The angular correlation of prompt neutrons and fission fragments in thermal neutron fission of U²³⁵

Abstract

The angular distributions of the prompt neutrons of specified mean energies emitted in the thermal neutron fission of U²³⁵ have been measured with respect to the direction of motion of light fragments. A combination of a gridded ionisation chamber and a gas scintillation counter system using Xenon gas was used to measure the energy of the fission fragments as well as their direction of motion relative to the electric field direction of the ionisation chamber. A fast neutron detector was placed along the electric field direction at a distance of one meter to measure the energy of the prompt neutrons by time of flight technique. The distributions obtained are compared with the distributions computed on the assumption of isotropic emission of neutrons with a Maxwellian emission spectrum. It is shown that the laboratory anisotropy N(0°)/N(90°) is an increasing function of the neutron energy in the laboratory system. The laboratory angular distributions are transformed to obtain the centre-of-mass angular distributions for two groups of prompt neutron emission energies. Anisotropy of the form (a+b cos²θ) is indicated for prompt neutrons with emission energies greater than 1 MeV. Anisotropy of emission is discussed in terms of the possibility of the fragments acquiring a large angular momentum at the instant of scission.