Zakopane 2026 Conference on Nuclear Physics

The emergence of collectivity can be explored by studying nuclei that differ from closed-shell configurations by a few particles or holes, involving both protons and neutrons. In this context, the regions around the doubly magic nuclei ²⁰⁸Pb and ¹³²Sn provide ideal testing grounds for such investigations. This can be achieved by measuring the electric quadrupole moments associated with the first pure E2 excitations, as well as the E2 components of the first mixed M1 + E2 transitions.

In the ²⁰⁸Pb region, for the ²⁰⁹Po (2p–1h), ²¹¹Rn (4p–1h), and ²¹³Ra (6p–1h) isotones, a systematic increase of the E2 transitions strength was found, originating from growing contributions of additional protons along the N=125 chain [1]. Similarly, in the ¹³²Sn region, an increase of collectivity was observed in ¹²⁹Sb for the states of the 2⁺(¹²⁸Sn)⊗πg_7/2 multiplet with respect to the ¹²⁸Sn core [2]. Recently, the lifetime of the analogous 2⁺(¹³⁰Sn)⊗πg_7/2 state in ¹³¹Sb was measured by our collaboration [3]. The deduced probability of the E2 transition showed a value similar to that of the ¹³⁰Sn core, in accordance with the weak-coupling limit, but in contrast with the ¹²⁹Sb results. To shed more light on this behavior, in the present work the electric quadrupole transition probabilities were investigated in the ¹³¹I and ¹³³I isotopes, which contain two additional protons compared with the ¹²⁹Sb and ¹³¹Sb nuclei, respectively.

The ¹³¹I and ¹³³I isotopes were produced during the thermal neutron induced fission of a ²³⁵U target at the Institut Laue-Langevin (ILL). The isotopes of interest were separated from all fission products using the LOHENGRIN spectrometer [4] according to their A/q and E/q mass-to-charge and energy-to-charge ratios. The 11/2⁺ and 5/2⁺ states, which decay via E2 and M1+E2 transitions, respectively, in ^131,133I, were populated via the β-decay of ^131,133Te isotopes. The lifetimes of the 11/2⁺ and 5/2⁺ levels were measured using the fast-timing technique [5] with two HPGe clover detectors and four LaBr₃(Ce) scintillators.

The lifetimes of the low-lying states in the ¹³³I isotope were measured for the first time in the present analysis. The corresponding states in ¹³¹I were remeasured with high precision. For both nuclei, a systematic increase in collectivity beyond the weak-coupling limit was observed. The experimental results will be supported by realistic shell-model calculations, providing deeper insight into the nature of the emergence of collectivity in the vicinity of the doubly magic ¹³²Sn nucleus.

References:

[1] M. Gerathy et al., Phys. Lett. B 823, 136738 (2021).
[2] T. J. Gray et al., Phys. Rev. Lett. 124, 032502 (2020).
[3] S. Bottoni et at., Phys. Rev. C 107, 014322 (2023).
[4] P. Armbruster et al., Nucl. Instrum. Methods 139, 213 (1976).
[5] H. Mach, R. Gill, and M. Moszyński, Nucl. Instrum. Methods Phys. Res., Sect. A 280, 49 (1989).