FISSION CROSS-SECTION MEASUREMENTS OF AM-241 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 slo wing-down spectrometer coupled to an electron linear accelerator, the cross section for the Am-241(n,f) reaction has been measured relative to that for the U-235(n,f) reaction in the energy range from 0.1 eV to 10 keV. To avoid the interference between the Am-241 and the U-235 re sonances, the fission cross section below 1 keV was measured relative to the B-10(n,alpha) reaction with a BF3 counter, and the result obtai ned was normalized to the absolute value by the U-235 reference data b etween 200 eV and I 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 a nd 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 th e energy region where the cross-section shapes show a pronounced struc ture. 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 th e energy range from 1 to 10 keV, the current result is 15 to 18% highe r 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 me asurement. The fission cross section for thermal neutrons was also mea sured 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 i s obtained relative to the reference value of 586.2 b for the U-235(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 o f the earlier experimental data to the current value are distributed b etween 0.89 and 1.02.