Measurement of the radiative neutron capture cross section of ${}^{206}\mathrm{Pb}$ and its astrophysical implications

The ($n,\gamma )$ cross section of ${}^{206}\mathrm{Pb}$ has been measured at the CERN n_TOF facility with high resolution in the energy range from 1 eV to 620 keV by using two optimized ${\mathrm{C}}_6{\mathrm{D}}_6$ detectors. In the investigated energy interval about 130 resonances could be observed, from which 61 had enough statistics to be reliably analyzed via the R-matrix analysis code SAMMY. Experimental uncertainties were minimized, in particular with respect to (i) angular distribution effects of the prompt capture γ-rays, and to (ii) the TOF-dependent background due to sample-scattered neutrons. Other background components were addressed by background measurements with an enriched ${}^{208}\mathrm{Pb}$ sample. The effect of the lower energy cutoff in the pulse height spectra of the ${\mathrm{C}}_6{\mathrm{D}}_6$ detectors was carefully corrected via Monte Carlo simulations. Compared to previous ${}^{206}\mathrm{Pb}$ values, the Maxwellian averaged capture cross sections derived from these data are about 20% and 9% lower at thermal energies of 5 keV and 30 keV, respectively. These new results have a direct impact on the $s$-process abundance of ${}^{206}\mathrm{Pb}$, which represents an important test for the interpretation of the cosmic clock based on the decay of ${}^{238}\mathrm{U}$.

Figure 1

Pulse height spectra for the 4.9 eV resonance in gold (grey) and for the 3.3 keV resonance in ${}^{206}\mathrm{Pb}$ (black), arbitrarily scaled. The dashed lines are the MC-calculated γ-ray spectra for the two resonances. The linear scale used in the inset illustrates the large difference between the simulated spectra below a threshold of 300 keV.

Figure 2

Level scheme and decay patterns for ${}^{207}\mathrm{Pb}$ [14]. All energies are in keV.

Figure 3

Ratio between the capture kernels reported in Refs. [15] (top), [14] (second), [27] (third), and [26] (bottom) and the kernels determined here.

Figure 4

(left) The bold red line represents an R-matrix fit to our experimental capture yield starting from the initial parameters (solid green line) in Ref. [22]. The dashed and dot-dashed curves correspond to the capture yields determined in Refs. [26] and [14], respectively. (right) The fitted capture yield in the 10–30 keV energy range (thin red line).

Figure 5

Maxwellian averaged ($n,\gamma$) cross sections for ${}^{206}\mathrm{Pb}$ from the resonance parameters of this work (bold red) compared to the IRMM measurement [26] (dashed), to the recommended data of Ref. [30] (grey), and to the compiled data of Ref. [22] (solid green).

Figure 6

Estimate of the radiogenic component of ${}^{206}\mathrm{Pb}$ using the Fowler's model with different nucleosynthetic assumptions (see labels in curves) and the $r$-process age ${\Delta }_r=t_U-4.6$ Gyr (vertical dashed line) derived from the age of the Universe $t_U$ [37].