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Evidence for a new nuclear ‘magic number’ from the level structure of 54Ca

Abstract

Atomic nuclei are finite quantum systems composed of two distinct types of fermion—protons and neutrons. In a manner similar to that of electrons orbiting in an atom, protons and neutrons in a nucleus form shell structures. In the case of stable, naturally occurring nuclei, large energy gaps exist between shells that fill completely when the proton or neutron number is equal to 2, 8, 20, 28, 50, 82 or 126 (ref. 1). Away from stability, however, these so-called ‘magic numbers’ are known to evolve in systems with a large imbalance of protons and neutrons. Although some of the standard shell closures can disappear, new ones are known to appear2,3. Studies aiming to identify and understand such behaviour are of major importance in the field of experimental and theoretical nuclear physics. Here we report a spectroscopic study of the neutron-rich nucleus 54Ca (a bound system composed of 20 protons and 34 neutrons) using proton knockout reactions involving fast radioactive projectiles. The results highlight the doubly magic nature of 54Ca and provide direct experimental evidence for the onset of a sizable subshell closure at neutron number 34 in isotopes far from stability.

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Figure 1: Schematic illustration highlighting the attractive interaction between the proton πf7/2 and neutron νf5/2 single-particle orbitals for N = 34 isotones.
Figure 2: Systematics of excited-state energies in even–even Ca isotopes and neighbouring nuclei.
Figure 3: Particle identification plots for radioisotopes.
Figure 4: Doppler-corrected γ-ray energy spectra.

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Acknowledgements

We thank the staff of the RIKEN Nishina Center accelerator complex for their contribution to the experiment. This work is part of the CNS-RIKEN joint research project on large-scale nuclear structure calculations.

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Contributions

D.S. performed offline data analyses and GEANT4 simulations, wrote the manuscript and prepared the figures. D.S. and S.T. designed the experiment. N.A., P.D., J.L., K.M., M.M., Y.S., D.S., S.T., R.T. and H.W. were responsible for setting up the γ-ray detector array. D.N. prepared the beamline ionization chambers. N.F. and H. Suzuki tuned the BigRIPS separator and the ZeroDegree spectrometer. H.B. was responsible for the data acquisition system. N.A., P.D., S.G., J.L., K.M., M.M., S.M., Y.S., P.-A.S., D.S., T.S., S.T., R.T., J.J.V.-D., H.W. and K.Y. checked data accumulation online and maintained operation of the experiment. M.H., T.O. and Y.U. performed the shell-model calculations. All authors discussed the results and commented on the manuscript.

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Correspondence to D. Steppenbeck.

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The authors declare no competing financial interests.

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Steppenbeck, D., Takeuchi, S., Aoi, N. et al. Evidence for a new nuclear ‘magic number’ from the level structure of 54Ca. Nature 502, 207–210 (2013). https://doi.org/10.1038/nature12522

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