We have performed precision laser spectroscopy on individual 6He (t(1/2)=0.8 s) atoms confined and cooled in a magneto-optical trap, and measured the isotope shift between 6He and 4He to be 43 194.772+/-0.056 MHz for the 2(3)S1-3(3)P2 transition. Based on this measurement and atomic theory, the nuclear charge radius of 6He is determined for the first time in a method independent of nuclear models to be 2.054+/-0.014 fm. The result is compared with the values predicted by a number of nuclear structure calculations and tests their ability to characterize this loosely bound halo nucleus.

Asymptotic normalization coefficients ͑ANCs͒ for 8 Li→ 7 Liϩn have been extracted from the neutron transfer reaction 13 C(7 Li, 8 Li) 12 C at 63 MeV. These are related to the ANCs in 8 B→ 7 Beϩp using charge symmetry. We extract ANCs for 8 B which are in very good agreement with those inferred from proton transfer and breakup experiments. We have also separated the contributions from the p 1/2 and p 3/2 components in the transfer. We find the astrophysical factor for the 7 Be(p,␥) 8 B reaction to be S 17 (0)ϭ17.6Ϯ1.7 eV b. This is the first time that the rate of a direct capture reaction of astrophysical interest has been determined through a measurement of the ANCs in the mirror system.

The ground and first excited states in 15 F were studied in resonant elastic scattering using the thick ͑CH 4 ͒ gas target method in inverse kinematics with a separated 14 O beam. An analysis of the excitation functions of the elastic scattering was carried out with the potential model. The quantum numbers 1 / 2 + (ground state) and 5/2 + (first excited state) were assigned to the lowest two states in 15 F. Also, the widths and the proton decay energies of the unbound levels were obtained. The analysis of the data indicates that a large diffuseness is needed in the Woods-Saxon potential in order to describe single-particle features in drip-line nuclei. Over the past decade it has become clear that drip-line nuclei demonstrate a number of phenomena which are not found in nuclei close to the line of stability. One such feature is the change in magic numbers, which are generated by a conventional Woods-Saxon potential with parameters fitted for stable nuclei [1][2][3]. One example is the intruder singleparticle 2s 1/2 state, which appears to be the ground state in 11 Be [4] and 11 N [5] instead of the 1p 1/2 state. As another example, it has been predicted [1-3] that the diffuseness of nuclear densities for intermediate mass nuclei increases dramatically when approaching the neutron drip line. These phenomena indicate that the parameters of the shell model potential can be unusual for nuclei at the borders of nuclear stability. Figure 1 shows how, for a light nucleus, the energies of the shell model levels in a Woods-Saxon potential depend on the ratio of the diffuseness to radius parameters. It can be seen in Fig. 1 that the 1p 1/2 and 2s 1/2 levels approach each other as the ratio increases. One way to explain the phenomena observed near the drip line is that the singleparticle potential changes its shape giving way to new shell structure. This effect can be tested by an analysis of the nucleon widths of the single-particle states in drip-line nuclei, which are mainly dependent upon the geometrical parameters of the well.15 F is a good system to check the considerations above. The lowest states in 15 F are unstable to proton decay and should have dominantly single-particle structure. The theoretical predictions for the single-particle spectroscopic factors are 0. O on hydrogen with the thick target inverse kinematics method [5,10,11]. An approach similar to Ref.[9] was used in the present experiment. However, there are important differences in the details of the two measurements. In the present work, a gas target, CH 4 , was used instead of a solid CH 2 target. The gas target results in a drastic decrease of the background (see below). Also, only the excitation function at 180°͑c.m.͒ in arbitrary units was measured in Ref. [9], while in the experiment reported here measurements were made at several angles. As a result, we have a better determination of the positions and the widths of the levels which allows us to make conclusions about the parameters of the interaction potential between 14 O and protons. The expe...

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We report on a study of the structure of the unbound nucleus 7 He utilizing the proton-removal reaction 2 H( 8 Li, 3 He) 7 He. Combining the present results with those of our prior measurements of the neutron-adding reaction 2 H( 6 He, p) 7 He, a consistent picture emerges for the low-lying excitations in 7 He. Specifically, the negative-parity sequence of resonances, in order of excitation energies, is consistent with 3/2 − , 1/2 − , and 5/2 − . The stable-beam reactions 2 H( 7 Li, t) 6 Li and 2 H( 7 Li, 3 He) 6 He were also measured. The results are compared with the predictions of nuclear structure models, including those of ab initio quantum Monte Carlo calculations.

The half-life, 3.8755(12) s, and superallowed branching ratio, 0.5315(12), for 22 Mg β-decay have been measured with high precision. The latter depended on γ-ray intensities being measured with an HPGe detector calibrated for relative efficiencies to an unprecedented 0.15%. Previous precise measurements of 0 + → 0 + transitions have been restricted to the nine that populate stable daughter nuclei. No more such cases exist, and any improvement in a critical CKM unitarity test must depend on precise measurements of more exotic nuclei. With this branching-ratio measurement, we show those to be possible for Tz = −1 parents. We obtain a corrected Ft-value of 3071(9) s, in good agreement with expectations.PACS numbers: 23.40. Bw, 23.40.Hc, 27.30+t, 29.40.Wk Superallowed 0 + → 0 + nuclear β-decay is a sensitive probe of the vector part of the weak interaction. Measurement of the f t-value for such a transition yields a direct determination of the vector coupling constant, G V , provided that small radiative corrections are properly accounted for. To date, the f t-values for nine 0Mn and 54 Co -have been measured with ∼ 0.1% precision or better, and these results yield fully consistent values for G V . With G V thus determined, it is possible to establish a very precise value for V ud , the up-down element of the Cabibbo-Kobayashi-Maskawa (CKM) quark-mixing matrix. Not only is this the most precise determination of V ud , it is the most precise result for any element in the CKM matrix. It also leads to the most demanding test available of CKM unitarity, a fundamental tenet of the minimal standard model. Strikingly, the test fails by more than two standard deviations [1, 2]: viz. V 2 ud + V 2 us + V 2 ub = 0.9968 ± 0.0014. Since recent results suggest that the value of V us may need to be revised [3], it is even more important that the value of V ud be known as precisely as possible.Since the uncertainty in V ud is dominated by the uncertainty in the small (∼ 1%) calculated correction terms that are applied to the data, any improvement in the statistical definition of the unitarity test must come from improvements in those terms, two of which depend on structure details for the nuclei involved. It has recently been argued [1] that the best way to validate the structuredependent correction terms is to measure additional superallowed transitions specifically selected to cover a wider range of correction-term magnitudes, demonstrating whether these transitions also produce consistent G V values. To this end, we focus on the even-even T z = −1 nuclei with 18 ≤ A ≤ 42, selected because their decays are between nuclei described by the same nuclear-model spaces as those used for some of the nine currently well known cases. Since the calculated correction terms are obtained in a completely consistent way, the tests of these terms will have a direct impact on current results for V ud . The T z = −1 nuclei, however, are all farther from stability than the currently known cases, have unstable daughters and exhibit multiple decay...