The neutron-induced reaction cross sections of 242m Am were measured at the Los Alamos Neutron Science Center using the Detector for Advanced Neutron-Capture Experiments array along with a compact parallel-plate avalanche counter for fission-fragment detection. A new neutron-capture cross section was determined, and the absolute scale was set according to a concurrent measurement of the well-known 242m Am(n,f) cross section. The (n,γ) cross section was measured from thermal to an incident energy of 1 eV at which point the data quality was limited by the reaction yield in the laboratory. Our new 242m Am fission cross section was normalized to ENDF/B-VII.1 to set the absolute scale, and it agreed well with the (n,f) cross section reported by Browne et al. from thermal energy to 1 keV. The average absolute capture-to-fission ratio was determined from thermal to En = 0.1 eV, and it was found to be 26(4)% as opposed to the ratio of 19% from the ENDF/B-VII.1 evaluation.
The cross section for neutron capture in the continuum region has been difficult to calculate accurately. Previous results for 238 U show that including an M1 scissors-mode contribution to the photon strength function resulted in very good agreement between calculation and measurement. This paper extends that analysis to 234,236 U by using gamma-ray spectra measured with the DANCE detector to constrain the photon strength function used to calculate the capture cross section. Calculations using a strong scissors-mode contribution reproduced the measured γ-ray spectra and were in excellent agreement with the reported cross sections for all three isotopes.
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