No abstract available

[en] It is believed that ²⁶Al, a short-lived (t_1/2 = 0.73 Ma) and now extinct radionuclide, was uniformly distributed in the nascent solar system (SS) with the initial ²⁶Al/²⁷Al ratio of ∼5.2 x 10^-5, suggesting an external, stellar origin rather than local, solar source. However, the stellar source of ²⁶Al and the manner in which it was injected into the SS remain controversial: the ²⁶Al could have been produced by an asymptotic giant branch star, a supernova, or a Wolf-Rayet star and injected either into the protosolar molecular cloud, protosolar cloud core, or protoplanetary disk. Corundum (Al₂O₃) is predicted to be the first condensate from a cooling gas of solar composition. Here we show that micron-sized corundum condensates from ¹⁶O-rich (Δ¹⁷O ∼ -25 per mille ) gas of solar composition recorded heterogeneous distribution of ²⁶Al at the birth of the SS: the inferred initial ²⁶Al/²⁷Al ratio ranges from ∼6.5x10^-5 to <2x10^-6; 52% of corundum grains measured are ²⁶Al-poor. Abundant ²⁶Al-poor, ¹⁶O-rich refractory objects include grossite- and hibonite-rich calcium-aluminum-rich inclusions (CAIs) in CH (high metal abundance and high iron concentration) chondrites, platy hibonite crystals in CM (Mighei-like) chondrites, and CAIs with fractionation and unidentified nuclear effects CAIs chondrites. Considering the apparently early and short duration (<0.3 Ma) of condensation of refractory ¹⁶O-rich solids in the SS, we infer that ²⁶Al was injected into the collapsing protosolar molecular cloud and later homogenized in the protoplanetary disk. The apparent lack of correlation between ²⁶Al abundance and O-isotope composition of corundum grains constrains the stellar source of ²⁶Al in the SS.

No abstract available

[en] We have measured the ²⁶Al(n,α₀)²³Mg and ²⁶Al(n,p₁)²⁶Mg^* cross sections from thermal energy to approximately 10 keV and 70 keV, respectively. These reactions are thought to be the major mechanisms for the destruction of ²⁶Al in many nucleosynthesis environments; hence, an accurate determination of their rates is important for understanding the observations of γ rays from open-quotes liveclose quotes ²⁶Al in our galaxy and of open-quotes extinctclose quotes ²⁶Al in meteorites. The astrophysical rate for the ²⁶Al(n,α₀)²³Mg reaction determined from our measurements is in good agreement with the rate determined via inverse measurements. On the other hand, the rate we determined for the ²⁶Al(n,p₁)²⁶Mg^* reaction is significantly larger than previously reported. In addition, we were able to determine this rate in the temperature range below 0.2 GK which was not covered by previous measurements. This lower temperature range may be important for understanding the production of ²⁶Al in Red Giant stars. Both of our rates are significantly different than the rates used in most nucleosynthesis calculations. We discuss the impact of our measurements on the nucleosynthesis of ²⁶Al. copyright 1997 The American Physical Society

No abstract available

[en] Several different lines of physical reasoning have converged on the importance of the radioactive nucleus ²⁶Al. The sciences of meteoritics, nucleosynthesis, gamma-ray astronomy, galactic chemical evolution, solar system formation, and interstellar chemistry all place this nucleus in a central position with possible profound implications. Perhaps more importantly the study of this radioactivity can unite these diverse fields in a complicated framework which will benefit all of them. This review traces the evolution of ideas concerning ²⁶Al in the context of these disciplines. ²⁶Al was first discussed for the possibility that its decay energy could melt meteorite parent bodies, and its daughter, ²⁶Mg, was later found in meteorites with enhanced abundance. It was also among the first radioactivities expected to be synthesized in interestingly large quantities in nulceosynthetic events. The first definitive detection of gamma-rays from an interstellar radioactivity is that of 1.809 MeV gamma-rays from ²⁶Al. This discovery has many implications, some of which are outlined here. The whole problem of isotopic anomalies in meteorites is greatly influenced by the specific issues surrounding excess ²⁶Mg, whether it represents in situ decay of ²⁶Al or memory of conditions of the ISM. The relationships among these ideas and their implications are examined. (orig.)

[en] The basic features of the ²²Ne and ²⁶Al nucleosynthesis in stellar outbursts have been explored in the most straightforward framework. Both isotopic anomalies of these two elements, recently observed in some meteoritic inclusions, may be accounted for in simple supernova events. The conditions encountered in classical models of novae outbursts are less favourable to such simultaneous production. (orig./WL) 891 WL/orig.- 892 ARA

[en] Non-magmatic iron meteorites, including the IIE group, can provide important insights into the history of metal-silicate differentiation and collisions on planetesimals. To better constrain the evolution of metal segregation and impacts on the IIE parent body, W isotopic data are reported for 10 IIE iron meteorites and a metal vein from the Portales Valley H6 chondrite. In addition, Pt isotopic data were obtained to quantify cosmic ray-induced neutron capture effects on W isotopes. After correction for these effects, the IIE iron meteorites exhibit variable pre-exposure 182W compositions, translating into Hf-W model age clusters of ~4–5 million years (Ma), ~10 Ma, ~15 Ma, and ~27 Ma after CAI formation. These distinct ¹⁸²W clusters likely represent samples from several discrete metallic melt pools on the IIE parent asteroid. The earliest metal segregation event at ~4–5 Ma was likely facilitated by ²⁶Al decay as an internal heat source. By contrast, the younger Hf-W model ages may not be chronologically meaningful, and probably reflect the effects of secondary mixing and re-equilibration of metal and silicates, likely facilitated by impacts on the IIE parent body. Thus, contrary to prior work, the Hf-W systematics of IIE iron meteorites do not require a protracted history of metal-silicate separation on the IIE parent body. Instead the results of this study are fully consistent with a single partial metal-silicate differentiation event driven by endogenic heating at ~4–5 Ma, followed by one or multiple impact events causing mixing and re-equilibration of metal and silicates at a later stage. As a result, the exact timing of these impact event(s) remains poorly constrained, but they most likely occurred in the first few tens of Ma of Solar System history.

[en] One of the most interesting questions concerns the nature of the weakly active nucleus of the Galaxy. Gamma spectroscopy contributes an important share to clarify the problem. Beside the measurement of the 0.511 MeV line arising from positron annihilation from the galactic center, a 1.81 MeV line was found, stemming from the decay of Al²⁶ and subsequent formation of Mg²⁶. A scenario for the processes going on in the galactic center is outlined. (UPO)

[en] The accelerator facility ALICE were used to count directly small quantities (approximately 5x10⁹ atoms) of the isotope ²⁶Al (half-life=7.3x10⁵ years). It is shown how measurement of this nuclide in marine sediments, in conjunction with similar measurements of ¹⁰Be (half-life=1.5x10⁶ years), can be used as an absolute dating technique. The method used here to avoid interference from the isobar ²⁶Mg is applicable to a number of other isobars of interesting long lived nuclides, thus expanding the potential application of the accelerator technique