Y. Tsunoda, T. Otsuka, and N. Shimizu. Quantum Phase Transition in the Shape of Zr isotopes. Phys. Rev. Lett. 117, 172502 (2016).
[4] ISOLDE Decay Station, CERN. Available online: https://isolde-ids.web.cern.ch/. Accessed on March 6, 2026.
[5] J.-M. Régis, G. Pascovici, J. Jolie, M. Rudigier. The mirror symmetric centroid difference method for picosecond lifetime measurements via γ–γ coincidences using very fast LaBr$_{3}$(Ce). Nucl. Instrum. Methods Phys. Res. A 622, 83–92 (2010).