skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Survival of nature's rarest isotope {sup 180}Ta under stellar conditions

Abstract

The nucleosynthesis of nature's rarest isotope {sup 180}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 underestimate the stellar rates dramatically because under stellar conditions intermediate states are mainly populated by excitations from thermally excited states in {sup 180m}Ta. Full thermalization of {sup 180}Ta is already achieved for typical s-process temperatures around kT=25 keV. Consequently, for the survival of {sup 180}Ta in the s-process fast convective mixing is required which has to transport freshly synthesized {sup 180}Ta to cooler regions. In supernova explosions {sup 180}Ta is synthesized by photon- or neutrino-induced reactions at temperatures above T{sub 9}=1 in thermal equilibrium; independent of the production mechanism, freeze-out from thermal equilibrium occurs at kT{approx_equal}40 keV, and only 35{+-}4% of the synthesized {sup 180}Ta survive in the isomeric state.

Authors:
 [1];  [2];  [3]
  1. Diakoniekrankenhaus Schwaebisch Hall, D-74523 Schwaebisch Hall (Germany)
  2. Forschungszentrum Karlsruhe, Institut fuer Kernphysik, P.O. Box 3640, D-76021 Karlsruhe (Germany)
  3. Dipartimento di Fisica Generale, Universita di Torino, Via P. Giuria 1, I-10125 Torino (Italy)
Publication Date:
OSTI Identifier:
20990928
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review. C, Nuclear Physics; Journal Volume: 75; Journal Issue: 1; Other Information: DOI: 10.1103/PhysRevC.75.012802; (c) 2007 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
73 NUCLEAR PHYSICS AND RADIATION PHYSICS; EXCITATION; EXCITED STATES; FREEZING OUT; GROUND STATES; KEV RANGE 10-100; MASS NUMBER; NEUTRINO-NUCLEON INTERACTIONS; NEUTRINOS; NUCLEOSYNTHESIS; PHOTON-NUCLEON INTERACTIONS; PHOTONS; PHOTONUCLEAR REACTIONS; REACTION KINETICS; S PROCESS; SUPERNOVAE; TANTALUM 180; THERMAL EQUILIBRIUM; THERMALIZATION

Citation Formats

Mohr, P., Kaeppeler, F., and Gallino, R.. Survival of nature's rarest isotope {sup 180}Ta under stellar conditions. United States: N. p., 2007. Web. doi:10.1103/PHYSREVC.75.012802.
Mohr, P., Kaeppeler, F., & Gallino, R.. Survival of nature's rarest isotope {sup 180}Ta under stellar conditions. United States. doi:10.1103/PHYSREVC.75.012802.
Mohr, P., Kaeppeler, F., and Gallino, R.. Mon . "Survival of nature's rarest isotope {sup 180}Ta under stellar conditions". United States. doi:10.1103/PHYSREVC.75.012802.
@article{osti_20990928,
title = {Survival of nature's rarest isotope {sup 180}Ta under stellar conditions},
author = {Mohr, P. and Kaeppeler, F. and Gallino, R.},
abstractNote = {The nucleosynthesis of nature's rarest isotope {sup 180}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 underestimate the stellar rates dramatically because under stellar conditions intermediate states are mainly populated by excitations from thermally excited states in {sup 180m}Ta. Full thermalization of {sup 180}Ta is already achieved for typical s-process temperatures around kT=25 keV. Consequently, for the survival of {sup 180}Ta in the s-process fast convective mixing is required which has to transport freshly synthesized {sup 180}Ta to cooler regions. In supernova explosions {sup 180}Ta is synthesized by photon- or neutrino-induced reactions at temperatures above T{sub 9}=1 in thermal equilibrium; independent of the production mechanism, freeze-out from thermal equilibrium occurs at kT{approx_equal}40 keV, and only 35{+-}4% of the synthesized {sup 180}Ta survive in the isomeric state.},
doi = {10.1103/PHYSREVC.75.012802},
journal = {Physical Review. C, Nuclear Physics},
number = 1,
volume = 75,
place = {United States},
year = {Mon Jan 15 00:00:00 EST 2007},
month = {Mon Jan 15 00:00:00 EST 2007}
}
  • The influence of the stellar plasma on the production and destruction of K isomers is studied for the examples {sup 176}Lu and {sup 180}Ta. Individual electromagnetic transitions are enhanced predominantly by nuclear excitation by electron capture, whereas the other mechanisms of electron scattering and nuclear excitation by electron transition give only minor contributions. It is found that individual transitions can be enhanced significantly for low transition energies below 100 keV. Transitions with higher energies above 200 keV are practically not affected. Although one low-energy transition in {sup 180}Ta is enhanced by up to a factor of 10, the stellar transitionmore » rates from low-K to high-K states via so-called intermediate states in {sup 176}Lu and {sup 180}Ta do not change significantly under s-process conditions. The s-process nucleosynthesis of {sup 176}Lu and {sup 180}Ta remains essentially unchanged.« less
  • The neutron capture cross sections of /sup 178/,/sup 179/,/sup 180/Hf were measured in the energy range 2.6 keV to 2 MeV. The average capture cross sections were derived and fitted in terms of strength functions. Resonance parameters for the observed resonances below 10 keV were determined by shape analysis. Maxwellian-averaged capture cross sections were computed for thermal energies with kT between 5 and 100 keV. The cross sections for kT = 30 keV were used to determine the population probability of the 8- isomeric level in /sup 180/Hf by neutron capture as (1.24 +- 0.06)% and the r-process abundance ofmore » /sup 180/Hf as 0.0290 (Si = 10/sup 6/). These quantities served to analyze s- and r-process nucleosynthesis of /sup 180/Ta, nature's rarest stable isotope.« less
  • The irradiation of an enriched sample of /sup 180/Ta/sup m/ with bremsstrahlung from a linear accelerator having an end point energy of 6 MeV has excited a strong channel for the reaction /sup 180/Ta/sup m/(..gamma..,..gamma..')/sup 180/Ta, which requires a total spin change of 8(h/2..pi..). An integrated cross section of 4.8 x 10/sup -25/ cm/sup 2/ keV has been found. This is a large value exceeding by 2 orders of magnitude known cross sections for (..gamma..,..gamma..') reactions producing isomers of other species.
  • The neutron capture cross sections of /sup 178,179,180/Hf were measured in the energy range 2.6 keV to 2 MeV. The average capture cross sections were calculated and fitted in terms of strength functions. Resonance parameters for the observed resonances below 10 keV were determined by a shape analysis. Maxwellian averaged capture cross sections were computed for thermal energies kT between 5 and 100 keV. The cross sections for kT = 30 keV were used to determine the population probability of the 8/sup -/ isomeric level in /sup 180/Hf by neutron capture as (1.24 +- 0.06) % and the r-process abundancemore » of /sup 180/Hf as 0.0290 (Si equivalent10/sup 6/). These quantities served to analyze s- and r-process nucleosynthesis of /sup 180/Ta.« less
  • Some effects of high temperature and density on the half-lives of /sup 180/Ta/sup g/ and /sup 180/Ta/sup m/ have been investigated. It is found that the half-life of /sup 180/Ta/sup g/ is greater in stellar environments than in the laboratory, while that of /sup 180/Ta/sup m/ may be much shorter than its laboratory value. The implications of these results for the nucleosynthesis of /sup 180/Ta are discussed.