${}^{179}\mathrm{Ta}(n,\gamma )$ cross-section measurement and the astrophysical origin of the ${}^{180}\mathrm{Ta}$ isotope

${}^{180\mathrm{m}}\mathrm{Ta}$ is nature's rarest (quasi) stable isotope and its astrophysical origin is an open question. A possible production site of this isotope is the slow neutron capture process in asymptotic giant branch stars, where it can be produced via neutron capture reactions on unstable ${}^{179}\mathrm{Ta}$. We report a new measurement of the ${}^{179}\mathrm{Ta}(n,\gamma ){}^{180}\mathrm{Ta}$ cross section at thermal-neutron energies via the activation technique. Our results for the thermal and resonance-integral cross sections are $952\pm 57$ and $2013\pm 148$ b, respectively. The thermal cross section is in good agreement with the only previous measurement [Phys. Rev. C 60, 025802 (1999)], while the resonance integral is different by a factor of $\approx 1.7$. While neutron energies in this work are smaller than the energies in a stellar environment, our results may lead to improvements in theoretical predictions of the stellar cross section.

Figure 1

$s$-process reaction network. Gray and white boxes show the stable and unstable isotopes respectively. Thick black arrows show the main $s$-process path. The red and green arrows show the weak branching paths suggested by Refs. [13] and [9], respectively. The orange arrows show all the other reactions in the network.

Figure 2

Schematic diagram of the detector setup comprising of two identical HPGe detectors D1 and D2 placed at distances $d_1$ and $d_2$ from the source holder, respectively.

Figure 3

Gamma-ray spectrum of activated ${}^{179}\mathrm{Ta}$ as recorded in detector D1 (see Fig. 2) over 12 h starting 5.5 h after the irradiation.

Figure 4

Detector efficiency as a function of energy for the two detectors D1 and D2 for the fluence monitors' spectroscopy setups after the first (no shield) and second (Cd shield) activations. Black circles show the measured values using radioactive sources. Error bars on the measurements are smaller than then marker size. Red dots are the simulated values obtained using geant3. The green line is the fit to the simulated values.

Figure 5

Epithermal vs thermal fluence plots as measured with different monitor isotopes (two samples of each) in the second activation (with Cd shield). The top and the right plots give the probability distribution of the ${\mathrm{\Phi }}_{\text{th}}$ and ${\mathrm{\Phi }}_{\text{epi}}$ solutions, respectively.