Neutron Total Cross Sections at 20 Mev

With the $\mathrm{T}(d, n){\mathrm{He}}^4$ reaction as a monoenergetic source of neutrons of about 20 Mev, the total cross sections of 13 elements have been measured by a transmission experiment. These cross sections vary approximately as $A^{\frac{2}{3}}$ as is to be expected from the continuum theory of nuclear reactions. The cross section for hydrogen at 19.93 Mev is 0.504±0.01 barn. This result, together with other results at lower energies, seems to require a Yukawa potential in both the singlet and triplet $n-p$ states and a singlet effective range that is lower than that obtained from $p-p$ scattering data.