Precision measurement of 11Li moments: influence of halo neutrons on the 9Li core.

The electric quadrupole moment and the magnetic moment of the 11Li halo nucleus have been measured with more than an order of magnitude higher precision than before, |Q| = 33.3(5) mb and mu = +3.6712(3)muN, revealing a 8.8(1.5)% increase of the quadrupole moment relative to that of 9Li. 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.

Decay of 1+ states as a new probe of the structure of 0+ shape isomers.

The nuclides 98Mo and 100Mo have been studied in photon-scattering experiments by using bremsstrahlung produced from electron beams with kinetic energies from 3.2 to 3.8 MeV. Six electromagnetic dipole transitions in 98Mo and 19 in 100Mo were observed for the first time in the energy range from 2 to 4 MeV. A specific feature in the two nuclides is the de-excitation of one state with spin J = 1 to the 0+ ground state as well as to the first excited 0+ state, which cannot be explained in standard models. We present a model that allows us to deduce the mixing coefficients for the two 0+ shape-isomeric states from the experimental ratio of the transition strengths from the J = 1 state to the 0+ ground state and to the 0+ excited state.

Measurement of the spin and magnetic moment of 31Mg: evidence for a strongly deformed intruder ground state.

Unambiguous values of the spin and magnetic moment of 31Mg are obtained by combining the results of a hyperfine-structure measurement and a beta-NMR measurement, both performed with an optically polarized ion beam. With a measured nuclear g factor and spin I=1/2, the magnetic moment mu(31Mg)=-0.88355(15)mu(N) is deduced. A revised level scheme of 31Mg (Z=12, N=19) with ground state spin/parity I(pi)=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(pi)=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.