Evidence for a spin-aligned neutron–proton paired phase from the level structure of 92Pd

Abstract

Shell structure and magic numbers in atomic nuclei were generally explained by pioneering work1 that introduced a strong spin–orbit interaction to the nuclear shell model potential. However, knowledge of nuclear forces and the mechanisms governing the structure of nuclei, in particular far from stability, is still incomplete. In nuclei with equal neutron and proton numbers (N = Z), enhanced correlations arise between neutrons and protons (two distinct types of fermions) that occupy orbitals with the same quantum numbers. Such correlations have been predicted to favour an unusual type of nuclear superfluidity, termed isoscalar neutron–proton pairing2,3,4,5,6, in addition to normal isovector pairing. Despite many experimental efforts, these predictions have not been confirmed. Here we report the experimental observation of excited states in the N = Z = 46 nucleus 92Pd. Gamma rays emitted following the 58Ni(36Ar,2n)92Pd fusion–evaporation reaction were identified using a combination of state-of-the-art high-resolution γ-ray, charged-particle and neutron detector systems. Our results reveal evidence for a spin-aligned, isoscalar neutron–proton coupling scheme, different from the previous prediction2,3,4,5,6. We suggest that this coupling scheme replaces normal superfluidity (characterized by seniority coupling7,8) in the ground and low-lying excited states of the heaviest N = Z nuclei. Such strong, isoscalar neutron–proton correlations would have a considerable impact on the nuclear level structure and possibly influence the dynamics of rapid proton capture in stellar nucleosynthesis.

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Figure 1: Schematic illustration of the two possible pairing schemes in nuclei.
Figure 2: Schematic illustration of the experimental set-up used to identify γ-ray transitions from excited states in 92 Pd.
Figure 3: Identification of γ-ray transitions in 92 Pd.
Figure 4: Illustration of the predicted ground-state wavefunctions of 92 Pd and 96 Pd, and comparison of calculated and experimental level energies in 92 Pd, 94 Pd and 96 Pd.

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Acknowledgements

This work was supported by the Swedish Research Council (contract nos 2007-4067 and 2008-5793), the Göran Gustafsson Foundation, the European Union Sixth Framework Programme ‘Integrating Infrastructure Initiative – Transnational Access’ (no. 506065; EURONS), the Hungarian Scientific Research Fund, OTKA (contract nos. K72566 and K68801), the UK Science and Technology Facilities Council (STFC), the Polish Ministry of Science and Higher Education (grant no. N N202 073935), the Spanish Ministerio de Ciencia e Innovación (contract no. FPA2007-66069), the Spanish Consolider-Ingenio 2010 Programme CPAN (CSD2007-00042), and Ankara University BIYEP project no. DPT 2005120140. We thank the EXOGAM collaboration for use of the germanium detector system, the DIAMANT collaboration for use of the charged particle detector system, the European γ-Ray Spectroscopy Pool for use of the neutron detector system, L. Einarsson and R. Seppälä for providing some of the targets used in this experiment and the GANIL staff for technical support and for providing the 36Ar beam.

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Contributions

G.d.F., S.B., E.C. and A.D. were responsible for setting up the EXOGAM germanium detectors and most of the related electronics and data acquisition system. J.N. was in charge of setting up the Neutron Wall detector system with its electronics. B.M.N., A.A., J.G., K.J., G.K., J.M., J.-N.S. and J.T. were responsible for the operation of the DIAMANT charged particle detector system with its associated electronics. All authors except J.B., J.G., K.J., G.K., R.L. and C.Q. participated in the measurements. B.C., F.G.M., T.B., K.A., K.L., E.C., G.d.F., A.Jo., M.P., R.W. and H.A.-A. performed the data analysis or were involved in different aspects of it. J.B. proposed the theoretical interpretation. The shell model calculations were performed by C.Q., R.L. and J.B. The manuscript text was prepared by B.C., with contributions from R.W., J.B., T.B., A.Jo., R.L., C.Q., A.A., G.d.A., E.C., Zs.D., F.G.M., A.Ju., S.M.L., R.M., B.M.N., J.N., M.P., F.R., D.S., M.S. and J.T. T.B. prepared the figures.

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Correspondence to B. Cederwall.

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Cederwall, B., Moradi, F., Bäck, T. et al. Evidence for a spin-aligned neutron–proton paired phase from the level structure of 92Pd. Nature 469, 68–71 (2011). https://doi.org/10.1038/nature09644

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