Measurements of the subthreshold neutron induced fission cross-sections of 234U, 237Np and 243Am

The results of measurements of the fission cross-sections and resonance parameters σ0Γf and Γf of 234U and 237Np together with the preliminary results of 243Am fission cross-section measurements carried out in the framework of the program of studying subthreshold fission at the IBR-30 pulsed neutron source in Dubna are presented. The measurements were conducted by the time-of-flight technique. Fission events in 234U and 237Np were detected by two multisectional ionisation chambers containing 234U (about 100 mg) and 237Np (about 1.5 g) highly purified from isotopes with a large fission cross-section, such as 235U and 239Pu. Each chamber also contains a 235U target for neutron flux measurements. 243Am cross-section measurements are conducted with a back-to-back ionisation chamber containing about 2 mg of 243Am and 1 mg of 235U. The fission width is calculated in the region where the TOF resolution is sufficient to resolve most resonances at the energy up to 200 eV for 234U and below 50 eV for 237Np. The cro...