Triple-differential cross sections of the (π+,pp) reaction on lithium isotopes

The (${\pi }^{+}$,pp) reactions on ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ and ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ have been studied at $T_{\pi }$=59.4 MeV with high resolution. The first triple-differential cross sections for these reactions are presented. The data are fitted to a T matrix and compared to the ${\pi }^{+}$d→pp reaction. A model in which the pion is absorbed on a ${}_{}{}^3{}_{}{}^{}\mathrm{S}_1^{}{}_{}{}^{}$ (pn) pair in the lithium nucleus describes many features of the data very well. An extrapolation of our data into unmeasured regions of phase space suggests that about 60% of the pion absorption cross section on ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ at 59.4 MeV goes into the (${\pi }^{+}$,pp) channel. One surprising feature of the data is that the ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$(${\pi }^{+}$,pp${)}^4$He ($2^{\mathrm{-}}$) transition at 22.1 MeV excitation is strongly populated, similar to what is observed in the ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$(d,α${)\mathrm{}}^4$He reaction. This transition involves removing one nucleon from the 1p shell and one from the 1s shell.