Speaker
Description
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 sharp change arises from the inversion of two distinct quantum nuclear configurations, 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 emphasises the importance of nuclear deformations and the variety of shapes present in neutron-rich nuclei such as strontium.
The IS709 experiment at the ISOLDE Decay Station (IDS) [4] aims to investigate the phenomenon of shape coexistence across the N=60 region in $^{96-102}$Sr isotopes, with particular emphasis on $^{100}$Sr. Excited nuclear states were populated via β and β–n decay of Rb beams and studied with 13 Clover HPGe detectors optimised for γ–γ angular correlation measurements, allowing a precise determination of transition multipolarities and spin assignments. In parallel, the SPectrometer for Electron DEtection (SPEDE) [5] was employed to measure internal conversion electrons, providing direct access to E0 transition strengths, which jointly enable the identification of excited 0$^{+}$ states.
In this contribution, we will present preliminary results from the IS709 experiment (September 2025). Together with the IS622 fast-timing lifetime measurements [6], this experiment provides a coherent and comprehensive picture of the nuclear structure and shape deformation in neutron-rich strontium isotopes in the N≈60 region, and further illustrates the broad range of capabilities and the versatility of the IDS setup.
[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] 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 10, 2025.
[5] P. Papadakis et al. The SPEDE spectrometer. Eur. Phys. J. A 54, 42 (2018).
[6] 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).
