|Natural abundance||trace (cosmogenic)|
|Decay mode||Decay energy (MeV)|
|Complete table of nuclides|
Aluminium-26, 26Al, is a radioactive isotope of the chemical element aluminium, decaying by either of the modes beta-plus or electron capture, both resulting in the stable nuclide magnesium-26. The half-life of 26Al is 7.17×105 years. This is far too short for the isotope to survive to the present, but a small amount of the nuclide is produced by collisions of argon atoms with cosmic ray protons.
Aluminium-26 also emits gamma rays and X-rays. The x-rays and Auger electrons are emitted by the excited atomic shell of the daughter 26Mg after the electron capture which typically leaves a hole in one of the lower sub-shells.
Because it is radioactive it should be stored behind at least 5 cm (2 in) of lead and special tools should be used for transfer, use, and storage. Contact with 26Al may result in radiological contamination.
Dating of meteorites
Aluminium-26 can be used to calculate the terrestrial age of meteorites. After the breakup of the meteorite parent body, it will be bombarded by cosmic rays, which will saturate it in aluminium-26. After falling to earth, 26Al production ceases, which means that the amount of 26Al in the sample can be used to calculate the date the meteorite fell to earth.
Occurrence in the interstellar medium
The isotope is mainly produced in supernovas ejecting many radioactive nuclides in the interstellar medium. The isotope is believed to provide enough heat to small planetary bodies so as to differentiate their interiors, such as has been the case in the early history of the asteroids 1 Ceres and 4 Vesta. This isotope also features in hypotheses regarding the equatorial bulge of Saturn's moon Iapetus.
Before 1954, the measured half-life of aluminium-26 was determined to be 6.3 seconds. After theoretical evidence occurred that this could be the half-life of a metastable state (isomer) of aluminium-26, the ground state was produced by bombardment of magnesium-26 and magnesium-25 with deuterons in the cyclotron of the University of Pittsburgh. The first half-life was determined to be in the range of 106 years.
The Fermi beta decay half-life of the aluminium-26 metastable state is of interest in the experimental testing of two components of the standard model, namely, the conserved-vector-current hypothesis and the required unitarity of the Cabibbo-Kobayashi-Maskawa matrix. The decay is superallowed. The 2011 measurement of the half life of Al-26(m) is 6346.54 ± 0.46(statistical) ± 0.60(system) milliseconds. In considering the known melting of small planetary 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.
Their search was undertaken because hitherto there was no known radioactive isotope of Al that might be useful as a tracer. Theoretical considerations suggested that a state of 26Al should exist. The life time of 26Al ( ) was not then known (104-106 yr.) The search for 26Al took place over many years, long after the discovery of extinct 129I ( by Reynolds (1960, Physical Review Letters v 4, p 8) which showed that contribution from stellar sources formed ~108 years before the Sun had contributed to the solar system mix. The asteroidal materials that provide meteorite samples were long known to be from the early solar system. Analysis of Ne in some meteorites showed that they had a widely variable isotopic composition. Then Black found that essentially pure 22Ne was one of the Ne components present which pointed to an extra solar origin of dust from red giant stars. Eberhardt found that the material obtained by essentially chemically destroying the meteorite matrix resulted in a colloidal sludge residue which contained almost pure 22Ne. This supported the idea that presolar dust grains were present in the sludge from the meteorite.
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. 26Al is only present today in solar system materials as the result of cosmic reactions on unshielded materials at an extremely low level. Thus, any original 26Al in the early Solar system is now extinct.
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 was shown by Lee, et al. The value (26Al/27Al ~ 5x10−5) 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 (Hutcheon & Hutchison). Other extinct radioactive nuclei, which clearly had a stellar origin, were then being discovered.
That 26Al was 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 of 26Al source. This represents a quasi steady state inventory corresponding to two solar masses of 26Al was distributed[clarification needed]. This discovery was greatly expanded on by observations from the Compton Gamma Ray Observatory using the COMPTEL telescope in the galaxy. Subsequently, the 60Fe lines (1.173 & 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, It was found that the carrier grains in micron size, acid resistant ultra-refractory materials (e.g. C, SiC) carried out 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  These studies on micron scale grains were possible as a result of the development of surface ion mass spectrometry at high mass resolution with a focused beam developed by G. Slodzian & R.Castaing with the CAMECA Co.
The production of 26Al by cosmic ray interactions in unshielded materials is used as a monitor of the time of exposure to cosmic rays. The amounts are far below the initial inventory that is found in very early solar system debris.
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