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Title: Solar abundance of {sup 176}Lu and s-process nucleosynthesis

Abstract

The isotopic composition of lutetium has been measured with high precision using a thermal ionization mass spectrometer whose linearity was verified by measuring an isotopically certified reference material for potassium prepared by the National Institute of Standards and Technology (NIST 985). The abundance sensitivity of the mass spectrometer for the measured ion beams of Lu{sup +} was examined to ensure the absence of tailing effects and interfering ion beams. The isotope fractionation of the measured {sup 176}Lu/{sup 175}Lu ratio was estimated with reference to the isotope fractionation of ytterbium (whose isotopes are in the same mass region as lutetium), which was recently measured in this laboratory using gravimetrically prepared solutions of the enriched isotopes {sup 171}Yb and {sup 176}Yb. This is the first reported publication in which the measured isotope ratio of Lu has been corrected for isotope fractionation. An accurate determination of the abundance of {sup 176}Lu is required because of the importance of this isotope in cosmochronometry, cosmothermometry, and s-process branching studies. An accurate abundance of {sup 176}Lu is also required as it is the parent nuclide of the {sup 176}Lu/{sup 176}Hf geochronometer. The measured isotopic composition of Lu, corrected for isotope fractionation, is {sup 176}Lu/{sup 175}Lu =more » 0.026680 {+-} 0.000013, which gives isotope abundances for {sup 175}Lu of 97.4013 {+-} 0.0012% and of {sup 176}Lu of 2.5987 {+-} 0.0012%. The isotope abundances and relative atomic masses of the two isotopes give an atomic weight of 174.9668 {+-} 0.0001, which is in good agreement with the present Standard Atomic Weight A{sub r}(Lu) = 174.967 {+-} 0.001, but with improved accuracy. An accurate assessment of the {sup 176}Lu/{sup 175}Lu ratio is important in order to calculate the Solar System abundances of {sup 175}Lu and {sup 176}Lu for astrophysical evaluations. The experimentally determined Solar System abundances for {sup 175}Lu and {sup 176}Lu of 0.0347918 {+-} 0.0000004 and 0.0009282 {+-} 0.0000004, respectively (as compared to silicon equals 10{sup 6} atoms), should now be used for these purposes. This determination of the isotopic composition of Lu also demonstrates that the presently accepted half-life of {sup 176}Lu needs to be reevaluated.« less

Authors:
;  [1]
  1. Department of Applied Physics, Curtin University of Technology, GPO Box U1987, Perth, Western Australia, 6845 (Australia)
Publication Date:
OSTI Identifier:
20771500
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review. C, Nuclear Physics; Journal Volume: 73; Journal Issue: 4; Other Information: DOI: 10.1103/PhysRevC.73.045806; (c) 2006 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; BRANCHING RATIO; ELEMENT ABUNDANCE; EVALUATION; FRACTIONATION; HAFNIUM 176; HALF-LIFE; ION BEAMS; ISOTOPE RATIO; LUTETIUM 175; LUTETIUM 176; NUCLEOSYNTHESIS; S PROCESS; SENSITIVITY; SOLAR SYSTEM; YTTERBIUM 171; YTTERBIUM 176

