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Fast-Timing Study of Nuclear Shape Deformation in $^{100}$Sr across N=60
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 and zirconium.
In the case of $^{100}$Sr (N=62), once shape inversion occurs at N=60, intruder states play a crucial role in understanding the structural evolution of the nucleus. These states refer to configurations where nucleons follow an orbital occupancy order that does not align with the predictions of the spherical shell model, underscoring the importance of deformation and collective effects.
To investigate shape transitions and nuclear structure in $^{100}$Sr, an experiment was conducted at the ISOLDE Decay Station (IDS) [4], populating their excited states via the beta decay of $^{100}$Rb. The fast-timing method [5], particularly through the use of γ-γ coincidences, enables the measurement of half-lives of excited states on the order of tens of picoseconds. A versatile detector system was employed, consisting of high-purity germanium (Clover-type) detectors for precise gamma-ray identification, plastic scintillators for beta particle detection, and LaBr$_{3}$(Ce) crystals, valued for their superior time resolution in measuring excited-state lifetimes.
This contribution presents new half-life measurements that resolve discrepancies from previous values and provide new insights into the nuclear structure of neutron-rich nuclei in the N≈60 region, furthering the understanding of the shape deformation phenomenon.
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[2] A. Poves. Shape coexistence in nuclei. J. Phys. G: Nucl. Part. Phys. 43, 020401 (2016).
[3] Tomoaki Togashi, Yusuke Tsunoda, Takaharu Otsuka, and Noritak 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 October 16, 2024.
[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).
