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HK: Physik der Hadronen und Kerne
HK 33: Postersitzung / Kern- und Teilchen- Astrophysik
HK 33.8: Poster
Wednesday, March 24, 1999, 10:15–12:00, Foyer
The stellar (n,γ) cross section of 179Ta — •C. Arlandini, M. Schumann, and F. Käppeler — Forschungszentrum Karlsruhe, Institut für Kernphysik, Postfach 3640, D–76021 Karlsruhe
Nature’s rarest isotope, 180Ta, is also the only isotope that exists in an isomeric state (t1/2>1.2× 1015 yr). Despite its low abundance its origin is still not understood. Among the suggested nucleosynthetic processes, the s process is the only one that allows a quantitative analysis, via the decay of the 5.5 h isomeric state of 180Hf, that is partially populated by the 179Hf(n,γ) reaction [1,2], and via 179Ta(n,γ), when the 179Hf branching opens up at s-process conditions [3]. The stellar (n,γ) cross section of the unstable nucleus 179Ta (t1/2=665 d) has been investigated for the first time. The required 179Ta was produced by the 180Hf(p,2n) reaction, and then chemically separated from the hafnium matrix by means of liquid-liquid acid chromatography. In this way, two samples of ∼1016 179Ta atoms could be obtained, which were irradiated in a quasi-stellar neutron spectrum simulating a thermal energy of kT=25 keV. The induced activity was measured by detecting the conversion electrons from the decay of the short-lived ground state of 180Ta in a Mini-Orange spectrometer. The preliminary results of the experiment and their astrophysical consequences are discussed.
[1] H. Beer, R. A. Ward , Nature 291, 308 (1981).
[2] H. Beer, R. L. Macklin , Phys. Rev. C 26, 1404 (1982).
[3] K. Yokoi, K. Takahashi , Nature 305, 198 (1983).