Radiative Neutron Capture on Lithium-7

The radiative neutron capture on lithium-7 is calculated model independently using a low energy halo effective field theory. The cross section is expressed in terms of scattering parameters directly related to the $S$-matrix element. The cross section depends on the poorly known $p$-wave effective range parameter $r_1$. This constitutes the leading order uncertainty in traditional model calculations. It is explicitly demonstrated by comparing with potential model calculations. A single parameter fit describes the low energy data extremely well and yields $r_1\approx -1.47$ fm$^{-1}$.