Studies of pear-shaped nuclei using accelerated radioactive beams

Abstract

There is strong circumstantial evidence that certain heavy, unstable atomic nuclei are ‘octupole deformed’, that is, distorted into a pear shape. This contrasts with the more prevalent rugby-ball shape of nuclei with reflection-symmetric, quadrupole deformations. The elusive octupole deformed nuclei are of importance for nuclear structure theory, and also in searches for physics beyond the standard model; any measurable electric-dipole moment (a signature of the latter) is expected to be amplified in such nuclei. Here we determine electric octupole transition strengths (a direct measure of octupole correlations) for short-lived isotopes of radon and radium. Coulomb excitation experiments were performed using accelerated beams of heavy, radioactive ions. Our data on 220Rn and 224Ra show clear evidence for stronger octupole deformation in the latter. The results enable discrimination between differing theoretical approaches to octupole correlations, and help to constrain suitable candidates for experimental studies of atomic electric-dipole moments that might reveal extensions to the standard model.

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Figure 1: Representative γ-ray spectra following the bombardment of 2 mg cm−2 60Ni and 120Sn targets by 220Rn and 224Ra.
Figure 2: Partial level-schemes for 220Rn and 224Ra, showing the excited states of interest for this work.
Figure 3: The values of the E2 and E3 intrinsic moments, Qλ (I, I′).
Figure 4: Graphical representation of the shapes of 220Rn and 224Ra.
Figure 5: Values of Qλ for low-lying transitions in nuclei as a function of N.

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Acknowledgements

The support of the ISOLDE Collaboration and technical teams is acknowledged. This work was supported by the following Research Councils: STFC (UK), BMBF(Germany; 05P12RDCIA, 06DA9036I, 06KY9136I and 06KY205I), HIC for FAIR (Germany), FWO-Vlaanderen (Belgium), Belgian Science Policy Office (IAP-BriX network P7/12), Academy of Finland (contract no. 131665), DOE (US; DE-AC52-07NA27344 and DE-FG02-04ER41331), NSF (US), MICINN (Spain; FPA2009-08958 and FIS2009-07277), Consolider-Ingenio 2010 Programmes (Spain; CPAN CSD2007-00042 and MULTIDARK CSD2009-00064), Polish Ministry for Science and Higher Education (grant no. 589/N-G-POOL/2009/0), EC via I3-EURONS (FP6 contract no. RII3-CT-2004-506065), MC Fellowship scheme (FP7 contract PIEF-GA-2008-219175) and IA-ENSAR (FP7 contract 262010).

Author information

Instrument set-up: M.A., C.B., A.B., T.D., H.D.W., L.P.G., J.K., J.P., P.R., M. Seidlitz, B.S., M.J.V. and N.W. DAQ and on-line analysis: A.B., L.P.G., R.L. and N.W. Data analysis and interpretation: L.P.G., P.A.B., D.C., A.B.H., M. Scheck and M.Z. REX development and set-up: F.W., D.V. and J.C. Primary target: T.S. Preparation of manuscript: P.A.B., L.P.G., T.C., A.B., D.G.J., Th.K., J.P., P.R., M. Scheck, P.V.D. and N.W. Theoretical interpretation: L.M.R. All authors except L.M.R. took part in the experiments.

Correspondence to P. A. Butler.

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Gaffney, L., Butler, P., Scheck, M. et al. Studies of pear-shaped nuclei using accelerated radioactive beams. Nature 497, 199–204 (2013). https://doi.org/10.1038/nature12073

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