Slow neutron capture origin for ¹⁸⁰Ta^m

Abstract

Most naturally-occurring heavy isotopes (A ≥ 74) are believed to be synthesized by the slow (s−) and rapid (r−) neutron capture processes¹, with the rest (some 30 neutron-deficient isotopes including ¹⁸⁰Ta) being conventionally classified as p-process elements. The very rare isotope ¹⁸⁰Ta (a nuclear isomeric state ¹⁸⁰Ta^m with a half-life ≥ 3×10¹³ yr)^2,3 is of particular astrophysical interest because of its uncertain origin. Various p-process models^4–6 (except the one eventually proposed in ref. 7) apparently fail to account even for the extremely small (2.78 × 10⁻⁶ per 10⁶ Si atoms) solar abundance⁸. Another example of this kind is ¹³⁸La. While preserving other more specific mechanisms (such as the cosmic-ray spallation reactions^9,10, or the local proton irradiation in the early Solar System^11,12) for the possible origin (s) of these rare isotopes, we show here that a significant amount of ¹⁸⁰Ta^m may indeed be produced in the s-process.