SOME THEORETICAL INVESTIGATION FOR THE STRUCTURE PHENOMENON OF FISSION NEUTRON SPECTRA

Abstract

An attempt is made to explain the observed structure phenomenon in the 2-3 Mev region of the spectra of the neutrons from the fission of U235 or Pu239 by the thermal neutron. Three possible causes for the effect are proposed and investigated. Firstly, . the effect of distribution of the nuclear temperature is studied. It is found that a continuous distribution of the nuclear temperature is unable to cause formation of any structure in the spectra, while two distinct nuclear temperatures would bring about such an effect; however, the value of the ratio of the two temperatures, i.e. T1/T2 ≥ 3 seems rather unreasonable. Next, in the evaporation formula of the energy spectra, the absorption cross section of the optical model is employed. In the case of fission, giant resonances of the absorption cross-section occur at various energies both for the light and for the heavy fragment. The calculation suggests that the structure phenomenon in the 2-3 Mev region of the spectra of the fission neutrons could very possibly be caused by the d-ware giant resonance in the absorption cross-section of the heavy fragments. By using the optical model absorption cross-section and taking a certain temperature distribution, the calculated spectra of the fission neutrons agree well with the experiment in magnitude as well as in position of the structure of the neutron spectra. Thirdly, the fact that the neutrons emitted by the fragments may be anisotopic, would also cause a structure phenomenon in fission neutron spectra; the effect is, however, rather small. Finally, some experiments for further clarification of the problem are proposed.