The experiments of Cook, McMillan, Peterson, and Sewell on the cross sections of nuclei for neutrons of about 90 Mev indicate that the nuclei are partially transparent to high energy neutrons. It is shown that the results can be explained quite satisfactorily using a nuclear radius $R=1.37\mathrm{}{\mathrm{A}}^{\frac{1}{2}}\times {10}^{-13\mathrm{}}$ ×${10}^{-13\mathrm{}}$ cm, a potential energy for the neutron in the nucleus of 31 Mev, and a mean free path for the neutron in nuclear matter of 4.5×${10}^{-13\mathrm{}}$ cm. This mean free path agrees with that estimated from the high energy $n-p$ cross section, but the results are not sensitive to the choice of mean free path.

• Received 19 January 1949

DOI:https://doi.org/10.1103/PhysRev.75.1352