Cross sections and analyzing powers for the(p→,n)reaction on3Heand4Heat 200 MeV

Abstract

Double-differential cross sections and analyzing powers ${A}_{y}$ for the $(\stackrel{\ensuremath{\rightarrow}}{p},n)$ reactions on ${}^{3}\mathrm{He}$ and ${}^{4}\mathrm{He}$ were measured at 200 MeV between ${\ensuremath{\theta}}_{\mathrm{lab}}=0\ifmmode^\circ\else\textdegree\fi{}$ and $44\ifmmode^\circ\else\textdegree\fi{}.$ The neutron spectra from ${}^{3}\mathrm{He}(\stackrel{\ensuremath{\rightarrow}}{p},n)$ are dominated by the quasifree scattering peak and show no evidence for resonances in the three-proton system. The spectra from ${}^{4}\mathrm{He}(\stackrel{\ensuremath{\rightarrow}}{p},n)$ 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 $L=1$ resonances in ${}^{4}\mathrm{Li},$ but there is no distinct quasifree peak in the measured spectra. The experimental ${A}_{y}$ for ${}^{3}\mathrm{He}(\stackrel{\ensuremath{\rightarrow}}{p},n),$ averaged over the experimental range of neutron energies, are similar to those for free nucleon-nucleon $(\mathrm{NN})$ scattering, whereas for ${}^{4}\mathrm{He}(\stackrel{\ensuremath{\rightarrow}}{p},n)$ 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 ${}^{4}\mathrm{He}(\stackrel{\ensuremath{\rightarrow}}{p},n),$ the model requires the use of an optical potential which has a strong $L=1$ potential resonance corresponding to the low-lying $L=1$ states in ${}^{4}\mathrm{Li}.$