Investigation of x-ray emission in fission of heavy nuclei by thermal neutrons

At present, essentially preliminary stage of the experiments, more or less complete and consistent data on the properties of characteristic radiation have been obtained only for spontaneous fission (252Cf), where there is no problem of a background from fission-inducing particles (quanta). The paper describes a series of experiments to measure the X-ray yields and energy distribution in fission induced by thermal neutrons. With a view to gaining a better understanding both of questions of the emission of characteristic radiation itself and of the properties of its charge distribution, following on earlier investigations into the fission of 235U and 239Pu, the authors studied X-rays from fragments of 233U. They measured the spectra and intensity of characteristic K-radiation relative to 235U and found that the energy of the quanta emitted by light fragments is less in the case of 233U than for 235U, while for heavy fragments it is greater. They attribute this difference in energy to a difference in charge, and compare this latter difference with current hypotheses. Certain conclusions are drawn regarding the influence of shell effects on charge distribution. The radiation intensity is found to be identical (to within ± 10%) for both nuclei. The unique possibility of measuring the prompt charge of fragments simultaneously with other fission parameters that is offered by the characteristic rays method was used to determine fragment charges in 235U fission events involving the ejection of long-range alpha particles (subsequently called ternary fission for short). For this purpose the energy spectra and yields of K-radiation in binary and ternary fission were measured under identical conditions. It was found that the radiation spectra for binary and ternary fission are very similar: they both have two peaks that are almost identical in respect of position and magnitude, which doubtless reflects the close similarity between the charge properties of the fragments formed by this process. Some differences, however, were observed in the spectra, in that the right-hand slopes of the radiation peaks for the light and heavy fragments from ternary fission are shifted in the direction of lower energies. It was found that the magnitude of the shifts corresponds to the ejection of alpha particles. The ratio of radiation yields (ternary to binary) was found to be close to unity: 0.9 ± 0.2. The fact that there are no appreciable differences in yield obviously points to there being no major differences in the spins of binary and ternary fission fragments. In view of the inconsistent data in the literature on K-radiation yields in the fission of 235U (the results obtained by individual authors differ by a factor of 2 to 3), the intensity of X-radiation from 235U fragments was measured relative to that obtained with fragments from the spontaneous fission of 252Cf, the yield for which is known most reliably. The comparison was made under conditions which were as identical as possible. (author)