Unexpectedly large charge radii of neutron-rich calcium isotopes

Garcia Ruiz, R. F.; Neugart, R.; Yordanov, D.T.; Wendt, K.A.; Blaum, K.; Frömmgen, N.; Schwenk, A.; Hagen, G.; Kreim, K.; Hammen, M.; Papuga, J.; Hebeler, K.; Bissell, M.L.; Jansen, G.R.; Ekström, A.; Nazarewicz, W.; Holt, J.D.; Neyens, G.; Papenbrock, T.; Nörtershäuser, W.; Kowalska, M.; Simonis, J.

doi:10.1038/nphys3645

Article
Report number	arXiv:1602.07906
Title	Unexpectedly large charge radii of neutron-rich calcium isotopes
Author(s)	Garcia Ruiz, R. F. (Leuven U.) ; Bissell, M.L. (Leuven U. ; Manchester U.) ; Blaum, K. (Heidelberg, Max Planck Inst.) ; Ekström, A. (Oak Ridge ; Tennessee U.) ; Frömmgen, N. (Mainz U., Inst. Kernchem.) ; Hagen, G. (Oak Ridge) ; Hammen, M. (Mainz U., Inst. Kernchem.) ; Hebeler, K. (Darmstadt, Tech. Hochsch. ; Darmstadt, EMMI) ; Holt, J.D. (TRIUMF) ; Jansen, G.R. (Oak Ridge ; Tennessee U.) ; Kowalska, M. (CERN) ; Kreim, K. (Heidelberg, Max Planck Inst.) ; Nazarewicz, W. (Oak Ridge ; Michigan State U. ; Michigan State U., NSCL ; Warsaw U.) ; Neugart, R. (Heidelberg, Max Planck Inst. ; Mainz U., Inst. Kernchem.) ; Neyens, G. (Leuven U.) ; Nörtershäuser, W. (Mainz U., Inst. Kernchem. ; Darmstadt, Tech. Hochsch.) ; Papenbrock, T. (Oak Ridge ; Tennessee U.) ; Papuga, J. (Leuven U.) ; Schwenk, A. (Heidelberg, Max Planck Inst. ; Darmstadt, Tech. Hochsch.) ; Simonis, J. (Darmstadt, Tech. Hochsch. ; Darmstadt, EMMI) ; Wendt, K.A. (Oak Ridge ; Tennessee U. ; Darmstadt, EMMI) ; Yordanov, D.T. (Heidelberg, Max Planck Inst. ; Orsay, IPN)
Publication	2016
Imprint	2016-02-25
Number of pages	6
Note	Submitted version. See original publication (doi:10.1038/nphys3645) for final version
In:	Nature Phys. 12 (2016) 594-598
DOI	10.1038/nphys3645
Subject category	Nuclear Physics - Experiment ; Nuclear Physics - Theory
Accelerator/Facility, Experiment	CERN ISOLDE
Abstract	Despite being a complex many-body system, the atomic nucleus exhibits simple structures for certain ‘magic’ numbers of protons and neutrons. The calcium chain in particular is both unique and puzzling: evidence of doubly magic features are known in 40,48Ca, and recently suggested in two radioactive isotopes, 52,54Ca. Although many properties of experimentally known calcium isotopes have been successfully described by nuclear theory, it is still a challenge to predict the evolution of their charge radii. Here we present the first measurements of the charge radii of 49,51,52Ca, obtained from laser spectroscopy experiments at ISOLDE, CERN. The experimental results are complemented by state-of-the-art theoretical calculations. The large and unexpected increase of the size of the neutron-rich calcium isotopes beyond N = 28 challenges the doubly magic nature of 52Ca and opens new intriguing questions on the evolution of nuclear sizes away from stability, which are of importance for our understanding of neutron-rich atomic nuclei.
Copyright/License	arXiv nonexclusive-distrib. 1.0

$High-resolution bunched collinear laser spectroscopy at ISOLDE, CERN. Short-lived calcium isotopes are produced from nuclear reactions of high-energy protons impacting on an uranium carbide target. Ca atoms were selectively ionized by using a three-step laser scheme \cite{marsh14}. Ions were extracted from the source and mass separated to be injected into a radiofrequency trap, where they are accumulated for typically 100 ms. Bunches of ions were extracted and redirected into the COLLAPS beam line to perform collinear laser spectroscopy experiments. At COLLAPS, the ions are superimposed with a continuous wavelength laser beam to scan the hyperfine structure in the $4s$ $^2S_{1/2}$ $\rightarrow$ $4p$ $^2P_{3/2}$ transition of Ca$^+$ (see text for more details).$ $Examples of hyperfine structure spectra measured for the Ca isotopes in the 393-nm $4s$ $^2S_{1/2} \rightarrow 4p$ $^2P_{3/2}$ ionic transition. The solid lines show the fit with a Voigt profile. Frequency values are relative to the centroid of $^{52}$Ca.$ $Charge radii of Ca isotopes. (a) Experimental charge radii compared to ab initio calculations with chiral EFT interactions NNLO$_{\rm sat}$, SRG1, SRG2, as well as DFT calculations with the UNEDF0 functional. Experimental error bars are smaller than the symbols. The absolute values were obtained from the reference radius of $^{40}$Ca ($R_{\rm ch}=3.478(2)$ fm) \cite{fricke}. The values of $^{39}$Ca and $^{41,42}$Ca are taken from Ref. \cite{vermeeren96} and Ref. \cite{martensson}, respectively. A systematic theoretical uncertainty of 1\% is estimated for the absolute values due to the truncation level of the coupled-cluster method and the finite basis-space employed in the computation. (b) Experimental rms charge radius in $^{52}$Ca relative to that in $^{48}$Ca compared to the ab initio results as well as those of representative density functional theory (DFT) and configuration interaction (CI) calculations. The systematic uncertainties in the theoretical predictions are largely cancelled when the differences between rms charge radii are calculated (dotted horizontal blue lines). Experimental uncertainties are represented by the horizontal red lines (statistical) and the gray shaded band (systematic).$ Show more plots

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