Differential Cross Sections of theLi6(N14, α)O16andLi7(N14, α)O17Reactions

Abstract

The angular distributions of $\ensuremath{\alpha}$ particles from the bombardment of ${\mathrm{Li}}^{6}$ and ${\mathrm{Li}}^{7}$ by 27.6-MeV incident nitrogen ions were measured from 0\ifmmode^\circ\else\textdegree\fi{} to 135\ifmmode^\circ\else\textdegree\fi{} c.m. Both $\ensuremath{\alpha}$ and recoil particles were detected in coincidence by silicon surface-barrier diodes. In the ${\mathrm{Li}}^{6}$ measurements the ${\mathrm{O}}^{16}$ recoil was left in its ground state; ${\mathrm{Li}}^{7}$ measurements did not resolve reactions leading to the ground state and to the 870-keV excited state of ${\mathrm{O}}^{17}$. The ${\mathrm{Li}}^{6}$ distribution exhibits a strong forward peak of approximately 75 \ensuremath{\mu}b/sr at 0\ifmmode^\circ\else\textdegree\fi{} c.m., falling off, with some structure, to about 10 \ensuremath{\mu}b/sr at 90\ifmmode^\circ\else\textdegree\fi{} c.m. at the backward angles, the cross section rises to a broad peak of 18 \ensuremath{\mu}b/sr at 112.5\ifmmode^\circ\else\textdegree\fi{} c.m., falling to 9 \ensuremath{\mu}b/sr at 135\ifmmode^\circ\else\textdegree\fi{} c.m. The ${\mathrm{Li}}^{7}$ distribution is of similar shape at forward angles, falling from 140 to 30 \ensuremath{\mu}b/sr, but at backward angles rises to 84 \ensuremath{\mu}b/sr at 135\ifmmode^\circ\else\textdegree\fi{} c.m. without clearly defined structure or symmetry with the cross section at forward angles. Calculation shows that the reactions do not proceed via a compound nucleus. The ${\mathrm{Li}}^{6}$ reaction is treated as a deuteron transfer reaction using a modified tunneling theory and a distorted-wave calculation.