Fission Neutron Spectrum ofU235

A high pressure ionization chamber has been used to measure the energy distribution of proton recoils from the neutrons which are emitted during the thermal neutron fission of ${\mathrm{U}}^{235}$. The energy spectrum of the fission neutrons is obtained by applying suitable wall corrections and by using 2.5-Mev $d-d$ neutrons for an energy calibration. Measurements were made in the range from 1 to 4 Mev, and within the experimental errors, the resulting fission spectrum follows Watt's [B. E. Watt, Phys. Rev. 87, 1037 (1952)] semi-empirical relation, $N\left(E\right)=e^{-E}sinh\surd \mathrm{}\left(2E\right)\mathrm{}$.