A new experimental method has been used to determine the isomeric ratio (IR) in neutron capture of 241Am in a differential experiment. Thin 241Am samples have been activated with subthermal monoenergetic neutrons of 14.75 meV and quasi-monoenergetic neutrons of ∼30 keV. The decay of the 242Am nuclei produced has been determined by observing the emitted beta spectrum in a mini-orange spectrometer. The measurements have been performed relative to gold. The ratio R1 = σγ (241Am→ 242gAm)/σγ(Au) was found to be R1 = 5.79 ± 0.33 at 14.75 meV and R1 = 2.73 ± 0.16 at ∼30 keV. The corresponding IRs, σγ(241Am→ 242gAm)/σγ(Am), are 0.92. ± 0.06 at 14.75 meV and 0.65 ± 0.05 at ∼30 keV. Detailed theoretical calculations of the total capture cross section, the IR, and the capture gamma-ray spectra were performed in the energy range from 1 to 1000 keV taking advantage of recently available information on the discrete level scheme of 242Am. With the present knowledge on the level scheme of 242Am, it seems to be difficult to reproduce the strong energy dependence of IR as indicated by the experimental results.
