Measurement of the ${}^{242m}\mathrm{Am}$ neutron-induced reaction cross sections

The neutron-induced reaction cross sections of ${}^{242m}\mathrm{Am}$ 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 ${}^{242m}\mathrm{Am}(n,f)$ cross section. The ($n,\gamma$) cross section was measured from thermal energy to an incident energy of 1 eV at which point the data quality was limited by the reaction yield in the laboratory. Our new ${}^{242m}\mathrm{Am}$ 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. (1984) from thermal energy to 1 keV. The average absolute capture-to-fission ratio was determined from thermal energy to $E_n=0.1$ eV, and it was found to be 26(4)$\%$ as opposed to the ratio of $19\%$ from the ENDF/B-VII.1 evaluation.

Figure 1

The nuclear reaction chain for isotopes of americium [5, 38, 54, 55]. CN is an abbreviation for compound nucleus.

Figure 2

The time difference between $\gamma$ rays detected by the DANCE array and charged particles detected by the PPAC for the ${}^{242m}\mathrm{Am}(n,f)$ reaction. The timing resolution is $\approx 1.5$ ns for the peak at $\approx -5$ ns. A 6-ns gate was placed around this peak. The discontinuity at 0 ns is an artifact of the timing algorithm.

Figure 3

The measured cluster multiplicity versus total $\gamma$-ray energy for the gated PPAC. The dashed box indicates the high-multiplicity ($M_{\mathrm{cl}}\ge 8$) region of the spectrum attributed to neutron-induced fission of ${}^{242m}\mathrm{Am}$ in the inclusive mode.

Figure 4

The $E_{\mathrm{sum}}$ spectrum collected in the inclusive mode (black) with the multiplicity condition $M_{\mathrm{cl}}\ge 8$ is compared to the PPAC $E_{\mathrm{sum}}$ (red) with the PPAC-DANCE coincident timing constraint and the same multiplicity threshold. These spectra were derived from the first period data with incident neutron energy bin $E_n=0.5$–1 eV.

Figure 5

Background subtractions of the scaled (a) fission, (b) presampled, and (c) environmental components were subtracted from inclusive $E_{\mathrm{sum}}$ spectra to isolate the ($n,\gamma$) channel, shown in panel (c). Only $E_{\mathrm{sum}}$ spectra for cluster multiplicity $M_{\mathrm{cl}}=4$ and incident neutron energy bin $E_n=0.1$–0.5 eV are shown. The ${}^{242m}\mathrm{Am}(n,\gamma ) Q$ value is 6365 keV [3, 57].

Figure 6

The background-subtracted $E_{\mathrm{sum}}$ spectrum for ${}^{242m}\mathrm{Am}(n,\gamma )$ at $E_n=0.1$–0.5 eV. Only cluster multiplicities 4 and 5 are shown, and the $E_{\mathrm{sum}}=5.0$–6.0 MeV region, representing the peak area, is shaded.

Figure 7

The $E_{\mathrm{sum}}$ spectrum for the ${}^{242m}\mathrm{Am}(n,\gamma )$ reaction channel, shown in orange, after the subtraction of scaled fission (blue), presampled (green), and environmental (purple) backgrounds from the inclusive (red) spectrum. Only the incident neutron energy bin $E_n=10$–100 eV with cluster multiplicity 4 is shown. The ${}^{242m}\mathrm{Am}(n,\gamma )$ signal deteriorates above $E_n=1$ eV.

Figure 8

The cluster multiplicity spectrum for the ${}^{242m}\mathrm{Am}(n,\gamma )$ reaction and energy bin $E_n=0.05$–0.1 eV. The solid line is a Poisson distribution fit to $M_{\mathrm{cl}}=2$–9 to estimate multiplicities 0 and 1.

Figure 9

The ${}^{242m}\mathrm{Am}(n,f)$ cross section (open black circles) is plotted alongside the Browne et al. [4] (red squares) data for incident neutron energy ranges of (a) thermal energy to 2.0 eV and (b) 1.0 eV to 1.0 keV. The absolute neutron-capture cross section (solid black circles) is plotted for $E_n$ from thermal energy to 1 eV.