Measurement of the Spin and Magnetic Moment ofMg31: Evidence for a Strongly Deformed Intruder Ground State

Abstract

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...

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“…The discovery of the "island of inversion" around 32 Mg and the vanishing of the N = 20 shell closure has profoundly affected our understanding of nuclear structure [1][2][3][4][5][6][7]. Dramatic changes in shell structure occur in this region, where nucleons occupy intruder states across the shell at low excitation energy and in the ground state.…”

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