Stellar neutron capture cross sections of the Ba isotopes

Abstract

The neutron capture cross sections of 134 Ba, 135 Ba, 136 Ba, and 137 Ba were measured in the energy range from 5 to 225 keV at the Karlsruhe 3.75 MV Van de Graaff accelerator. Neutrons were produced via the 7 Li(p,nfBe reaction by bombarding metallic Li targets with a pulsed proton beam. Capture events were registered with the Karlsruhe 47r Barium Fluoride Detector. Several runs have been performed under different experimental conditions to study the systematic uncertainties, which resulted mainly from the large ratios of total to capture cross sections of up to 400. The cross section ratios were determined with an overall uncertainty of "'3%, an improvement by factors of five to eight compared to existing data. Severe discrepancies were found with respect to previous results. Maxwellian averaged neutron capture cross sections were calculated for thermal energies between kT = 10 ke V and 100 ke V. These stellar cross sections were used in an s-process analysis. For the s-only isotopes 134 Ba and 136 Ba the Ns<O">ratio was determined to 0.875±0.025. Hence, a significant brauehing of the s-process path at 134 Cs can be claimed for the first time, in cantrast to predictions from the classical approach. This brauehing yields information on the s-process temperature, indicating values araund T 8 = 2. The new cross sections arealso important for the interpretation of barium isotopic anomalies, which were recently discovered in SiC grains of carbonaceous chondrite meteorites. Tagether with the results from previous experiments on tellurium and samarium, a general improvement of the Ns<O">systematics in the mass range A = 120 to 150 is achieved. This allows for a more reliable separation of s-and r-process yields, resulting in an improved assignment of the respective contributions to eiemental barium that is required for comparison with stellar observations. ZUSAMMENFASSUNG

Stellar neutron capture cross sections of the Gd isotopes

“…The required input for this calculation are the individual neutron capture cascades and their relative contributions to the total capture cross section as well as the detector efficiency for monoenergetic [-rays in the energy range up to 10 MeV. Since this correction is limiting the accuracy of the present experimental technique, an attempt was made to compare these corrections with two independent sets of capture cascades and capture [-ray spectra of the involved isotopes, one calculated by G. Reffo according to the statistical and optical models [19] as in the previous measurements with the 4?T BaF 2 detector [1,2,3], and a second set calculated by M. Uhl [20]. Both calculations are based on the Hauser Feshbach approach, but the way how the gamma cascades are calculated is different.…”

“…Hence, even a small misalignment of the detector could have caused the observed deviations. Correspondingly, this e:ffect was avoided in the barium experiment [3], where the larger sample diameter of 22 mm resulted in a significantly better agreement with the evaluated total cross section of carbon. In view of this difficulty only rough estimates for the total cross sections of the gadolinium isotopes were obtained.…”

“…The adopted values of the correction factors F 1 , in the lower part of Table 8 are the average of both calculations, except for 158 Gd. The small number of capture cascades given by Reffo caused strong fluctuations in the capture 1-ray spectrum and, 1 The experimental data are not corrected for threshold effects and cross talking between neighboring detector modules and should be used only for discussing relative differences between isotopes 2 Number of capture cascades covering 95% of the cross section 3 Percentage of cross section covered by 10000 cascades consequently, in the spectrum fraction f. Since these fl.uctuations seem to result from numerical effects and are not justified by the systematic trend in the even gadolinium isotopes, only the data of Uhl were used for this isotope.…”

“…After tellurium [1], samarium [2), and barium [3), gadolinium is the forth of the six elements with two even s-only isotopes. As described in the above references these are particularly important cases where accurate cross sections allow to investigate the related s-process branchings in great detail, in particular with respect to the physical conditions in the helium burning zones of Red Giant stars.…”

“…For the branchings at the tellurium, samarium, and barium isotopes [1,2,3], the brauehing factors, fn, were small and of the order of 0.1. This implies that only a small part of the s-process flow is bypassing the lighter s-only isotope and that the N 8 <u>values of both s-isotopes are about equal.…”