Partial-wave analysis of $n+{}^{241}\mathrm{Am}$ reaction cross sections in the resonance region

Cross sections for neutron-induced reactions of ${}^{241}\text{Am}$ in the resonance region have been evaluated. Results of time-of-flight cross section experiments carried out at the GELINA, LANSCE, ORELA and Saclay facilities have been combined with optical model calculations to derive consistent cross sections from the thermal energy region up to the continuum region. Resolved resonance parameters were derived from a resonance shape analysis of transmissions, capture yields, and fission yields in the energy region up to 150 eV using the refit code. From a statistical analysis of these parameters, a neutron strength function (${10}^4{S}_0=1.01\pm 0.12$), mean level spacing ($D_0=0.60\pm 0.01$ eV) and average radiation width ($\langle {\mathrm{\Gamma }}_{{\gamma }_0}\rangle =43.3\pm 1.1$ meV) for $s$-wave resonances were obtained. Neutron strength functions for higher partial waves ($l>0$) together with channel and effective scattering radii were deduced from calculations based on a complex mean-field optical model potential, applying an equivalent hard-sphere scattering radius approximation.

Figure 1

Comparison of the channel radius calculated with the expressions (20) and (22). The reduced radius $r_0$ and the surface diffuseness $a$ of the real part of the volume potential are calculated with Eqs (24) and (25).

Figure 2

Matter density distribution (a) and real part of the volume potential (b) for the nuclear system ${}^{241}\text{Am}+n$. The densities ${\rho }_n$ and ${\rho }_p$ are taken from the AMEDE database [39]. The channel radii $a_c$ are taken from Table 2.

Figure 3

Energy dependence of the $J$-dependent phase shift for $l=0,1,2$ calculated with the optical model code ecis for the nuclear system ${}^{241}\text{Am}+n$. The open circles represent the equivalent hard-sphere phase shift calculated with Eq. (26).

Figure 4

Comparison of the $l$-dependent phase shift calculated with the hard-sphere approximation ($a_0=9.52$ fm, $a_1=7.20$ fm, and $a_2=8.76$ fm) and calculated with the optical model code ecis for the nuclear system ${}^{241}\text{Am}+n$ in log-log and log-lin scales.

Figure 5

Low neutron energy part of the capture yield and transmission data measured at the IRMM. The theoretical curves were calculated with the refit code. The solid line was obtained by using $\mathrm{Im}\left[{\mathcal{R}}_c\right]=(7.6\pm 0.5)\times {10}^{-3}$ and $\mathrm{Re}\left[{\mathcal{R}}_c\right]=0$. The dashed line was obtained with the resonance parameters reported in Ref. [5]. The open circles indicate the thermal values.

Figure 6

Theoretical capture yield and transmission calculated with the refit code up to 27 eV.

Figure 7

Theoretical fission cross section reconstructed up to 27 eV.

Figure 8

Capture yield calculated with the refit code compared to Jandel's data [6] up to 3 eV.

Figure 9

Ratio of the neutron widths reported in Ref. [5] (a), in Ref. [59] (b), and compiled in the Evaluated Nuclear Data library JEFF-311 (c) to our results.

Figure 10

Comparison of the individual radiation widths determined in the present work with results reported in the literature.

Figure 11

Comparisons of the estima calculations to the experimental distributions established from the neutron resonance shape analysis (NRSA). Plot (a) represents the cumulative Porter-Thomas integral distribution, plot (b) stands for the cumulative number of the $s$-wave resonances, and plot (c) is the cumulative distribution of the reduced neutron widths.

Figure 12

${}^{241}\text{Am}$ total cross section obtained in this work (ecis calculations) and compared with EXFOR data [47, 63].

Figure 13

Neutron strength functions and distant level parameters obtained with the SPRT method [Eqs. (17) and (16)] for the nuclear system ${}^{241}\text{Am}+n$. Channel radii calculated in the equivalent hard-sphere approximation and in the ENDF convention are reported in Tables 1 and 2.

Figure 14

Comparison of the neutron transmission coefficients provided by the ecis code and calculated with Eq. (35) for the nuclear system ${}^{241}\text{Am}+n$ in log-log and log-lin scales. The coupled channel calculations were performed with the optical model parameters listed in Tables 6 and 7.

Figure 15

Comparison of the theoretical ${}^{241}\text{Am}$ capture cross section (talys) with data retrieved from the EXFOR data base [6, 51, 68] time the square root of the incident neutron energy. No normalization factors were applied to the data.

Figure 16

Comparison of the ${}^{241}\text{Am}$ capture cross section calculated with conrad and talys up to 300 keV.