High-resolution measurement and analysis of the fission cross-section of 239Pu

The fission cross-section of 239Pu was measured below 35 keV, using the 45-MeV linear accelerator as pulsed neutron source. The nominal resolution was approximately 1 ns/m and the 23Pu deposits were brought to the temperature of liquid nitrogen in a gaseous scintillator. This cross-section was analysed together with the total cross-section by the mean-squares method with the single-level Breit-Wigner formalism. The resolution of this measurement enabled us to extend the detailed resonance analysis to 450 eV. In a large number of cases, the analysis of shape leads to the determination of the resonance spin. The results confirm and refine the existence of two resonance families, having very different mean values of fission width corresponding to two spin states J = 0+ and J = 1+. The study of the autocorrelation coefficient of the fission cross-section up to 2 keV energy shows that this coefficient is incompatible with a statistical model in which the properties of the parameters are independent of energy. The present study shows that there exists an intermediate structure whose mean spacing is about 450 eV. It is interpreted as being due to fission below the threshold of the 1+ channel for which the coupling with more intense fission at some energies corresponds to the 1+ intermediate states situated in the potential well of the two-hump fission barrier calculated by Strutinsky. This interpretation is confirmed by the fact that the narrow resonances (hence probably of spin J = 1+) have a much smaller fission width between 450 eV and 650 eV (about 10 MeV) than below 400 eV (about 40 MeV). This effect thus explains the diminution of the fission cross-section towards 500 eV. It is discussed particularly from the point of view of the properties of the 1+ transition state and the (n, γf) reaction. The results obtained with other fissile nuclei (235U and 241Pu) are also presented. (author)