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Meson exchange currents (MEC{close_quote}s) can induce an enhancement of the B(M1) strength in nuclei over the corresponding Gamow-Teller strength. Such an increase was observed in {sup 28}Si [C. L{umlt u}ttge {ital et al.}, Phys. Rev. C {bold 53}, 127 (1996)], but the experimental ratio was much larger than shell model predictions using the unified sd-shell interaction and corrections to the g factors which incorporate MEC effects. It is pointed out that, when taking the Gamow-Teller strength from a reanalysis of the {sup 28}Si(p,n) data as well as from a recent {sup 28}Si({sup 3}He,t) experiment, the derived ratio is now in good agreement with the shell model results. This finding provides strong support that the observed enhancement is due to meson exchange currents. {copyright} {ital 1997} {ital The American Physical Society}

Total cross sections for the $^{14}\mathrm{N}(p,\ensuremath{\alpha})^{11}\mathrm{C}$ reaction have been measured by using sealed gas targets and detecting annihilation radiation from the positron decay of $^{11}\mathrm{C}$. These measurements agree with the recent work of Jacobs et al. above 5.0 MeV and extend the measured cross sections to 3.8 MeV, where the cross section is a factor of 1000 smaller than the higher-energy cross sections. The results determine the low-energy normalization of the inverse $^{11}\mathrm{C}(\ensuremath{\alpha},p)^{14}\mathrm{N}$ reaction, reducing the $^{14}\mathrm{N}$ production in big bang model calculations relative to results obtained by using estimated cross sections.NUCLEAR REACTIONS $^{14}\mathrm{N}(p,\ensuremath{\alpha})$, $E=3.8\ensuremath{-}6.4$ MeV, measured $\ensuremath{\sigma}(E)$. Inferred $\ensuremath{\sigma}(E)$ and astrophysical cross section factor for inverse reaction. Sealed gas target.

Gamow-Teller transition probabilities are extracted for eight nuclei with masses between $A=13 \mathrm{and} 39$ from medium-energy ($p, n$) reactions via the distorted-wave impulse approximation, and compared with experimental $\ensuremath{\beta}$-decay and with free-nucleon transition probabilities. These comparisons indicate strongly that the renormalization of the Gamow-Teller operator needed for ($p, n$) reactions on finite nuclei is different from that needed for $\ensuremath{\beta}$ decay.

Inclusive double-differential cross sections for neutron production were measured at 337 MeV/nucleon in the reaction $\mathrm{Ne}+\mathrm{U}\ensuremath{\rightarrow}n+X$. Neutrons were detected with energies between \ensuremath{\sim} 12 and \ensuremath{\sim} 600 MeV at laboratory angles of 30\ifmmode^\circ\else\textdegree\fi{}, 45\ifmmode^\circ\else\textdegree\fi{}, 60\ifmmode^\circ\else\textdegree\fi{}, and 90\ifmmode^\circ\else\textdegree\fi{}. The neutron spectra above \ensuremath{\sim} 30 MeV are comparable with the predictions of a "firestreak" calculation. The neutron spectra differ from proton spectra reported for U and Pb.