There is sparse direct experimental evidence that atomic nuclei can exhibit stable 'pear' shapes arising from strong octupole correlations. In order to investigate the nature of octupole collectivity in radium isotopes, electric octupole (E3) matrix elements have been determined for transitions in 222,228 Ra nuclei using the method of sub-barrier, multi-step Coulomb excitation. Beams of the radioactive radium isotopes were provided by the HIE-ISOLDE facility at CERN. The observed pattern of E3 matrix elements for different nuclear transitions is explained by describing 222 Ra as pear-shaped with stable octupole deformation, while 228 Ra behaves like an octupole vibrator.

There is a large body of evidence that atomic nuclei can undergo octupole distortion and assume the shape of a pear. This phenomenon is important for measurements of electric-dipole moments of atoms, which would indicate CP violation and hence probe physics beyond the Standard Model of particle physics. Isotopes of both radon and radium have been identified as candidates for such measurements. Here, we observed the low-lying quantum states in 224Rn and 226Rn by accelerating beams of these radioactive nuclei. We show that radon isotopes undergo octupole vibrations but do not possess static pear-shapes in their ground states. We conclude that radon atoms provide less favourable conditions for the enhancement of a measurable atomic electric-dipole moment.

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We would like to make readers aware that after the publication of this article the sort code was updated. This resulted in more gamma-gamma data, particularly for high-spin transitions. By performing additional analysis we confirm the energies of most of the states, have identified several new states, and have updated one of the states that was incorrectly represented in its energy in the original paper. The figures (Figs. 1-4) and table (Table 1) are updated along with their captions. The overall conclusions of the paper remain unaffected. The new data confirm our original finding, that 224,226 Rn behave as octupole vibrators in which the octupole phonon is aligned to the rotational axis. We conclude that there are no isotopes of radon that have static octupole deformation, so that any parity doublets in the odd-mass neighbours will not be closely spaced in energy. This means that radon atoms will provide less favourable conditions for the enhancement of a measurable atomic electric-dipole moment. Prior to this work, less was known about the energies and spins of excited states in 224,226 Rn. The spectra of γ-rays time-correlated with scattered beam and target recoils are shown in Fig. 1. The E2 γ-ray transitions within the ground-state positive-parity band can be clearly identified. The other relatively intense γ-rays observed in these spectra are assumed to have E1 multipolarity, depopulating the odd-spin negative-parity members of the octupole band. In order to determine which states are connected by these transitions, pairs of time-correlated ('coincident') γ-rays were examined. In this analysis, the energy spectrum of γ-rays coincident with one particular transition is generated by requiring that the energy of this gating transition lies in a specific range. Typical spectra obtained this way are shown in Fig. 2. Each spectrum corresponds to a particular gating transition, background subtracted, so that the peaks observed in the spectrum arise from γ-ray transitions in coincidence with that transition. The data in these γ-γ spectra have been significantly enhanced by modifying the sort code that converts raw data from the Miniball spectrometer to Root analysis files. This modification enables γ-γ data to be included when both heavy ions are detected in the silicon detector array, in addition to single-ion events. Since both conditions for ion detection were already considered for single γ-ray events, this modification does not affect the total spectra shown in (Fig. 1), but considerably enhances the statistics of the γ-γ gated spectra shown in Fig. 2. The additional γ-γ data have allowed the authors to extend the level schemes by one additional state in each of the positive-parity bands in 224,226 Rn and in the negative-parity band in 224 Rn over that reported originally. More importantly, we are able to determine the probable energy of the 7 − , 9 − and 11 − states in 226 Rn, see Fig. 2. By extrapolating this band to lower spin states on the basis of its rotational-like behaviour, we are able to estimate th...

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