Collectivity in the light radon nuclei measured directly via Coulomb excitation

Abstract

Background: Shape coexistence in heavy nuclei poses a strong challenge to state-of-the-art nuclear models, where several competing shape minima are found close to the ground state. A classic region for investigating this phenomenon is in the region around $Z=82$ and the neutron mid-shell at $N=104$. Purpose: Evidence for shape coexistence has been inferred from $\hat{I}\pm$-decay measurements, laser spectroscopy and in-beam measurements. While the latter allow the pattern of excited states and rotational band structures to be mapped out, a detailed understanding of shape coexistence can only come from measurements of electromagnetic matrix elements. Method: Secondary, radioactive ion beams of ${}^{202}$Rn and ${}^{204}$Rn were studied by means of low-energy Coulomb excitation at the REX-ISOLDE facility in CERN. Results: The electric-quadrupole ($E2$) matrix element connecting the ground state and first-excited $2_1^{+}$ state was extracted for both ${}^{202}$Rn and ${}^{204}$Rn, corresponding to $B(E2;2_1^{+}\to 2_1^{+})=29_{-8}^{+8}$ W.u. and $43_{-12}^{+17}$ W.u., respectively. Additionally, $E2$ matrix elements connecting the $2_1^{+}$ state with the $4_1^{+}$ and $2_2^{+}$ states were determined in ${}^{202}$Rn. No excited $0^{+}$ states were observed in the current data set, possibly due to a limited population of second-order processes at the currently-available beam energies. Conclusions: The results are discussed in terms of collectivity and the deformation of both nuclei studied is deduced to be weak, as expected from the low-lying level-energy schemes. Comparisons are also made to state-of-the-art beyond-mean-field model calculations and the magnitude of the transitional quadrupole moments are well reproduced.

12 pages, 14 figures