Improved Value for the Gamow-Teller Strength of the

A record number of ^{100}Sn nuclei was detected and new isotopic species toward the proton dripline were discovered at the RIKEN Nishina Center. Decay spectroscopy was performed with the high-efficiency detector arrays WAS3ABi and EURICA. Both the half-life and the ß-decay end point energy of ^{100}Sn were measured more precisely than the literature values. The value and the uncertainty of the resulting strength for the pure 0^{+}→1^{+} Gamow-Teller decay was improved to B_{GT}=4.4_{-0.7}^{+0.9}. A discrimination between different model calculations was possible for the first time, and the level scheme of ^{100}In is investigated further.

New Isotopes and Proton Emitters-Crossing the Drip Line in the Vicinity of

Several new isotopes, ^{96}In, ^{94}Cd, ^{92}Ag, and ^{90}Pd, have been identified at the RIKEN Nishina Center. The study of proton drip-line nuclei in the vicinity of ^{100}Sn led to the discovery of new proton emitters ^{93}Ag and ^{89}Rh with half-lives in the submicrosecond range. The systematics of the half-lives of odd-Z nuclei with T_{z}=-1/2 toward ^{99}Sn shows a stabilizing effect of the Z=50 shell closure. Production cross sections for nuclei in the vicinity of ^{100}Sn measured at different energies and target thicknesses were compared to the cross sections calculated by epax taking into account contributions of secondary reactions in the primary target.

Search for superscreening effects in a superconductor.

The decay of (19)O(ß(-)) and (19)Ne(ß(+)) implanted in niobium in its superconducting and metallic phases was measured using purified radioactive beams produced by the SPIRAL GANIL facility. Half-lives and branching ratios measured in the two phases are consistent within a 1σ error bar. This measurement casts strong doubts on the predicted strong electron screening in a superconductor, the so-called superscreening. The measured difference in screening potential energy is 110(90) eV for (19)Ne and 400(320) eV for (19)O. Precise determinations of the half-lives were obtained for (19)O, 26.476(9) s, and for (19)Ne, 17.254(5) s.

Discovery of doubly magic 48Ni

In an experiment at the SISSI/LISE3 facility of GANIL, we used the projectile fragmentation of a primary 58Ni26+ beam at 74.5 MeV/nucleon with an average current of 3 &mgr;A on a natural nickel target to produce very neutron-deficient isotopes. In a 10-day experiment, 287 42Cr isotopes, 53 45Fe isotopes, 106 49Ni isotopes, and 4 48Ni isotopes were unambiguously identified. The doubly magic nucleus 48Ni, observed for the first time, is the most proton-rich isotope ever identified with an isospin projection T(z) = -4. It is probably the last doubly magic nucleus with "classical" shell closures accessible for present-day facilities. Its observation allows us to deduce a lower limit for the half-life of 48Ni of 0.5 &mgr;s.

Shape coexistence and the N = 28 shell closure far from stability

The masses of 31 neutron-rich nuclei in the range A = 29-47 have been measured. The precision of 19 masses has been significantly improved and 12 masses were measured for the first time. The neutron-rich Cl, S, and P isotopes are seen to exhibit a change in shell structure around N = 28. Comparison with shell model and relativistic mean field calculations demonstrate that the observed effects arise from deformed prolate ground state configurations associated with shape coexistence. Evidence for shape coexistence is provided by the observation of an isomer in 43S.

Magnetic moment of the fragmentation-aligned 61Fe (9/2(+)) isomer.

We report on the g factor measurement of an isomer in the neutron-rich (61)(26)Fe (E(*)=861 keV and T(1/2)=239(5) ns). The isomer was produced and spin aligned via a projectile-fragmentation reaction at intermediate energy, the time dependent perturbed angular distribution method being used for the measurement of the g factor. For the first time, due to significant improvements of the experimental technique, an appreciable residual alignment of the nuclear spin ensemble has been observed, allowing a precise determination of its g factor, including the sign: g=-0.229(2). In this way we open the possibility to study moments of very neutron-rich short-lived isomers, not accessible via other production and spin-orientation methods.

New shape isomer in the self-conjugate nucleus 72Kr.

A new isomeric 0(+) state was identified as the first excited state in the self-conjugate (N=Z) nucleus 72Kr. By combining for the first time conversion-electron and gamma-ray spectroscopy with the production of metastable states in high-energy fragmentation, the electric-monopole decay of the new isomer to the ground state was established. The new 0(+) state is understood as the band head of the known prolate rotational structure, which strongly supports the interpretation that 72Kr is one of the rare nuclei having an oblate-deformed ground state. This observation gives in fact the first evidence for a shape isomer in a N=Z nucleus.

Two-proton radioactivity of 45Fe.

In an experiment at the SISSI-LISE3 facility of GANIL, the decay of the proton drip line nucleus 45Fe has been studied. Fragment-implantation events have been correlated with radioactive decay events in a 16x16 pixel silicon-strip detector. The decay-energy spectrum of 45Fe implants shows a distinct peak at (1.14+/-0.04) MeV with a half-life of T(1/2)=(4.7(+3.4)(-1.4)) ms. None of the events in this peak is in coincidence with beta particles. For a longer correlation interval, daughter decays of the two-proton daughter 43Cr can be observed after 45Fe implantation. The decay energy for 45Fe agrees nicely with several theoretical predictions for two-proton radioactivity.

(68)(28)Ni(40): Magicity versus superfluidity.

The neutron-rich (66,68)Ni have been produced at GANIL via interactions of a 65.9A MeV 70Zn beam with a 58Ni target. Their reduced transition probability B(E2;0(+)(1)-->2+) has been measured for the first time by Coulomb excitation in a (208)Pb target at intermediate energy. The B(E2) value for (68)Ni(40) is unexpectedly small. An analysis in terms of large scale shell model calculations stresses the importance of proton core excitations to reproduce the B(E2) values and indicates the erosion of the N = 40 harmonic-oscillator subshell by neutron-pair scattering.

Halo structure of (14)Be.

The two-neutron halo nucleus (14)Be has been investigated in a kinematically complete measurement of the fragments ((12)Be and neutrons) produced in dissociation at 35 MeV/nucleon on C and Pb targets. Two-neutron removal cross sections, neutron angular distributions, and invariant mass spectra were measured, and the contributions from electromagnetic dissociation (EMD) were deduced. Comparison with three-body model calculations suggests that the halo wave function contains a large nu(2s(1/2))(2) admixture. The EMD invariant mass spectrum exhibited enhanced strength near threshold consistent with a nonresonant soft-dipole excitation.