The fission cross section of 228Th near threshold

Abstract The fission cross section and fragment angular distribution for the reaction 228 Th(n, f) have been measured over the neutron energy range 350 keV to 1200 keV at values of the neutron energy resolution in the range 20 keV to 37 keV. An absolute determination of the fission cross section at 1635 keV gave the value 370 ± 32 mb to which the data are normalised. Forty cross-section data points are available but only twenty-two of these allow a trustworthy anisotropy to be quoted. The data confirm the structure noted by Vorotnikov et al . near 450 keV and at 1.1 MeV but also reveal a pronounced resonance at 760 keV. The fragment anisotropy values are predominantly forward peaked and indicate a spin projection K = 1 2 for all resonance groups. The way in which the results reflect on our understanding of the fission potential barrier is noted.