66.7-keV gamma-line intensity of Tm-171 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 Tm-171 is weak. Hence, the Tm-171 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 Tm-171, 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 Er-170 sample was activated with thermal neutrons at the TRIGA (Training, Research, Isotopes, General Atomics) research reactor at the Johannes Gutenberg-Universitat Mainz. The activation resulted in an easily quantifiable number of Er-171 nuclei that subsequently decayed to Tm-171. Result: The intensity of the 66.7-keV gamma line of the Tm-171 decay was measured to I-gamma = (0.144 +/- 0.010)%. Conclusions: Our result is in good agreement with the value found in the literature.