The ${}^{14}\mathrm{C}$($n,\gamma$) cross section between 10 keV and 1 MeV

The neutron capture cross section of ${}^{14}\mathrm{C}$ is of relevance for several nucleosynthesis scenarios such as inhomogeneous Big Bang models, neutron induced CNO cycles, and neutrino driven wind models for the $r$ process. The ${}^{14}\mathrm{C}$($n,\gamma$) reaction is also important for the validation of the Coulomb dissociation method, where the ($n,\gamma$) cross section can be indirectly obtained via the time-reversed process. So far, the example of ${}^{14}\mathrm{C}$ is the only case with neutrons where both, direct measurement and indirect Coulomb dissociation, have been applied. Unfortunately, the interpretation is obscured by discrepancies between several experiments and theory. Therefore, we report on new direct measurements of the ${}^{14}\mathrm{C}$($n,\gamma$) reaction with neutron energies ranging from 20 to 800 keV.

Figure 1

The γ-ray efficiency of the HPGe detector used during the cyclic activation. The efficiency was measured using calibration sources and the ${}^{27}\mathrm{Al}$($p,\gamma$)${}^{28}\mathrm{Si}$ reaction. The red dashed and blue solid curves correspond to GEANT simulations assuming 50 cm and 6 mm distance between sample and detector, respectively. The extrapolation from low to high energies results in slightly smaller values for the 6 mm case.

Figure 2

Comparison of a simulated neutron spectrum (solid, blue line) with experimental data (dashed, black line) [23] in arbitrary units.

Figure 3

Neutron spectra for Runs I-IV (top to bottom) in arbitrary units. The curves correspond to the simulated spectra for the gold foils as well as for the ${}^{14}\mathrm{C}$ sample. The second neutron group around 200 keV in the bottom panel results from the ${}^7\mathrm{Li}$(p,n)${}^7\mathrm{Be}$${}^{*}$ reaction.

Figure 4

Measured γ-ray spectrum after 22 h of cyclic activation during Run I (23.3 keV MACS).

Figure 5

The same data as shown in Fig. 4, but focused on the decay lines of ${}^{15}\mathrm{C}$.

Figure 6

Time dependence of the ${}^{15}\mathrm{C}$ activity.

Figure 7

Theoretical ${}^{14}\mathrm{C}$($n,\gamma ){}^{15}$C cross section, fitted to the experimental data, as described in the text.

Figure 8

Reaction rates for ${}^{14}\mathrm{C}$($n,\gamma$)${}^{15}\mathrm{C}$ as a function of stellar temparature $T_9$ given in ${10}^9$ K.

Figure 9

Comparison between the present results and previous data.

Figure 10

Comparison between this measurement (shaded band) and previous cross section results at $E_{\mathrm{c}.\mathrm{m}.}=23.3$ keV. Open squares refer to theoretical estimates while full circles refer to experiments including Coulomb-breakup studies. The only open circle refers to the measurement by Beer et al. before the renormalization based on the new mass and line intensity information (see text). The respective references from left to right are [7, 8, 33] (theoretical) and [9, 10, 11, 12, 34, 35] (experimental).