We have measured the cross section for neutron radiative capture by Li at thermal neutron energy as 45.4+3.0 mb. We have compared this value, and the available fast-neutron-capture cross-section data on Li, with directand valence-capture theory. The mechanism of direct capture involving simple neutron-orbital transitions within a potential well can adequately account for the magnitude of the thermal-neutron-capture cross sections and the shape of the fast-neutron-capture cross sections. We recommend that the capture cross sections from the theoretical calculation, instead of those from a recent measurement, be used for nuclear astrophysics calculations. If the fast-neutron-capture data are normalized at 25 keV to the theoretical value, the magnetic-dipole (M 1) radiation width of the 255-keV p-wave resonance can be deduced. It is in agreement with the value calculated from a valence theory for M1 capture, thus lending support for this neutron-capture mechanism as an important one, at least for light nuclei. We also find some evidence from our analysis of the total cross-section data for a possible energy dependence of the potential required to describe the almost pure single-particle s-wave resonances underlying the Li cross section.

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The level scheme of " Sn has been studied by combining the results of '"Sn(n, y)" Sn and " Sn(n, n'y}" Sn experiments. Both experiments were performed using isotopically enriched samples and Ge y-ray detectors. Based on the thresholds of y-ray excitation functions measured for the " Sn(n, n'y) reaction and the precise y-ray energies from the capture reaction, 100 levels were observed below 4.3 MeV excitation energy. Approximately half of these were not known previously. Forty-eight of these levels have unique or tentative spin-parity assignments, and for ten more the spin has been restricted to a single value. The spin-parity for most other levels below 4.3 MeV excitation has been restricted to a few values. These spin-parity assignments and limitations were derived mainly from (n, n y) angular distribution measurements, together with additional information obtained from the cross section magnitudes in both experiments. Above 4.3 MeV excitation energy, 55 additional levels are proposed, based only on the '"Sn(n, y) results. No J information is available for these higher-lying levels beyond the fact that they most probably all have J~4. The level scheme below 4.3 MeV from the current work, together with known high-spin levels up to 5.4 MeV seen in other experiments, are compared to the combined predictions of the two-broken-pair model, the interacting boson model, and the deformed collective model. In addition, several states have been phenomenologically identified as proton 1p-1h and collective quadrupole-octupole two-phonon excitations. It is concluded from the good agreement between experiment and these models that all levels in " Sn with J~6 up to an excitation of 4.0 MeV and J 3 up to 4.3 MeV may have been experimentally identified. The nearest-neighbor spacing distribution is intermediate between that of a Gaussian orthogonal ensemble and that of a Poisson distribution, with a slight preference for the former. The neutron separation energy was determined to be 9563.47+0.11 keV.

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