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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.

References

  1. Goeppert Mayer, M. On closed shells in nuclei. II. Phys. Rev. 75, 1969–1970 (1949)

    ADS  CAS  Article  Google Scholar 

  2. Engel, J., Langanke, K. & Vogel, P. Pairing and isospin symmetry in proton-rich nuclei. Phys. Lett. B 389, 211–216 (1996)

    ADS  CAS  Article  Google Scholar 

  3. Goodman, A. L. Restoration of axial symmetry of the equilibrium shape of 24Mg by pairing correlations. Adv. Nucl. Phys. 11, 260–263 (1979)

    Google Scholar 

  4. Engel, J. et al. Neutron-proton correlations in an exactly solvable model. Phys. Rev. C 55, 1781–1788 (1997)

    ADS  CAS  Article  Google Scholar 

  5. Satula, W. & Wyss, R. Competition between T = 0 and T = 1 pairing in proton-rich nuclei. Phys. Lett. B 393, 1–6 (1997)

    ADS  CAS  Article  Google Scholar 

  6. Civitarese, O., Reboiro, M. & Vogel, P. Neutron-proton pairing in the BCS approach. Phys. Rev. C 56, 1840–1843 (1997)

    ADS  CAS  Article  Google Scholar 

  7. Talmi, I. Generalized seniority and structure of semi-magic nuclei. Nucl. Phys. A 172, 1–24 (1971)

    ADS  CAS  Article  Google Scholar 

  8. Talmi, I. Generalized seniority states with definite isospin. Nucl. Phys. A 686, 217–240 (2001)

    ADS  Article  Google Scholar 

  9. Macchiavelli, A. O. et al. Is there np pairing in N = Z nuclei? Phys. Rev. C 61, 041303(R) (2000)

    ADS  Article  Google Scholar 

  10. Afanasjev, A. & Frauendorf, S. Description of rotating N = Z nuclei in terms of isovector pairing. Phys. Rev. C 71, 064318 (2005)

    ADS  Article  Google Scholar 

  11. Danos, M. & Gillet, V. Stretch scheme, a shell model description of deformed nuclei. Phys. Rev. 161, 1034–1044 (1967)

    ADS  CAS  Article  Google Scholar 

  12. Ma˘rginean, N. et al. Yrast isomers in 95Ag, 95Pd, and 94Pd. Phys. Rev. C 67, 061301 (2003)

    ADS  Article  Google Scholar 

  13. Alber, D., Bertschat, H. H., Grawe, H., Haas, H. & Spellmeyer, B. Nuclear structure studies of the neutron deficient N = 50 nucleus 96Pd. Z. Phys. A 332, 129–135 (1989)

    ADS  CAS  Google Scholar 

  14. Schatz, H. et al. End point of the rp process on accreting neutron stars. Phys. Rev. Lett. 86, 3471–3474 (2001)

    ADS  CAS  Article  Google Scholar 

  15. Clement, R. R. C. et al. New approach for measuring properties of rp-process nuclei. Phys. Rev. Lett. 92, 172502 (2004)

    ADS  CAS  Article  Google Scholar 

  16. Scheurer, J. N. et al. Improvements in the in-beam γ-ray spectroscopy provided by an ancillary detector coupled to a Ge -spectrometer: the DIAMANT-EUROGAM II example. Nucl. Instrum. Methods Phys. Res. A 385, 501–510 (1997)

    ADS  CAS  Article  Google Scholar 

  17. Gál, J. et al. The VXI electronics of the DIAMANT particle detector array. Nucl. Instrum. Methods Phys. Res. A 516, 502–510 (2004)

    ADS  Article  Google Scholar 

  18. Skeppstedt, Ö. et al. The EUROBALL neutron wall design and performance tests of neutron detectors. Nucl. Instrum. Methods Phys. Res. A 421, 531–541 (1999)

    ADS  CAS  Article  Google Scholar 

  19. Azaiez, F. EXOGAM: A γ-ray spectrometer for radioactive beams. Nucl. Phys. A 654, 1003c–1008c (1999)

    ADS  Article  Google Scholar 

  20. Simpson, J. et al. The EXOGAM array: a radioactive beam gamma-ray spectrometer. Heavy Ion Phys. 11, 159–188 (2000); see also 〈http://pro.ganil-spiral2.eu/laboratory/detectors/exogam

    CAS  Google Scholar 

  21. O'Leary, C. D. et al. Neutron-proton pairing, Coulomb effects and shape coexistence in odd-odd N = Z 46V. Phys. Lett. B 459, 73–80 (1999)

    ADS  CAS  Article  Google Scholar 

  22. Lenzi, S. M. et al. Band termination in the N = Z odd-odd nucleus 46V. Phys. Rev. C 60, 021303(R) (1999)

    ADS  Article  Google Scholar 

  23. Heese, J. et al. High spin states and shell model description of the neutron deficient nuclei 90Ru and 91Ru. Phys. Rev. C 49, 1896–1903 (1994)

    ADS  CAS  Article  Google Scholar 

  24. Ma˘rginean, N. et al. Identification of excited states and shell model description of the N = Z+1 nucleus 91Rh. Phys. Rev. C 72, 014302 (2005)

    ADS  Article  Google Scholar 

Download references

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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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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This file contains Supplementary Information comprising: Experimental details and Theoretical approach. The file also contains additional references. (PDF 112 kb)

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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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