Emission of prompt gamma rays in the thermal-neutron fission of U²³⁵

Abstract

The angular distributions of prompt-fission gamma rays with respect to the direction of selected fission fragments in the thermal-neutron fission of U²³⁵ have been studied for two different gamma-energy groups, using a gridded ionization-chamber scintillation-detector assembly and time-of-flight method to eliminate fission-neutron background. From the measured angular distributions with respect to the direction of the selected light fragments, the values of the laboratory anisotropy {N(0°)-N(90°)}/N(0°) for gamma rays of energies greater than 180 and 510 keV are found to be 14.9% and 12.2%, respectively. For the ratio of the intensities of the gamma rays of energies greater than 180 keV along the direction of the selected light and heavy fragments, a value equal to (0.985±0.024) has been obtained, showing that the heavy fragments emit somewhat more gamma rays than the light fragments. These measurements definitely show the existence of a significant anisotropy of emission of the gamma rays in the emitting-fragment system, suggesting the presence of significant angular momenta of the fragments correlated with the fission axis which also lead to an enhanced emission of the gamma rays.