Measurement and analysis of the ${}^{241}\mathrm{Am}$ neutron capture cross section at the n_TOF facility at CERN

The ${}^{241}\mathrm{Am}(n,\gamma$) cross section has been measured at the n_TOF facility at CERN with the n_TOF ${\mathrm{BaF}}_2$ Total Absorption Calorimeter in the energy range between 0.2 eV and 10 keV. Our results are analyzed as resolved resonances up to 700 eV, allowing a more detailed description of the cross section than in the current evaluations, which contain resolved resonances only up to 150–160 eV. The cross section in the unresolved resonance region is perfectly consistent with the predictions based on the average resonance parameters deduced from the resolved resonances, thus obtaining a consistent description of the cross section in the full neutron energy range under study. Below 20 eV, our results are in reasonable agreement with JEFF-3.2 as well as with the most recent direct measurements of the resonance integral, and differ up to 20–30% with other experimental data. Between 20 eV and 1 keV, the disagreement with other experimental data and evaluations gradually decreases, in general, with the neutron energy. Above 1 keV, we find compatible results with previously existing values.

Figure 1

Schematic view of part of the n_TOF Total Absorption Calorimeter. Modules in front of the beam pipe and the upper hemisphere of the neutron absorber are not shown.

Figure 2

Total deposited energy in the TAC during the ${}^{241}\mathrm{Am}(n,\gamma$) and different background measurements. The data correspond to neutron energies between 1 and 10 eV and no cuts on $m_{\mathrm{cr}}$ have been applied to the detected events.

Figure 3

Number of events detected per pulse and per unit per unit lethargy ($\mathrm{\Delta }\mathrm{LnE}$, which means that the bin contents have been divided by the natural logarithm of the ratio between the upper and lower bin limits) in the ${}^{241}\mathrm{Am}(n,\gamma$) measurement as a function of the neutron energy, together with different background measurements. Only $m_{\mathrm{cr}}>1$ and $2.5<E_{\mathrm{sum}}<6$ MeV events are shown.

Figure 4

Experimental ${}^{241}\mathrm{Am}(n,\gamma$) yield (non-background-subtracted) together with the calculated background, in blue, and the background obtained if the Sm impurities are not removed, in red.

Figure 5

Experimental (solid lines) and simulated (dotted lines) deposited energy spectra from ${}^{241}\mathrm{Am}$ capture cascades for different $m_{\mathrm{cr}}$ cuts. The experimental data were obtained from the strong ${}^{241}\mathrm{Am}$ resonance at $\sim 0.3$ eV.

Figure 6

Deposited energy spectra (with $m_{\mathrm{cr}}>2$) from ${}^{241}\mathrm{Am}$ capture cascades for different resonances. The spectrum corresponding to the first resonance at 0.3 eV (solid lines) is compared to the spectra of the next eight resonances (dotted lines) at (a) 0.6, (b) 1.3, (c) 1.9, (d) 2.4, (e) 2.6, (f) 4.0, (g) 5.0, and (h) 5.4 eV. All the spectra have been normalized to the area.

Figure 7

One minus the fraction of events lost ($f_{\mathrm{losses}}$) due to the pile-up induced by the background, as a function of the neutron energy and for three different $E_{\mathrm{sum}}$ and $m_{\mathrm{cr}}$ cuts.

Figure 8

(b) The experimental ${}^{197}\mathrm{Au}(n,\gamma$) yields obtained for the three different samples, sample 1 ($250\mu \mathrm{m}$ thick, 12.2 mm diameter), sample 2 ($50\mu \mathrm{m}$ thick, 12.2 mm diameter), and sample 3 ($50\mu \mathrm{m}$ thick, 45 mm diameter), and the two beam pulse intensities, high (HIP) and low (LIP). The yields have been normalized to reproduce the results obtained from sammy using JEFF-3.2. (a) The same experimental data without performing pile-up corrections.

Figure 9

Standard deviation of the capture yield obtained from the six different $E_{\mathrm{sum}}$ and $m_{\mathrm{cr}}$ cuts mentioned in the text as a function of the neutron energy, together with the uncertainties due to counting statistics.

Figure 10

Examples of the fitted n_TOF capture yield, in different energy ranges. The data points correspond to the experimental capture yield and the solid line to the fit performed with sammy.

Figure 11

Cumulative distributions of the neutron widths compared with the value of $S_0$ obtained in this work. The two blue lines represent the uncertainty due to the number of resonances used in the determination of $S_0$.

Figure 12

Estimation of the number of missing resonances, in this case 8.5%, performed in the 0–40 eV energy range. The experimental points were fitted to Eq. (2).

Figure 13

Measured (markers) and fitted (solid line) n_TOF capture cross section in the URR. The uncertainty of $S_0$ is shown as a shadowed area. The cross sections provided by the JEFF-3.2, ENDF/B-VII.1, and JENDL-4.0 evaluations, which are very similar, are also plotted.

Figure 14

Ratio of the $g{\mathrm{\Gamma }}_n$ obtained in this work and the ones obtained in previous measurements, in the low neutron energy range. Only uncertainties due to counting statistics have been considered.

Figure 15

Ratio between the capture cross section (${\int }_{E_1}^{E_2}\sigma \left(E\right)dE$) obtained in this work and the ones obtained in previous measurements and evaluations. Only uncertainties due to counting statistics have been considered in all cases but Jandel et al., where uncertainties due to systematic effects are also included.