Comprehensive ${}^{242\mathrm{m}}\mathrm{Am}$ neutron-induced reaction cross sections and resonance parameters

The ${}^{242}\mathrm{Am}$ metastable isomer's neutron-induced destruction mechanisms were studied at the Los Alamos Neutron Science Center using the Detector for Advanced Neutron-Capture Experiments array with a compact parallel-plate avalanche counter. New ${}^{242\mathrm{m}}\mathrm{Am}$ neutron-capture cross sections were determined from 100 meV to 10 keV, and the absolute scale was set with respect to a concurrent measurement of the well-known ${}^{242\mathrm{m}}\mathrm{Am}$ neutron-induced-fission cross section. The new fission cross section spans an energy range from 100 meV to 1 MeV and was normalized to the ENDF/B-VII.1 evaluated cross section to set the absolute scale. Our ${}^{242\mathrm{m}}\mathrm{Am}(n,f)$ cross section agrees well with the cross section of Browne et al. [Phys. Rev. C 29, 2188 (1984)] over this large energy interval. The new neutron-capture cross section measurement complements and agrees well with our recent results reported below 1 eV in Buckner et al. [Phys. Rev. C 95, 024610 (2017)]. This new work comprises the most comprehensive study of ${}^{242\mathrm{m}}\mathrm{Am}(n,\gamma )$ above thermal energy. Neutron-induced resonance energies and parameters were deduced with the sammy $R$-matrix code for incident neutron energies up to 45 eV, and the new average ${\mathrm{\Gamma }}_{\gamma }$ is $13\%$ higher than the evaluated average $\gamma$ width.

Figure 1

The characteristic neutron-capture reaction channel $E_{\mathrm{sum}}$, shown in orange, after the subtraction of the fission (blue), presampled (green), and environmental (purple) spectra from the inclusive (dashed red) $E_{\mathrm{sum}}$. The spectra for the incident neutron energy bin $0.1\text{–}1$ eV, with cluster multiplicities 4 and 5, are shown. Negative values and $E_{\mathrm{sum}}$ uncertainties are not shown in this figure to make it easier to distinguish between subtraction components.

Figure 2

The data quality for the ${}^{242\mathrm{m}}\mathrm{Am}(n,\gamma )$ reaction channel (orange) after fission (blue), presampled (green), and environmental (purple) background subtraction from the inclusive (red) spectrum. Incident neutron energy bins (a) $1\text{–}10$ eV, (b) $10\text{–}100$ eV, and (c) $E_n=100\text{–}1000$ eV with cluster multiplicities 4 and 5 are shown. Negative values and $E_{\mathrm{sum}}$ uncertainties are not shown here to allow the subtraction components to be easily distinguished, but are included in the final analysis.

Figure 3

The current ${}^{242\mathrm{m}}\mathrm{Am}(n,\gamma )$ cross section (filled blue circles) and the ${}^{242\mathrm{m}}\mathrm{Am}(n,f)$ cross section (open black circles) are plotted alongside the Browne et al. [5] (red squares) data for incident neutron energy ranges of (a) 10 meV to 2 eV, (b) 1 eV to 300 eV, and (c) 300 eV to 1 MeV. The neutron-capture cross sections $<100$ meV from Buckner et al. [30] (open black triangles), are included in (a). The fission cross section between 1 and 20 keV trends lower than that of Browne et al. [5] by $\approx 20\%$, but all reported values are within 1.5 standard deviations of the previous measurement. Excluded cross sections at $\approx 300$ eV and $\approx 25$ keV coincide with aluminum resonances.