Speaker
Description
Shape coexistence refers to a phenomenon where multiple shapes occur within the same nucleus. While this phenomenon has been proposed to exist in almost all nuclei\,[1], neutron-deficient Pb isotopes near the N=104 mid-shell have been a primary focus for detailed investigations of shape coexistence for decades. In particular, neutron-deficient Pb isotopes, such as $^{184,186,188,190}$Pb, exhibit experimental evidence of three distinct shapes\,[2-4]. Within the shell-model framework, these three shapes are associated with 0p-0h (spherical), 2p-2h (oblate) and 4p-4h (prolate) proton excitation across the Z=82 shell.
In the last few years, spectroscopic studies performed at the Accelerator Laboratory of Jyväskylä have advanced the understanding of the shape coexistence phenomenon in neutron-deficient Pb isotopes. These experiments allowed us to reassign states between different collective configurations and provided insight into the mixing between these configurations.
This presentation will go through the experimental techniques employed at Accelerator laboratory of Jyväskylä and observables that can be measured. Some of these techniques have been used for studying the Pb isotopes.
The $^{186,188,190}$Pb nuclei have been investigated using two complementary in-beam experimental techniques [5-8]. One utilised a combined $\gamma$-ray and conversion electron spectrometer, SAGE, which is sensitive to E0 transitions and allow us to assess the mixing between configurations. The other employed a plunger device for measuring lifetimes, which enabled assessment of the collectivity of different structures.
The most recent result at Jyväskylä focused on $^{184}$Pb utilising recoil-decay-tagging method in conjunction with the JUROGAMII germanium detector array [9]. This experiment led to the identification of non-yrast structure for $^{184}$Pb for the first time. Observing the non-yrast structures in this nucleus is currently at the limit of feasibility, as its production via fusion-evaporation reaction yields low cross-section.
This presentation will summarise the latest in-beam experiments around mid-shell Pb isotopes performed at the Accelerator Laboratory in Jyväskylä. The experimental results will hopefully stimulate new theoretical developments in this region.
References
[1] Heyde, K. & Wood, J. L. Phys. Scr. 91, 083008 (2016)
[2] J. F. C.Cocks et al., Eur. Phys. J. A, Hadrons Nucl. 3 (1998)
[3] A. N.Andreyev et al., Nature 405 430-433 (2000)
[4] N. Bijnens et al, Z. Phys. A, 356 1 (1999)
[5] T. Grahn et al., Phys. Rev. Lett. 97 062501 (2006)
[6] J. Ojala et al., Commun. Phys. 5 213 (2022)
[7] P. Papadakis et al., Phys. Lett. B 858 139048 (2024)
[8] A. Montes-Plaza et al., Comm.phys. 8 8 (2025)
[9] J. Ojala et al., Phys. Lett B 867 139601 (2025)