Stellar neutron capture on promethium: Implications for the s-process neutron density

The unstable isotope ¹⁴⁷Pm represents an important branch point in the s-process reaction path. This paper reports on the successful determination of the stellar (n, γ) cross section via the activation technique. The experiment was difficult because the relatively short ¹⁴⁷Pm half-life of 2.62 yr enforced the sample mass to be restricted to 28 ng or 10¹⁴ atoms only. By means of a modular, high-efficiency Ge Clover array the low induced activity could be identified in spite of considerable backgrounds from various impurities. Both partial cross sections feeding the 5.37 day ground state and the 41.3 day isomer in ¹⁴⁷Pm were determined independently, yielding a total (n, γ) cross section of 709 ± 100 mbarn at a thermal energy of kT = 30 keV. The (n, γ) cross sections of the additional branch point isotopes ¹⁴⁷Nd and ¹⁴⁸Pm as well as the effect of thermally excited states were obtained by detailed statistical model calculations. The present results allowed considerably refined analyses of the s-process branchings at A = 147/148, which are probing the neutron density in the He-burning shell of low-mass asymptotic giant branch stars.