Unambiguous values of the spin and magnetic moment of 31 Mg are obtained by combining the results of a hyperfine-structure measurement and a -NMR measurement, both performed with an optically polarized ion beam. With a measured nuclear g factor and spin I 1=2, the magnetic moment 31 Mg ÿ0:8835515 N is deduced. A revised level scheme of 31 Mg (Z 12, N 19) with ground state spin/parity I 1=2 is presented, revealing the coexistence of 1p-1h and 2p-2h intruder states below 500 keV. Advanced shell-model calculations and the Nilsson model suggest that the I 1=2 ground state is a strongly prolate deformed intruder state. This result plays a key role for the understanding of nuclear structure changes due to the disappearance of the N 20 shell gap in neutron-rich nuclei. Since Mayer and Jensen established the concept of shell structure in atomic nuclei, magic nucleon numbers have played a decisive role in describing the nuclear system [1]. About a quarter century later, the discovery of the anomalous ground state properties of 31 Na [2,3] suggested that the magic shell structure can be broken. Shell-model calculations allowing particle-hole (p-h) excitations across the N 20 shell gap proposed that a group of nuclei with deformed ground states appears between Z 10-12 and N 20-22. Because the p-h excited intruder states come lower in energy than the normal shell-model states, this region has been called the ''island of inversion '' [4]. In fact, -decay experiments [5,6] It has been suggested that the N 20 shell gap is changing from one nucleus to another [11,12] due to changes in the proton-neutron interaction. The boundary of the island of inversion can thus be shifted or smeared out, and intruder ground states might appear outside the earlier defined boundaries. Since the size of the shell gap is related to the single-particle energies [determined mainly by the monopole part of the nucleon-nucleon (NN) interaction], the mapping of the boundary is linked to one of the most basic and unanswered questions in present day nuclear structure physics: the microscopic mechanism to determine the monopole part of the NN interaction.We present in this Letter a measurement of the ground state spin and magnetic moment of the exotic even-odd nucleus 31 Mg (Z 12, N 19). The earlier observed anomalous lifetime and the branching intensities in its decay have never been explained [5,13], although the high level density suggested the presence of intruder states at low excitation energy [14]. However, unambiguous spin/ parity assignments are needed in order to establish the coexistence of normal sd-shell states with 1p-1h and 2p-2h intruder states. In addition to the ground state spin and parity, the magnetic moment value and sign provides direct information on the odd-neutron configuration.The spin and magnetic moment of 31 Mg are measured by combining the results from two experimental techniques, based on the atomic hyperfine structure and on the nuclear interaction with external magnetic fields. Both methods rely on an optically polarized...

The ground-state spins and magnetic moments of neutron-rich 27 Mg, 29 Mg, and 31 Mg were measured for the first time with laser and β-NMR spectroscopy at ISOLDE/CERN. The hyperfine structure of 27 Mgobserved in fluorescence-confirms previous assignments of the spin I = 1/2 and reveals the magnetic moment µ I ( 27 Mg) = −0.4107(15) µ N . The hyperfine structure and nuclear magnetic resonance of optically polarized 29 Mg-observed in the asymmetry of its β decay after implantation in a cubic crystal-give I = 3/2 and µ I ( 29 Mg) = +0.9780(6) µ N . For 31 Mg they yield together I = 1/2 and µ I ( 31 Mg) = −0.88355(15) µ N , where the negative magnetic moment provides evidence for a positive parity. The results for 27 Mg and 29 Mg agree well with shell-model calculations confined only to the sd model space, whereas the ground state of 31 Mg involves large contributions from neutrons in the pf shell, which places this nucleus inside the "island of inversion."

The electric quadrupole moment and the magnetic moment of the 11 Li halo nucleus have been measured with more than an order of magnitude higher precision than before, |Q| = 33.3(5) mb and µ = 3.6712(3) µN , revealing a 8.8(1.5)% increase of the quadrupole moment relative to that of 9 Li. This result is compared to various models that aim at describing the halo properties. In the shell model an increased quadrupole moment points to a significant occupation of the 1d orbits, whereas in a simple halo picture this can be explained by relating the quadrupole moments of the proton distribution to the charge radii. Advanced models so far fail to reproduce simultaneously the trends observed in the radii and quadrupole moments of the lithium isotopes.PACS numbers: 21.10. Ky, 27.30.+t, 21.60.Cs Since nuclear physicists could produce and investigate bound systems of nucleons in many possible combinations, a wealth of isotopes with unexpected properties have been discovered. For example, some neutron-rich isotopes of light elements, such as 11 Li, were found to have exceptionally large radii [1]. Upon discovery in 1985, this phenomenon was attributed to either large deformation or to a long tail in the matter distribution [2]. Deformation was soon excluded by the spin and magnetic moment of 11 Li belonging to a spherical πp 3/2 state [3]. Considering the weak binding of the last two neutrons [4], one could conclude that such a nuclear system consists of a core with two loosely bound neutrons around it [5]. This is the concept of 'halo' nuclei which has been related to similar phenomena in atomic and molecular physics [6], showing the universality of the concept. To fully unravel the mechanisms leading to the existence of halo nuclei, many types of experiments have been devoted to the investigation of their properties. An observable that gives information on the nuclear charge deformation is the spectroscopic quadrupole moment. By comparing the quadrupole moment of 11 Li to that of 9 Li, one can investigate how the two halo neutrons modify the deformation of the core which contains the three protons. Already fifteen years ago, a first attempt to do so giving Q( 11 Li)/Q( 9 Li) = 1.14(16), suggested just a slight increase in agreement with the halo concept [7]. Unbiasedly, for a nucleus with a neutron magic number of N = 8 one would expect a minimum value of the quadrupole such as for 13 B [8]. If 11 Li has a larger quadrupole moment than 9 Li, it can not be considered as semi-magic, and the two halo neutrons have to be responsible for an expansion or polarization of the proton distribution in the core. The latter, in terms of the shell model, must be understood by an excitation of halo neutrons to the 1d orbits, and a precise value of the 11 Li quadrupole moment may provide evidence for this. The effect of a more extended charge distribution can be estimated on the basis of a recent laser spectroscopy measurement of the charge radius [9]. The increase of the charge radius for 11 Li, as well as other properties of Li isoto...

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