Neutron-capture cross sections for osmium isotopes and the age of the universe

Abstract

The neutron-capture cross sections for '"'""""'~'"Os have been measured by the neutron-time-of-flight technique from 0.5 eV to 150 keV. Nuclear level spacings were extracted from the low-energy (resonance-regionj data below -1 keV, and average cross sections from the data above 1 keV. The ratio of the Maxwellian-weighted average cross sections for '"Os and '"Os near 30 keV is a vital parameter for the determination of the duration of nucleosynthesis prior to the formation of the solar system, and thus for the determination of the age of the universe by the nuclear-dating technique. The present result of 10.8+, ",F10' yr for the duration of nucleosynthesis is in concordance with the value obtained from U-Th dating. The result for the age of the universe {17~3 &(10' yrj agrees with the value obtained from the globular-cluster method, but clearly exceeds the most recent determination of the Hubble time.NUCLEAB REACTIONS~~~~~Qs(yg p), E= 2-j.5& &0 sured 0(n, y); Maxwelban-averaged 0(k7= 30 keV); extracted average level spacing", deduced age of universe for Be-Os clock. I~INTRODUCTIONBecause the half-life of '8'He is very long (-43 x 109 yr), because both 'MOs and '8'Os are pure s-process nuclei (i.e. , they are shielded by '@W and '8'He, respectively, from the r-process), and because these heavy metals are present in a relatively undisturbed state in primordial meteorites, the ' YHe -' 'Os nuclear p-decay clock constitutes probably the best basis for a radiogenic determination of the age of the elements.This method was suggested originally by Clayton, ' who pointed out its advantages, particularly over those methods that depend upon a knowledge of the r-process production rates, Clayton also pointed out the key role played by the neutron-capture cross sections for '@Os and '8'Os (or rather, by their ratio) in this determination. Such measurements, however, would have been beset with formidable experimental difficulties until the sufficiently massive and isotopicaQy pure samples of these rare osmium isotopes were manufactured for the measurements reported here. ' Subsequent measurements using these enriched samples have been. reported by Browne et al. and by Winters et al. 4 The results of all three measurements are compared in Sec. IV.In addition to the required ratio of Mamvellianaveraged 30-keg cross sections, a correction factor is needed in order to account for neutron capture by ' 'Os nuclei in their 9.8-keg first excited state, which also is populated significantly in. a stellar environment. A Hauser-Feshbach calculations of this correction factor has been made by Woosley and Fowler. 8 Among the important input parameters for this calculation are the average level spacings for '8'Os ('@Os+n) and '880s ('8'Os + n). We have been able to extract improved values for these level spacings as well from our low-energy (&1 keV) data.These results, together with values for the halflife of '8'He and the elemental abundance ratio of osmium and rhenium in carbonaceous chxondites and the use of an expon. ential model of the ...

Neutron physics of the Re/Os clock. III. Resonance analyses and stellar (n,γ) cross sections ofOs186,187,1

“…5 with the experimental results obtained at n TOF [18] and in previous measurements [39][40][41]. One finds that the use of the deformed OMP sets does not provide a better description of the capture cross sections in the energy range of interest compared to the spherical OMP approach.…”

Neutron physics of the Re/Os clock. I. Measurement of the (n,γ) cross sections ofOs186,187,188Mosconi1, Fujii2, Mengoni3 et al. 2010Phys. Rev. C311160View full textAdd to dashboardCite

“…Because of their smaller cross sections 190 Os and 192 Os contribute only a rather small part to the correction. The more significant contribution of 189 Os was obtained with [15], Browne and Berman [16], and Segawa et al [17]. The present data are averaged over 20 bins per energy decade.…”

“…The code for isotopic corrections was validated by tests with cross sections from a database, which were artificially contaminated with several impurities. The contributions of 190,192 Os were considered by the cross-section data of Browne and Berman [16], which have been confirmed at 25 keV by a recent activation measurement [42]. At higher energies, these data were extrapolated by means of the energy dependence adopted from the JEFF-3.1 evaluation [43].…”

“…In 186 Os, the SEF is much smaller, because only the first excited state at 137.2 keV, which is considerably less populated, contributes to the SEF correction. Previous measurements of the (n,γ ) cross sections for 186,187 Os were reported in limited energy ranges only, from 2.75 keV to 2.65 MeV [15], from 1 to 135 keV [16], and from 5 to 90 keV [17]. Below 1 keV, experimental information is rather incomplete [18], leading to large uncertainties in the MACSs at the low thermal energies typical of the s process during the interpulse phases of asymptotic giant branch (AGB) stars [19].…”

“…Below 1 keV, experimental information is rather incomplete [18], leading to large uncertainties in the MACSs at the low thermal energies typical of the s process during the interpulse phases of asymptotic giant branch (AGB) stars [19]. The MACS at low thermal energies are particularly important because the interpulse phase contributes most of the s-process neutron exposure via the 13 C(α,n) 16 O reaction [20]. Among the available cross section sets, the MACSs of 186 Os exhibit some discrepancies, especially at energies close to the first excited state of 186 Os at 130 keV, where a pronounced step in the cross section is expected.…”