Neutron-capture nucleosynthesis of nature's rarest stable isotope

Survival of nature's rarest isotopeTa180under stellar conditions

“…It is interesting to note that the results for the effective stellar half-life of 180 Ta are not affected if the 5 + state decays preferentially by the above discussed E2 transition to the 7 + state -provided that there is a 1 % branching to states which decay finally to the ground state because of the symmetric roles of the 9 − isomer and 1 + ground state in the stellar integrated cross section I * σ in Eq. (8). At the relevant thermal energy of kT = 26 keV one finds that the effective stellar half-life of 180 Ta is about 11 h from the above mentioned model which includes the 1 + ground state, the 9 − isomer, and the IMS which con-nects the ground state and the isomer.…”

“…But the s-process may also contribute to the production of 180 Ta via β-decay of thermally excited 179 Hf to 179 Ta and subsequent 179 Ta(n,γ) 180 Ta neutron capture [7], or via 179 Hf(n,γ) 180m Hf(β − ) 180 Ta [8]. Even the rprocess might produce 180 Ta via the 180m Hf(β − ) 180 Ta decay [8,9]; however, the isomer decay branches in A = 180 nuclei are too small to explain the stellar abundance of 180 Ta [10,11,12,13]. The astrophysical sites for the nucleosynthesis of 180 Ta are still uncertain.…”

Measurement of the?-decay branch in180Lu to180m Hf for Investigation of the r-process nucleosynthesis of180m Ta

“…According to those works, s-process nucleosynthesis of 18~ is possible via a small /?-branching in the decay of the ~8~ isomer populated through neutron capture on a79Hf [14,15] or via excited state p-decay of 179Hf to a79Ta and subsequent neutron capture on 665 d-179Ta. Post-r-process formation is mediated by weak successive pbranchings in the decays of ~8~ and 18~ [14,15]. While s-process production by neutron capture on 179Hf and subsequent 18~ decay was found to account for about 10~ of the solar 18~ abundance [15], no definite contribution from the s-process via neutron capture on 179Ta could be calculated because of the lack of measured capture cross sections and accurate data on s-process neutron density and temperature [16].…”

“…Recently its production has been discussed in terms of small branchings in the common stellar s-and post-r-processes [14][15][16]. According to those works, s-process nucleosynthesis of 18~ is possible via a small /?-branching in the decay of the ~8~ isomer populated through neutron capture on a79Hf [14,15] or via excited state p-decay of 179Hf to a79Ta and subsequent neutron capture on 665 d-179Ta. Post-r-process formation is mediated by weak successive pbranchings in the decays of ~8~ and 18~ [14,15].…”

Neutron Capture on180Tam: Clue for ans-Process Origin of Nature’s Rarest

“…Figure 1 shows that the main s-process reaction path (thick arrows) is bypassing 180 Ta m via the ͑n, g͒ sequence along the stable hafnium isotopes, with only marginal feeding by two minor branchings. The decay of the 8 2 isomer in 180 Hf, which is weakly populated by neutron capture on 179 Hf, has been shown to account for about 20% of the observed 180 Ta m abundance [4,5]. This contribution is well determined by the partial cross section to 180 Hf m .…”