Subthreshold Neutron-Induced Fission inU238

In an attempt to observe subthreshold fission in ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$, a sample with very low ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ content was exposed to the intense neutron beam of the Physics-8 nuclear explosion. A high-sensitivity oscilloscope setting allowed individual fission-fragment pulses from a semiconductor detector to be counted. In the region from 37 to 327 eV, the average fission width for eight resonances was determined to be (0.03 ± 0.05) × ${10}^{-6\mathrm{}}$ eV. In the keV region, the average fission cross section was measured to be 61 ± 24 μb from ${10}^4$ to 3 × ${10}^4$ eV and 36 ± 14 μb from 3 × ${10}^4$ to ${10}^5$ eV.