Measurement of the ${}^{159}\mathrm{Tb}(n,\gamma )$ cross section at the CSNS Back-n facility

The stellar ($n,\gamma$) cross section data for the mass numbers around $A\approx 160$ are of key importance to nucleosynthesis in the main component of the slow neutron capture process, which occurs in the thermally pulsing asymptotic giant branch (TP-AGB). The new measurement of ($n,\gamma$) cross sections for ${}^{159}\mathrm{Tb}$ was performed using the ${\mathrm{C}}_6{\mathrm{D}}_6$ detector system at the back streaming white neutron beam line (Back-n) of the China spallation neutron source with neutron energies ranging from 1 eV to 1 MeV. Experimental resonance capture kernels are reported up to 1.2 keV neutron energy with this capture measurement. Maxwellian-averaged cross sections (MACS) are derived from the measured ${}^{159}\mathrm{Tb}(n,\gamma )$ cross sections at $kT=5$–100 keV and are in good agreement with the recommended data of KADoNiS-v0.3 and JEFF-3.3, while KADoNiS-v1.0 and ENDF-VIII.0 significantly overestimate the present MACS up to $40\%$ and $20\%$, respectively. A sensitive test of the $s$-process nucleosynthesis is also performed with the stellar evolution code mesa. Significant changes in abundances around $A\approx 160$ are observed between the ENDF/B-VIII.0 and present measured rate of ${}^{159}\mathrm{Tb}(n,\gamma ){}^{160}\mathrm{Tb}$ in the mesa simulation.

Figure 1

The $s$-process reaction path in the region of terbium. Black solid boxes indicate stable; blue empty boxes indicate unstable isotopes. Arrows to the right and the next higher elements represent neutron capture reactions and ${\beta }^{-}$ decays, respectively. The isotope ${}^{163}\mathrm{Dy}$ is a terrestrially stable nucleus, which becomes unstable at stellar temperature [8].

Figure 2

${}^{159}\mathrm{Tb}(n,\gamma )$ capture yield with filters and background components are shown as a function of neutron energy. Black and blue solid lines indicate the ${}^{159}\mathrm{Tb}$ sample with ${}^{181}\mathrm{Ta}+{}^{59}\mathrm{Co}$ neutron filters and total background which is made of scattered neutrons (green dash-dotted lines) and in-beam $\gamma$ rays (red dashed lines).

Figure 3

The sammy fits to the experimental capture yields of ${}^{159}\mathrm{Tb}(n,\gamma )$.

Figure 4

Comparisons of the measured neutron capture cross section on ${}^{159}\mathrm{Tb}$ of this work (black full circles) with previous measurements [11, 12, 13, 14, 15, 16] (symbols), evaluations [47, 51] (dashed lines), and talys-1.9 calculations [53] in the 2 keV to 1 MeV energy region.

Figure 5

The ${}^{159}\mathrm{Tb}(n,\gamma )$ cross sections used in the calculation of present MACS.

Figure 6

(top) The MACS of ${}^{159}\mathrm{Tb}(n,\gamma )$ and (bottom) the ratios between present experimental data and others [17, 19, 47, 51].

Figure 7

(Top) Time evolution of nuclear abundances on the test trajectory with $T=8.9\times {10}^7$ K and $\rho$ = $7.7\times {10}^3\mathrm{g}/{\mathrm{cm}}^3$. (Middle) The abundance differences between the default case of the JINA REACLIB rate for ${}^{159}\mathrm{Tb}(n,\gamma ){}^{160}\mathrm{Tb}$, which is derived from KADoNiS-v0.3, and the case of ENDF/B-VIII.0. (Bottom) Same physical quantities as in the middle one but the reaction rates of this work instead of ENDF/B-VIII.0 data are used in calculations.