Fission Cross-Section Measurements of ²⁴¹Am Between 0.1 eV and 10 keV with Lead Slowing-Down Spectrometer and at Thermal Neutron Energy

Making use of back-to-back type double fission chambers and a lead slowing-down spectrometer coupled to an electron linear accelerator, the cross section for the ²⁴¹Am(n,f) reaction has been measured relative to that for the ²³⁵U(n,f) reaction in the energy range from 0.1 eV to 10 keV. To avoid the interference between the ²⁴¹ Am and the ²³⁵U resonances, the fission cross section below 1 keV was measured relative to the ¹⁰B(n, α) reaction with a BF₃ counter, and the result obtained was normalized to the absolute value by the ²³⁵U reference data between 200 eV and 1 keV The measured result has been compared with (a) the evaluated nuclear data contained in the ENDF/B-VI and JENDL-3.2 libraries and (b) the existing experimental data, with the evaluated and measured data being broadened using the energy resolution function of the spectrometer.There is general agreement between the evaluated data and this measurement, although some discrepancies are found in the energy region where the cross-section shapes show a pronounced structure. The JENDL-3.2 data are underestimated by a factor in the range 1.2 to 2.3 between 22 and 140 eV, while the more recently measured data by Dabbs, Johnson, and Bemis and the evaluated data in ENDF/B-VI are in good agreement with the measurement within the uncertainties. In the energy range from 1 to 10 keV, the current result is 15 to 18% higher than the evaluations and the data of Dabbs, Johnson, and Bemis. Some of the earlier experimental data that were measured over part of the relevant energy region are not always in agreement with the current measurement. The fission cross section for thermal neutrons was also measured in a pure Maxwellian neutron spectrum field with double fission chambers. The derived result at 0.0253 eV is 3.15 ± 0.097 b, which is obtained relative to the reference value of 586.2 b for the ²³⁵U(n,f) reaction. The ENDF/B-VI data are in good agreement with the current measurement, while the JENDL-3.2 value is lower by 4.2%. The ratios of the earlier experimental data to the current value are distributed between 0.89 and 1.02.