A precision $n-\alpha$ scattering experiment has been carried out in the energy interval 2-3 Mev. The angular distribution of neutrons elastically scattered by alpha particles has been measured at angles smaller than those considered in similar experiments. The phase-shift analysis of the data has led to the following results: (a) No $D$ waves are detectable. (b) The phase shifts for the interaction in the $L=0\mathrm{}$ and $L=1\mathrm{}$, $J=\frac{3}{2}$ state agree very well with the values quoted in the literature. (c) The phase shift arising from the interaction in the $L=1\mathrm{}$, $J=\frac{1}{2}$ state turns out to be much smaller than expected; it has been found ${{\delta }_{\frac{1}{2}}}^1=4\mathrm{}°$ at 2.37 Mev and ${{\delta }_{\frac{1}{2}}}^1=8\mathrm{}°$ at 2.87 Mev, instead of ∼20° in this energy interval. It is emphasized that such an unusual experimental result should be carefully considered in theoretical investigations concerning the spin-orbit potential, and in experimental researches using helium as a polarization analyzer.

• Received 17 July 1961

DOI:https://doi.org/10.1103/PhysRev.125.318