Emission of prompt neutrons in the thermal neutron fission of U²³⁵

Abstract

The energy distributions of the prompt neutrons emitted along the direction of motion of the light and heavy fragments have been measured in the laboratory system by the time-of-flight method. A gridded ionization chamber is used to measure the kinetic energies of the fission fragments and their directions of motion with the electric field direction of the ion chamber. A photomultiplier associated with the chamber detects the scintillations in the gas by the passage of a fragment and gives the zero-time pulse. A fast neutron detector placed along the electric field direction of the ion chamber determines the time-of-flight of the prompt neutrons in coincidence with the selected fission events. From the energy distributions in the laboratory system, the emission spectra of the prompt neutrons from the selected light and heavy fragments have been obtained. It is found that for the selected region, the light fragments emit about 30% more neutrons than the heavy fragments and also the average emission energy from the light fragments is about 32% more than that from the heavy fragments. The average nuclear temperatures of the fragments determined from the emission spectra are consistent with the average excitation energies expected from the number of neutrons and gamma rays emitted from the fragments. It is established that the emission spectrum from each fragment is a superposition of the various evaporation spectra corresponding to a distribution of nuclear temperatures. A linear distribution of temperatures up to a certain maximum temperature is found to fit the observed emission spectra. The angular correlations of the prompt neutrons of four different average energies and the light fission fragments have also been determined experimentally. The analysis shows that about 10% of the prompt neutrons are not emitted from the moving fragments. These prescission neutrons have an evaporation-like spectrum and an average energy of about 3.2 MeV. It is proposed that these neutrons are evaporated from the excited fissioning nucleus at stages between the saddle point and the scission.