In considering the known melting of small bodies in the early Solar System,
H. C. Urey noted that the naturally occurring long-lived radioactive nuclei (40K, 238U, 235U and 232Th) were insufficient heat sources. He proposed that the heat sources from short lived nuclei from newly formed stars might be the source and identified 26Al as the most likely choice. This proposal was made well before the general problems of
stellar nucleosynthesis of the nuclei were known or understood. This conjecture was based on the discovery of 26Al in a Mg target by Simanton, Rightmire, Long & Kohman. The
Allende meteorite, which fell in 1969, contained abundant
calcium–aluminium-rich inclusions (CAIs). These are very refractory materials and were interpreted as being condensates from a hot
solar nebula. then discovered that the oxygen in these objects was enhanced in 16O by ~5% while the 17O/18O was the same as terrestrial. This clearly showed a large effect in an abundant element that might be nuclear, possibly from a stellar source. These objects were then found to contain strontium with very low 87Sr/86Sr indicating that they were a few million years older than previously analyzed meteoritic material and that this type of material would merit a search for 26Al. To establish the presence of 26Al in very ancient materials requires demonstrating that samples must contain clear excesses of 26Mg/24Mg which correlates with the ratio of 27Al/24Mg. The stable 27Al is then a surrogate for extinct 26Al. The different 27Al/24Mg ratios are coupled to different chemical phases in a sample and are the result of normal chemical separation processes associated with the growth of the crystals in the CAIs. Clear evidence of the presence of 26Al at an abundance ratio of 5×10−5 (relative to 27Al, the standard way of quantifying this isotope) was shown by Lee et al. The value (26Al/27Al ~ 5) has now been generally established as the high value in early Solar System samples and has been generally used as a refined time scale chronometer for the early Solar System. Lower values imply a more recent time of formation. If this 26Al is the result of pre-solar stellar sources, then this implies a close connection in time between the formation of the Solar System and the production in some exploding star. Many materials which had been presumed to be very early (e.g. chondrules) appear to have formed a few million years later. Other extinct radioactive nuclei, which clearly had a stellar origin, were then being discovered. That 26Al is present in the interstellar medium as a major
gamma ray source was not explored until the development of the high-energy astronomical observatory program. The
HEAO-3 spacecraft with cooled Ge detectors allowed the clear detection of 1.808 MeV gamma lines from the central part of the galaxy from a distributed 26Al source. Subsequently, the 60Fe lines (1.173 MeV and 1.333 Mev) were also detected showing the relative rates of decays from 60Fe to 26Al to be 60Fe/26Al ~ 0.11. In pursuit of the carriers of 22Ne in the sludge produced by chemical destruction of some meteorites, carrier grains in micron size, acid-resistant ultra-refractory materials (e.g. C,
SiC) were found by E. Anders & the Chicago group. The carrier grains were clearly shown to be circumstellar condensates from earlier stars and often contained very large enhancements in 26Mg/24Mg from the decay of 26Al with 26Al/27Al sometimes approaching 0.2. The production of 26Al by
cosmic ray interactions in unshielded materials (meteorites) is used as a monitor of the last time of exposure to cosmic rays. The maximum amount detected are far below the initial inventory that was found in the very early solar system. == Metastable state ==