Measurement of ${}^{208}\mathrm{Pb}(n,\gamma ){}^{209}\mathrm{Pb}$ Maxwellian averaged neutron capture cross section

The doubly magic ${}^{208}\mathrm{Pb}$ nucleus is a bottleneck at the termination of the $s$-process path due to its very low neutron capture cross section. This cross section is also important for the decomposition of $s$, $r$ processes and U/Th radiogenic decay contributions to the Pb-Bi solar abundances. The ${}^{208}\mathrm{Pb}(n,\gamma ){}^{209}\mathrm{Pb}$ cross section was measured at the Soreq Applied Research Accelerator Facility Phase I using an intense quasi-Maxwellian neutron source produced by irradiation of the liquid-lithium target with a 1.5-mA continuous-wave proton beam at 1.94 MeV. The cross section was measured by counting the $\beta$ activity from the irradiated lead target. The measurement allowed us to evaluate the Maxwellian averaged cross section (MACS) at 30 keV obtaining a value of 0.33(2) mb. This has been compared with the earlier activation and time-of-flight measurements found in the literature. The MACS cross-sectional value of the ${}^{63}\mathrm{Cu}(n,\gamma ){}^{64}\mathrm{Cu}$ reaction was determined in the same experiment and is compared to a recent published value.

Figure 1

Beam logarithm during the irradiation. The counting rate of a fission chamber is used as a beam current monitor (see Refs. [10, 12] for details).

Figure 2

(a) The $\gamma$ spectrum of the lead foil taken in contact with the Ge detector immediately after irradiation. (b) The $\beta$ counting rate of the lead sample as a function of time together with the fitting curve. Time is presented in ${}^{209}\mathrm{Pb}$ mean lifetime units.

Figure 3

Experimental $\beta$ spectra taken from ${}^{64}\mathrm{Cu}$ (diamonds), ${}^{209}\mathrm{Pb}$ (circles), and ${}^{198}\mathrm{Au}$ (triangles) are compared with the results of the geant simulations. The experimental spectra and simulations are normalized by arbitrary factors to reduce the density of the figure.

Figure 4

(a) A $\gamma$ spectrum taken with the copper sample. The 1345.8-keV $\gamma$ ray is shown in the inset. (b) The $\gamma$ spectrum of one of the gold samples.

Figure 5

An autoradiograph of one of the gold samples. The yellow square corresponds to a $25\times 25\mathrm{m}{\mathrm{m}}^2$ gold foil and is centered on the ion-optical axis of the proton (and neutron) beam. The vertical color scale is linear.

Figure 6

A simulated neutron spectrum on the targets is compared with fitted MB distributions.

Figure 7

(a) The evaluation of the ${}^{208}\mathrm{Pb}(n,\gamma ){}^{209}\mathrm{Pb}$ excitation functions from the endf and jendl libraries. (b) Comparison the MACS cross section calculated with the endf and jendl evaluated cross sections with the experimental data taken from Ref. [29].

Figure 8

Summary of the MACS ${}^{208}\mathrm{Pb}(n,\gamma ){}^{209}\mathrm{Pb}$ cross-sectional values at 30 keV taken from Ref. [29]. The error bar for Ratzel [8] is taken from Ref. [29]. The original experimental error for 25 keV was 0.02 mb, see Ratzel [8].