Neutron-capture nucleosynthesis of nature's rarest stable isotope

Abstract

Many attempts^1–6 have been made to explain the solar abundances of the stable, heavy odd–odd nuclei: ⁵⁰V, ¹³⁸La, ¹⁷⁶Lu and ¹⁸⁰Ta. Among these, ¹⁸⁰Ta (actually an isomeric state, ¹⁸⁰Ta^m) has the distinction of being the rarest, stable naturally occurring isotope (0.012% of all Ta) with a solar abundance⁷ of only 2.46×10⁻⁶ (per 10⁶ Si atoms) and a half life of >10¹³ yr against β decay. Although the variety of mechanisms previously proposed can in some special cases account adequately for the small abundance of ¹⁸⁰Ta^m, it is difficult to test quantitatively those special types of synthesis invented only to explain such a small number of nuclei. In contrast, we show here from our experimental work that ¹⁸⁰Ta^m can also be accounted for by relatively small branchings in the much more common slow (s)-and rapid (r)-processes of stellar nucleosynthesis by neutron capture.