Abstract
Many properties of the atomic nucleus, such as vibrations, rotations and incompressibility, can be interpreted as due to a two-component quantum liquid of protons and neutrons. Electron scattering measurements on stable nuclei demonstrate that their central densities are saturated, as for liquid drops. In exotic nuclei near the limits of mass and charge, with large imbalances in their proton and neutron numbers, the possibility of a depleted central density, or a ‘bubble’ structure, has been discussed in a recurrent manner since the 1970s. Here we report first experimental evidence that points to a depletion of the central density of protons in the short-lived nucleus 34Si. The proton-to-neutron density asymmetry in 34Si offers the possibility to place constraints on the density and isospin dependence of the spin–orbit force—on which nuclear models have disagreed for decades—and on its stabilizing effect towards limits of nuclear existence.
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Acknowledgements
This work is supported by the National Science Foundation (NSF) under Grant Nos. PHY-1102511 and PHY-1306297, the OTKA Contract No. K100835, and by the Institut Universitaire de France. GRETINA was funded by the US DOE—Office of Science. Operation of the array at NSCL is supported by the NSF under Cooperative Agreement PHY-1102511 (NSCL) and the DOE under grant DE-AC02-05CH11231 (LBNL). J.A.T. acknowledges support of the Science and Technology Facility Council (UK) grant ST/L005743.
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A.M. performed the offline data analysis, A.Lem., D.W. and K.W. performed on-line data analysis and checked the integrity of data taking. K.W. and A.Lem. performed GEANT4 simulations and wrote parts of the offline sorting code. A.G. and J.A.T. performed reaction theory calculations. D.B. operated the S800 spectrometer. D.W. and F.Re. were responsible for the setting up, calibration and operation of the Gretina array. H.I. and K.W. helped to set up the Gretina array. The manuscript was prepared by O.S., A.M., J.A.T., A.G., A.Lem. and E.K. J.-P.E. performed relativistic mean field calculations. Z.D. and D.S. contributed to the offline data analysis of the γ-ray spectra and C.B., R.B., E.K., A.Lem., A.Lep., H.I., T.R., F.Ro., M.S., M.V. and S.R.S. checked data accumulation on-line. O.S. proposed the experiment and supervised the analysis.
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Mutschler, A., Lemasson, A., Sorlin, O. et al. A proton density bubble in the doubly magic 34Si nucleus. Nature Phys 13, 152–156 (2017). https://doi.org/10.1038/nphys3916
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DOI: https://doi.org/10.1038/nphys3916
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