title: Proton-neutron pairing correlations in the self-conjugate nucleus (38)K probed via a direct measurement of the isomer shift.
abstract: A marked difference in the nuclear charge radius was observed between the I^{π}=3^{+} ground state and the I^{π}=0^{+} isomer of ^{38}K and is qualitatively explained using an intuitive picture of proton-neutron pairing. In a high-precision measurement of the isomer shift using bunched-beam collinear laser spectroscopy at CERN-ISOLDE, a change in the mean-square charge radius of ⟨r_{c}^{2}⟩(^{38}K^{m})-⟨r_{c}^{2}⟩(^{38}K^{g})=0.100(6)  fm^{2} was obtained. This is an order of magnitude more accurate than the result of a previous indirect measurement from which it was concluded that both long-lived states in ^{38}K have similar charge radii. Our observation leads to a substantially different understanding since the difference in charge radius is, moreover, opposite in sign to previously reported theoretical predictions. It is demonstrated that the observed isomer shift can be reproduced by large-scale shell-model calculations including proton and neutron excitations across the N,Z=20 shell gaps, confirming the significance of cross-shell correlations in the region of ^{40}Ca.
authors.affiliation: KU Leuven, Instituut voor Kern-en Stralingsfysica, B-3001 Leuven, Belgium.
authors.forename: M L
authors.initials: ML
authors.lastname: Bissell
authors.affiliation: KU Leuven, Instituut voor Kern-en Stralingsfysica, B-3001 Leuven, Belgium.
authors.forename: J
authors.initials: J
authors.lastname: Papuga
authors.affiliation: Université de Strasbourg, IPHC, 23 rue du Loess 67037 Strasbourg, France CNRS, UMR7178, 67037 Strasbourg, France and GSI Helmholtzzentrum für Schwerionenforschung, D-64291 Darmstadt, Germany and Laboratoire de Physique Mathématique et Subatomique, Constantine 1 University, Constantine 25000, Algeria.
authors.forename: H
authors.initials: H
authors.lastname: Naïdja
authors.affiliation: Max-Plank-Institut für Kernphysik, D-69117 Heidelberg, Germany.
authors.forename: K
authors.initials: K
authors.lastname: Kreim
authors.affiliation: Max-Plank-Institut für Kernphysik, D-69117 Heidelberg, Germany.
authors.forename: K
authors.initials: K
authors.lastname: Blaum
authors.affiliation: KU Leuven, Instituut voor Kern-en Stralingsfysica, B-3001 Leuven, Belgium.
authors.forename: M
authors.initials: M
authors.lastname: De Rydt
authors.affiliation: KU Leuven, Instituut voor Kern-en Stralingsfysica, B-3001 Leuven, Belgium.
authors.forename: R F
authors.initials: RF
authors.lastname: Garcia Ruiz
authors.affiliation: KU Leuven, Instituut voor Kern-en Stralingsfysica, B-3001 Leuven, Belgium.
authors.forename: H
authors.initials: H
authors.lastname: Heylen
authors.affiliation: Physics Department, CERN, CH-1211 Geneva 23, Switzerland.
authors.forename: M
authors.initials: M
authors.lastname: Kowalska
authors.affiliation: Max-Plank-Institut für Kernphysik, D-69117 Heidelberg, Germany and Institut für Kernchemie, Johannes Gutenberg-Universität Mainz, D-55128 Mainz, Germany.
authors.forename: R
authors.initials: R
authors.lastname: Neugart
authors.affiliation: KU Leuven, Instituut voor Kern-en Stralingsfysica, B-3001 Leuven, Belgium.
authors.forename: G
authors.initials: G
authors.lastname: Neyens
authors.affiliation: Institut für Kernchemie, Johannes Gutenberg-Universität Mainz, D-55128 Mainz, Germany and Institut für Kernphysik, TU Darmstadt, D-64289 Darmstadt, Germany.
authors.forename: W
authors.initials: W
authors.lastname: Nörtershäuser
authors.affiliation: Université de Strasbourg, IPHC, 23 rue du Loess 67037 Strasbourg, France CNRS, UMR7178, 67037 Strasbourg, France.
authors.forename: F
authors.initials: F
authors.lastname: Nowacki
authors.affiliation: KU Leuven, Instituut voor Kern-en Stralingsfysica, B-3001 Leuven, Belgium.
authors.forename: M M
authors.initials: MM
authors.lastname: Rajabali
authors.affiliation: GSI Helmholtzzentrum für Schwerionenforschung, D-64291 Darmstadt, Germany and Helmholtz-Institut Mainz, Johannes Gutenberg-Universität Mainz, 55099 Mainz, Germany.
authors.forename: R
authors.initials: R
authors.lastname: Sanchez
authors.affiliation: Université de Strasbourg, IPHC, 23 rue du Loess 67037 Strasbourg, France CNRS, UMR7178, 67037 Strasbourg, France.
authors.forename: K
authors.initials: K
authors.lastname: Sieja
authors.affiliation: Max-Plank-Institut für Kernphysik, D-69117 Heidelberg, Germany.
authors.forename: D T
authors.initials: DT
authors.lastname: Yordanov