Survival of Nature’s Rarest Isotope Ta under Stellar Conditions
نویسندگان
چکیده
The nucleosynthesis of nature’s rarest isotope Ta depends sensitively on the temperature of the astrophysical environment because of depopulation of the long-living isomeric state via intermediate states to the short-living ground state by thermal photons. Reaction rates for this transition have been measured in the laboratory. These ground state rates understimate the stellar rates dramatically because under stellar conditions intermediate states are mainly populated by excitations from thermally excited states in Ta. Full thermalization of Ta is already achieved for typical s-process temperatures around kT = 25 keV. Consequently, for the survival of Ta in the s-process fast convective mixing is required which has to transport freshly synthesized Ta to cooler regions. In supernova explosions Ta is synthesized by photonor neutrino-induced reactions at temperatures above T9 = 1 in thermal equilibrium; independent of the production mechanism, freeze-out from thermal equilibrium occurs at kT ≈ 40 keV, and only 35±4% of the synthesized Ta survive in the isomeric state.
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تاریخ انتشار 2006