Experimental determination of the neutron resonance peak of ${}^{162}\mathrm{Er}$ at 67.8 eV

${}^{162}\mathrm{Er}$ is one of the $p$ nuclei in nuclear astrophysics, and its ($n,\gamma$) cross sections are important input parameters in nuclear astrophysics network calculations. Resonance data for ${}^{162}\mathrm{Er}(n,\gamma ){}^{163}\mathrm{Er}$ at 67.8 eV is currently not present in the EXFOR database; however, it is included in the ENDF/B VIII.0 database. The ($n,\gamma$) cross section of ${}^{\mathrm{nat}}\mathrm{Er}$ has been measured in the energy range of 1–100 eV at the Back-n facility in the China Spallation Neutron Source. However, due to the influence of the in-beam $\gamma$ background of the Back-n Facility, it is has not been possible to observe the neutron resonance peak of ${}^{162}\mathrm{Er}$ at 67.8 eV. A general simulation method is proposed for quantifying the in-beam $\gamma$-ray background. By subtracting the in-beam $\gamma$-ray background, the neutron-capture yields of ${}^{162}\mathrm{Er}$ are obtained within the energy range of 20–100 eV. The neutron resonance parameters of ${}^{162}\mathrm{Er}$ at 67.8 eV are then extracted successfully. It is found that ${\mathrm{\Gamma }}_{\gamma }=101.15\pm 10.08$ meV and ${\mathrm{\Gamma }}_n=2.79\pm 0.28$ meV, which are consistent with the ENDF/B VIII.0 database. However, the 51.4-eV resonance peak of ${}^{162}\mathrm{Er}(n,\gamma$) is currently unobserved.

Figure 1

(a) The photograph of the ${\mathrm{C}}_6{\mathrm{D}}_6$ detector system. (b) Detector system's construction in simulation. (c) Backward detector layout to minimize background from neutron scattering by the target.

Figure 2

The time structure of the in-beam $\gamma$-ray background. Different line styles represent the experimental and fitted values of different backgrounds, and yellow, blue and red indicate the results of experiments 1–3, respectively. The solid black line indicates the fitted values of $B_{s\gamma }$.

Figure 3

Shape uncertainties of the in-beam $\gamma$-ray background change with time (neutron energy). The maximum occurs at 20 eV with a value of 34.9%.

Figure 4

Experimental spectra for Ag target with and without the Al filter, together with $B_{s\gamma }^{\mathrm{fit}}, C_n{B}_{sn}$, and $C_n{B}_{sn}+C_{\gamma }{B}_{s\gamma }^{\mathrm{fit}}$. Here $C_n$ and $C_{\gamma }$ are the coefficients of the neutron and $\gamma$-ray scattering backgrounds, respectively. The value of $C_n$ can be obtained from Ref. [7] and the determination of $C_{\gamma }$ is discussed later. The inset shows the enlarged spectrum for the Ag target plus the Al filter, which is in the energy range between 30 and 100 keV. Red dots with error bars represent the simulated counting spectrum at the two ${}^{27}\mathrm{Al}$ absorption peaks mentioned above.

Figure 5

The uncertainty of ${\delta }_{Y_w}$ as a function of $E_n$.

Figure 6

The experimental capture yields and the fitted ones obtained with the sammy code.

Figure 7

Comparison between the ${\mathrm{\Gamma }}_n$ values obtained from the ENDF/B-VIII.0 database, previous data [7], and this work. The inset shows a range of 10 to 40 meV for the $y$ axis.