Double-differential cross sections and analyzing powers $A_y$ for the $(\overrightarrow{p},n)$ reactions on ${}^3\mathrm{He}$ and ${}^4\mathrm{He}$ were measured at 200 MeV between ${\theta }_{\mathrm{lab}}=0\mathrm{}°$ and $44°.$ The neutron spectra from ${}^3\mathrm{He}(\overrightarrow{p},n)$ are dominated by the quasifree scattering peak and show no evidence for resonances in the three-proton system. The spectra from ${}^4\mathrm{He}(\overrightarrow{p},n)$ exhibit strong resonance behavior in the $p^3\mathrm{He}$ system at low relative $p^3\mathrm{He}$ energies owing to the excitation of known $L=1\mathrm{}$ resonances in ${}^4\mathrm{Li},$ but there is no distinct quasifree peak in the measured spectra. The experimental $A_y$ for ${}^3\mathrm{He}(\overrightarrow{p},n),$ averaged over the experimental range of neutron energies, are similar to those for free nucleon-nucleon $\mathrm{}\left(\mathrm{NN}\right)\mathrm{}$ scattering, whereas for ${}^4\mathrm{He}(\overrightarrow{p},n)$ the $A_y$ are generally larger than the free values. The cross sections at far forward angles for both ${}^3\mathrm{He}$ and ${}^4\mathrm{He}$ appear to be suppressed relative to the free NN cross sections by Pauli blocking. At most angles, the shapes of the cross section spectra from both ${}^3\mathrm{He}$ and ${}^4\mathrm{He}$ are reproduced by distorted-wave impulse approximation (DWIA) calculations using a quasifree scattering model. Specifically for ${}^4\mathrm{He}(\overrightarrow{p},n),$ the model requires the use of an optical potential which has a strong $L=1\mathrm{}$ potential resonance corresponding to the low-lying $L=1\mathrm{}$ states in ${}^4\mathrm{Li}.$

• Received 31 December 1997

DOI:https://doi.org/10.1103/PhysRevC.58.645