Subthreshold Fission Induced by Neutrons onU238

Subthreshold fission is observed in ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}(n, f)$ at 0.720, 1.210, 2.5, 7.5, 11, 15, 27, and 35 keV. Fission widths are obtained for the resonances at 720 and 1210 eV. The average fission cross section is 87 ± 26 μb for $10<~E_n<~30$ keV and 40 ± 12 μb for $30<~E_n<~100$ keV. The second potential minimum is deduced to lie ≈ 2.2 MeV above the first minimum.