Energy Distribution of Mass-97 Fission Fragments from Thermal-Neutron Fission of Uranium-235

The energy distribution of mass-97 fragments from thermal neutron induced fission of ${\mathrm{U}}^{235}$ was measured with a high-resolution magnetic spectrograph. The fragments originate in a thin plating of ${\mathrm{U}}^{235}$ near the center of the Oak Ridge National Laboratory graphite reactor and travel 16 feet to a wedge magnet which analyzes and focuses them at the focal plane 6 feet beyond. There they are caught in an aluminum foil which, after the irradiation, is cut into strips each of which is radiochemically analyzed for ${\mathrm{Zr}}^{97}$. The $H\rho$ distribution is complicated by the large energy width which leads to overlapping of momentum distributions from successive charges. An analysis of the shapes of the $H\rho$ distributions obtained with different relative charge populations determines that the width of the energy distributions is (11.4±0.8)% corrected for broadening due to prompt neutron emission. This result is in agreement with measurements of the distributions of the number of neutrons per fission but is in sharp disagreement with the predictions of Fong's theory of the fission process. The most probable energy is 174.7±2 Mev for the mass-97 fission mode, and about 164.5±3 Mev for the mass-91 mode.