Citation Formats

Laeter, J.R. de, and Bukilic, N. Solar abundance of {sup 176}Lu and s-process nucleosynthesis. United States: N. p., 2006. Web. doi:10.1103/PhysRevC.73.045806.
Laeter, J.R. de, & Bukilic, N. Solar abundance of {sup 176}Lu and s-process nucleosynthesis. United States. doi:10.1103/PhysRevC.73.045806.
Laeter, J.R. de, and Bukilic, N. Sat . "Solar abundance of {sup 176}Lu and s-process nucleosynthesis". United States. doi:10.1103/PhysRevC.73.045806.
@article{osti_20771500,
title = {Solar abundance of {sup 176}Lu and s-process nucleosynthesis},
author = {Laeter, J.R. de and Bukilic, N.},
abstractNote = {The isotopic composition of lutetium has been measured with high precision using a thermal ionization mass spectrometer whose linearity was verified by measuring an isotopically certified reference material for potassium prepared by the National Institute of Standards and Technology (NIST 985). The abundance sensitivity of the mass spectrometer for the measured ion beams of Lu{sup +} was examined to ensure the absence of tailing effects and interfering ion beams. The isotope fractionation of the measured {sup 176}Lu/{sup 175}Lu ratio was estimated with reference to the isotope fractionation of ytterbium (whose isotopes are in the same mass region as lutetium), which was recently measured in this laboratory using gravimetrically prepared solutions of the enriched isotopes {sup 171}Yb and {sup 176}Yb. This is the first reported publication in which the measured isotope ratio of Lu has been corrected for isotope fractionation. An accurate determination of the abundance of {sup 176}Lu is required because of the importance of this isotope in cosmochronometry, cosmothermometry, and s-process branching studies. An accurate abundance of {sup 176}Lu is also required as it is the parent nuclide of the {sup 176}Lu/{sup 176}Hf geochronometer. The measured isotopic composition of Lu, corrected for isotope fractionation, is {sup 176}Lu/{sup 175}Lu = 0.026680 {+-} 0.000013, which gives isotope abundances for {sup 175}Lu of 97.4013 {+-} 0.0012% and of {sup 176}Lu of 2.5987 {+-} 0.0012%. The isotope abundances and relative atomic masses of the two isotopes give an atomic weight of 174.9668 {+-} 0.0001, which is in good agreement with the present Standard Atomic Weight A{sub r}(Lu) = 174.967 {+-} 0.001, but with improved accuracy. An accurate assessment of the {sup 176}Lu/{sup 175}Lu ratio is important in order to calculate the Solar System abundances of {sup 175}Lu and {sup 176}Lu for astrophysical evaluations. The experimentally determined Solar System abundances for {sup 175}Lu and {sup 176}Lu of 0.0347918 {+-} 0.0000004 and 0.0009282 {+-} 0.0000004, respectively (as compared to silicon equals 10{sup 6} atoms), should now be used for these purposes. This determination of the isotopic composition of Lu also demonstrates that the presently accepted half-life of {sup 176}Lu needs to be reevaluated.},
doi = {10.1103/PhysRevC.73.045806},
journal = {Physical Review. C, Nuclear Physics},
number = 4,
volume = 73,
place = {United States},
year = {Sat Apr 15 00:00:00 EDT 2006},
month = {Sat Apr 15 00:00:00 EDT 2006}
}
  • The s-process branching at mass number A=176 depends on the coupling between the high-K ground state and a low-lying low-K isomer in {sup 176}Lu. This coupling is based on electromagnetic transitions via intermediate states at higher energies. The properties of the lowest experimentally confirmed intermediate state at 839 keV are reviewed, and the transition rate between low-K and high-K states under stellar conditions is calculated on the basis of new experimental data for the 839-keV state. Properties of further candidates for intermediate states are briefly analyzed. It is found that the coupling between the high-K ground state and the low-Kmore » isomer in {sup 176}Lu is at least one order of magnitude stronger than previously assumed, leading to crucial consequences for the interpretation of the {sup 176}Lu/{sup 176}Hf pair as an s-process thermometer.« less
  • The effects of photoexcitation and positron annihilation-excitation of /sup 176/Lu/sup g/ to /sup 176/Lu/sup m/ have been investigated. It is found that as a result of these two processes alone, /sup 176/Lu/sup g/ and /sup 176/Lu/sup m/ are in thermal equilibrium at temperatures > or =3.5 x 10/sup 8/ K. This implies that /sup 176/Lu is not a reliable s-process chronometer.
  • 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
  • Gamma-ray branches that connect high-K states to low-K states in the s-process nucleus {sup 176}Lu were observed, thus providing a link between the 58 Gyr, 7{sup -} ground state and the 5.3 h, 1{sup -} isomeric state. High sensitivity and unambiguous placement were achieved through the study of the decay of the 58 {micro}s K{pi} = 14{sup +} isomer using {gamma}-{gamma}-coincidence measurements. The large number of decay paths from the isomer provides a means of populating a broad selection of states from above, resulting, paradoxically, in higher sensitivity than in cases where low-spin input reactions are used. The out-of bandmore » decay widths important for excitation processes in stars are quantified.« less