66.7-keV $\gamma$-line intensity of ${}^{171}\mathrm{Tm}$ determined via neutron activation

Background: About 50% of the heavy elements are produced in stars during the slow neutron capture process. The analysis of branching points allows to set constraints on the temperature and the neutron density in the interior of stars. The temperature dependence of the branch point ${}^{171}\mathrm{Tm}$ is weak. Hence, the ${}^{171}\mathrm{Tm}$ neutron capture cross section can be used to constrain the neutron density during the main component of the s process in thermally pulsing asymptotic giant branch stars.

Purpose: In order to perform neutron capture experiments on ${}^{171}\mathrm{Tm}$, sample material has to be produced and characterized. The characterization is done by $\gamma$ spectroscopy, relying on the intensities of the involved $\gamma$ lines. Only the 66.7-keV $\gamma$ line can be observed whose intensity was uncertain so far.

Method: An enriched ${}^{170}\mathrm{Er}$ sample was activated with thermal neutrons at the TRIGA (Training, Research, Isotopes, General Atomics) research reactor at the Johannes Gutenberg-Universität Mainz. The activation resulted in an easily quantifiable number of ${}^{171}\mathrm{Er}$ nuclei that subsequently decayed to ${}^{171}\mathrm{Tm}$.

Result: The intensity of the 66.7-keV $\gamma$ line of the ${}^{171}\mathrm{Tm}$ decay was measured to $I_{\gamma }=(0.144\pm 0.010)\%$.

Conclusions: Our result is in good agreement with the value found in the literature.

Figure 1

The $s$-process region between Er and Yb with the mass flow path depicted by gray arrows. Secondary paths are represented by dashed lines. The radioactive isotopes ${}^{170}\mathrm{Tm}$ and ${}^{171}\mathrm{Tm}$ act as branching points. The red arrows depict the activation of ${}^{170}\mathrm{Er}$, which leads to ${}^{171}\mathrm{Er}$. Counting its $\gamma$ emission allows to determine the number of nuclei that finally decay to ${}^{171}\mathrm{Tm}$.

Figure 2

The most prominent $\gamma$-ray emission lines of ${}^{171}\mathrm{Er}$ at 308 keV and 295 keV after a counting time of 3 h.

Figure 3

Resulting spectrum from the same sample as in Fig. 2 after about 6 days of the ${}^{171}\mathrm{Tm}$ counting. Besides ${}^{171}\mathrm{Tm}$ at 66.7 keV, $\gamma$ lines of other constituents of the sample material are visible, most notably isotopes of Tm, Er, and Yb. Below 65 keV x rays from various species overlap. No contributions by isotopes other than listed in Table 1 could be identified via spectroscopic analysis.