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Abstract
Octupole deformations are predicted to occur in nuclei in the region $130\leq N \leq 140$ and $85 \leq Z \leq 92$ [1]. Nuclei that exhibit octupole deformations are of interest for ongoing searches for permanent atomic electric-dipole-moments and beyond Standard Model physics.
Recent studies suggest the octupole deformed region north-east of $^{208}$Pb extends down to At ($Z=85$) [2]. However, no evidence for octupole deformation has been observed in the Po ($Z=84$) isotopes up to $N=135$ [3].
To investigate the lower boundary of the octupole deformed region north-east of $^{208}$Pb, decay spectroscopy studies of $^{219,220}$Po were performed at CERN ISOLDE with the ISOLDE Decay Station (IDS) and the Alpha SETup (ASET). The Laser Ion Source and Trap (LIST) [4] was utilized to reduce isobaric contamination from surface ionised francium isotopes, which previously prevented measurements at ISOLDE in this region.
In this work, we further delineate the octupole deformed region north-east of $^{208}$Pb by presenting improved values for the half-life and $\alpha$-$\beta$ branching ratio of $^{219}$Po ($N=135$) and first values for these quantities for $^{220}$Po ($N=136$). From these results we calculate the alpha decay hindrance factors and discuss their implications on nuclear structure in this region.
References
[1] Y. Cao, S. E. Agbemava, A. V. Afanasjev, W. Nazarewicz, and E. Olsen, “Landscape of pear-shaped even-even nuclei,” Physical Review C, vol. 102, no. 2, p. 024 311, Aug. 10, 2020, issn: 2469-9985, 2469-9993. doi: 10 . 1103 / PhysRevC . 102 . 024311. Accessed: Oct. 10, 2025. [Online]. Available: https://link.aps.org/doi/10.1103/PhysRevC.102.024311.
[2] A. E. Barzakh et al., “Inverse odd-even staggering in nuclear charge radii and possible octupole collectivity in At 217 , 218 , 219 revealed by in-source laser spectroscopy,” Physical Review C, vol. 99,no. 5, p. 054 317, May 14, 2019, issn: 2469-9985, 2469-9993. doi:10.1103/PhysRevC.99.054317. Accessed: Oct. 10, 2025. [Online]. Available: https://link.aps.org/doi/10.1103/PhysRevC.99.054317.
[3] D. A. Fink et al., “In-Source Laser Spectroscopy with the Laser Ion Source and Trap: First Direct Study of the Ground-State Properties of Po 217 , 219,” Physical Review X, vol. 5, no. 1, p. 011 018, Feb. 20, 2015, issn: 2160-3308. doi: 10.1103/PhysRevX.5.011018. Accessed: Jul. 23, 2024. [Online]. Available: https://link.aps.org/doi/10.1103/PhysRevX.5.011018.
[4] R. Heinke et al., “First on-line application of the high-resolution spectroscopy laser ion source PI-LIST at ISOLDE,” Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, vol. 541, pp. 8–12, Aug. 2023, issn: 0168583X. doi: 10.1016/j.nimb.2023.04.057. Accessed: Jun. 22, 2024. [Online]. Available: https://linkinghub.elsevier.com/retrieve/pii/S0168583X23001945
