CERN Accélérateur de science

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 midshell at 𝑁=104.

Purpose: Evidence for shape coexistence has been inferred from 𝛼-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 in CERN.

Results: The electric-quadrupole (𝐸⁢2) matrix element connecting the ground state and first excited 2+
1 state was extracted for both  
202
 Rn 
  and  
204
 Rn 
 , corresponding to 𝐵⁡(𝐸⁢2;2+
1→0+
1)=29+8
−8 and 43+17
−12 W.u., respectively. Additionally, 𝐸⁢2 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 owing 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.