Fission Yield by Fast Neutrons

The fission cross section of uranium was measured for neutrons produced in the following reactions: Rn+Be, $D+C$, $D+D$, $D+\mathrm{Be}$, $D+B$, $D+\mathrm{Li}$. From the fact that the mean cross section has about the same value for neutrons of the $D+D$, $D+\mathrm{Be}$, $D+\mathrm{B}$ reactions, it was concluded that the fission cross section has a value ${\sigma }_f$ which remains nearly constant between 1 and 10 Mev. For neutrons of the Rn+Be and $D+\mathrm{C}$ reactions the fission cross sections seem to be respectively, about $\frac{1}{5}{\sigma }_f$ and $\frac{1}{8}{\sigma }_f$. Finally, for neutrons of the $D+\mathrm{Li}$ reaction the mean cross section is equal to $1.4{\sigma }_f$. This fact was interpreted by N. Bohr as due to successive transformations which are possible for energies of the impinging neutrons larger than 10 Mev. A similar increase of the fission cross section was observed also in case of thorium, in very good agreement with the theoretical prediction by Bohr.