ABSTRACT
The composition of the early Solar System can be inferred from meteorites. Many elements heavier than iron were formed by the rapid neutron capture process (r-process), but the astrophysical sources where this occurred remain poorly understood. We demonstrate that the near-identical half-lives [Formula: see text] of the radioactive r-process nuclei iodine-129 and curium-247 preserve their ratio, irrespective of the time between production and incorporation into the Solar System. We constrain the last r-process source by comparing the measured meteoritic ratio 129I/247Cm = 438 ± 184 with nucleosynthesis calculations based on neutron star merger and magneto-rotational supernova simulations. Moderately neutron-rich conditions, often found in merger disk ejecta simulations, are most consistent with the meteoritic value. Uncertain nuclear physics data limit our confidence in this conclusion.
ABSTRACT
The 146Sm/144Sm ratio in the early solar system has been constrained by Nd/Sm isotope ratios in meteoritic material. Predictions of 146Sm and 144Sm production in the γ process in massive stars are at odds with these constraints, and this is partly due to deficiencies in the prediction of the reaction rates involved. The production ratio depends almost exclusively on the (γ,n)/(γ,α) branching at 148Gd. A measurement of 144Sm(α,γ)148Gd at low energy had discovered considerable discrepancies between cross-section predictions and the data. Although this reaction cross section mainly depends on the optical α+nucleus potential, no global optical potential has yet been found that can consistently describe the results of this and similar α-induced reactions at the low energies encountered in astrophysical environments. The untypically large deviation in 144Sm(α,γ) and the unusual energy dependence can be explained, however, by low-energy Coulomb excitation, which is competing with compound nucleus formation at very low energies. Considering this additional reaction channel, the cross sections can be described with the usual optical potential variations, compatible with findings for (n, α) reactions in this mass range. Low-energy (α, γ) and (α, n) data on other nuclei can also be consistently explained in this way. Since Coulomb excitation does not affect α emission, the 148Gd(γ,α) rate is much higher than previously assumed. This leads to very small 146Sm/144Sm stellar production ratios, in even more pronounced conflict with the meteorite data.
ABSTRACT
BACKGROUND AND PURPOSE: It has been suggested that the actual dose rate of an irradiating source may be a distinct influencing factor for the biological effect after brachytherapy with ruthenium-106 for uveal melanoma. The purpose of this study was to investigate a hypothesized impact of the dose rate on the clinical and echographic course after brachytherapy. PATIENTS AND METHODS: In total, 45 patients were included in this retrospective study. According to the actual dose rate, two groups were defined: group 1 with a dose rate <4 Gy/h and group 2 with a dose rate >or=4 Gy/h. Regarding age, tumor height, basal diameter, scleral and apical dose, differences between the groups were not significant. Clinical parameters, including early and late side effects, and echographic courses were compared. RESULTS: A significantly lower metastatic rate was found in group 2. Using univariate Cox proportional hazards regression, only dose rate predicted metastatic spread significantly (p<0.05), while in a multivariate analysis, using age at the time of treatment, greatest tumor height and greatest basal diameter as covariates, the variable dose rate was of borderline significance (p=0.077). Patients in group 2 had more early side effects and more pronounced visual decline, but these differences were of borderline significance with p-values of 0.072 and 0.064, respectively. CONCLUSION: These data suggest that a higher dose rate may confer a lower risk for metastatic spread, but may be associated with more side effects and more pronounced visual decline.
