Accelerator mass spectrometry measurements of the ${}^{13}\mathrm{C}(n,\gamma ){}^{14}\mathrm{C}$ and ${}^{14}\mathrm{N}(n,p){}^{14}\mathrm{C}$ cross sections

The technique of accelerator mass spectrometry (AMS), offering a complementary tool for sensitive studies of key reactions in nuclear astrophysics, was applied for measurements of the ${}^{13}\mathrm{C}\left(n,\gamma \right){}^{14}\mathrm{C}$ and the ${}^{14}\mathrm{N}\left(n,p\right){}^{14}\mathrm{C}$ cross sections, which act as a neutron poison in $s$-process nucleosynthesis. Solid samples were irradiated at Karlsruhe Institute of Technology with neutrons closely resembling a Maxwell-Boltzmann distribution for $kT=25$ keV, and also at higher energies between $E_n=123$ and 182 keV. After neutron irradiation the produced amount of ${}^{14}\mathrm{C}$ in the samples was measured by AMS at the Vienna Environmental Research Accelerator (VERA) facility. For both reactions the present results provide important improvements compared to previous experimental data, which were strongly discordant in the astrophysically relevant energy range and missing for the comparably strong resonances above 100 keV. For ${}^{13}\mathrm{C}\left(n,\gamma \right)$ we find a four times smaller cross section around $kT=25$ keV than a previous measurement. For ${}^{14}\mathrm{N}\left(n,p\right)$, the present data suggest two times lower cross sections between 100 and 200 keV than had been obtained in previous experiments and data evaluations. The effect of the new stellar cross sections on the $s$ process in low-mass asymptotic giant branch stars was studied for stellar models of $2M_{\odot }$ initial mass, and solar and $1/{10}^{\mathrm{th}}$ solar metallicity.

Figure 1

Energy production and related $s$-process sites in thermally pulsing low-mass AGB stars are associated with recurrent H and He burning episodes. Short highly convective thermal instabilities during He shell flashes are separated by long radiative phases of H burning.

Figure 2

The $s$-process network in the ${}^{13}\mathrm{C}$ pocket for the main reactions among the CNO isotopes (the investigated cases are indicated by full arrows).

Figure 3

Schematic sketch of the setup used for the activations at the Karlsuhe Van de Graaff.

Figure 4

Experimental neutron energy distributions used in the ${}^{14}\mathrm{N}$ measurement. Apart from minor differences, the same spectra were applied in the ${}^{13}\mathrm{C}$ runs (see Table 2).

Figure 5

Schematic layout of the AMS facility VERA. Negative ions $\left({}^{12,13,14}{\mathrm{C}}^{-}\right)$ were extracted from the ion source and after low-energy mass analysis injected into the tandem accelerator. After gas stripping in the terminal and further acceleration, ions with charge $3^{+}$ and an energy of 12 MeV were selected with the analyzing magnet. The stable ${}^{12}\mathrm{C}$ and ${}^{13}\mathrm{C}$ ions were counted as current with Faraday cups, whereas the low-intensity ${}^{14}\mathrm{C}$ fraction in the beam was subjected to further background suppression by the electrostatic analyzer and were eventually recorded with one of the energy detectors (A, B or C).

Figure 6

The ${}^{14}\mathrm{N}\left(n,p\right){}^{14}\mathrm{C}$ cross section between 1 and 400 keV. The plot shows a comparison of the present results (full squares) with existing experimental data [13, 14, 15, 16] and with the evaluated cross section in the JEFF-3.2 library [50]. In general, there is very good agreement with existing data except for the values of Brehm et al. [13]. The solid line represents a best-fit cross section, yielding average values (FWHM indicated by open boxes) in good agreement with the experimental results.

Figure 7

The ${}^{13}\mathrm{C}\left(n,\gamma \right){}^{14}\mathrm{C}$ cross section between 1 and 300 keV. The solid line represents a best-fit cross section obtained with the prescription described in the text. Folded with the neutron energy spectra of this work, this best fit yields average values (open boxes indicating the FWHM) in good agreement with the experimental results. Because the $p$- and $d$-wave components of the DRC contribution were neglected in the evaluation, the JEFF 3.2 cross section (dashed line) is too small in the astrophysically relevant region below the resonance at 152.4 keV.

Figure 8

Comparison of the present MACS results with data obtained from the evaluated cross sections in the JEFF-3.2 library [50].