On the slowing down of neutrons in various substances containing hydrogen

Abstract Fermi and his collaborators have recently reported that a very large increase in the yield of the radioactivity induced in certain substances by neutron bombardment can be obtained, when the energy of the neutrons has been reduced by elastic collisions with hydrogen nuclei. In order to determine as far as possible the nature of the processes involved, we have performed a series of experiments, using substances containing various proportions of other elements (which absorb or scatter the neutrons) in addition to hydrogen. At the present state of knowledge it is impossible to give an exact account of the phenomena, since, for example, the way in which the cross-sections for scattering and absorption of neutrons by various elements vary with the velocity of the neutrons is not known, but we believe that these experiments lead to a description which is consistent with all the known facts, as well as being highly plausible, even though we have not been able to exclude the possibility of alternative explanations.