Collectivity in the light radon nuclei measured directly via Coulomb excitation

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.

Authors: L. P. Gaffney, A. P. Robinson, D. G. Jenkins, A. N. Andreyev, M. Bender, A. Blazhev, N. Bree, B. Bruyneel, P. A. Butler, T. E. Cocolios, T. Davinson, A. N. Deacon, H. De Witte, D. DiJulio, J. Diriken, A. Ekstrom, Ch. Fransen, S. J. Freeman, K. Geibel, T. Grahn, B. Hadinia, M. Hass, P. -H. Heenen, H. Hess, M. Huyse, U. Jakobsson, N. Kesteloot, J. Konki, Th. Kröll, V. Kumar, O. Ivanov, S. Martin-Haugh, D. Mucher, R. Orlandi, J. Pakarinen, A. Petts, P. Peura, P. Rahkila, P. Reiter, M. Scheck, M. Seidlitz, K. Singh, J. F. Smith, J. Van de Walle, P. Van Duppen, D. Voulot, R. Wadsworth, N. Warr, F. Wenander, K. Wimmer, K. Wrzosek-Lipska, M. Zielińska

Doi: 10.1103/PhysRevC.91.064313

Issue number:

Journal: Phys. Rev. C

Page number: 064313

Published on: 2015-06-22

Year: