| General | |
|---|---|
| Symbol | 26Al |
| Names | aluminium-26 |
| Protons(Z) | 13 |
| Neutrons(N) | 13 |
| Nuclide data | |
| Natural abundance | trace (cosmogenic) |
| Half-life(t1/2) | 7.17×105 years[1] |
| Spin | 5+ |
| Decay modes | |
| Decay mode | Decay energy (MeV) |
| β+ | 4.004[2] |
| ε | 4.004 |
| Isotopes of aluminium Complete table of nuclides | |
Aluminium-26 (26Al,Al-26) is aradioactive isotope of thechemical elementaluminium, decaying by eitherpositron emission orelectron capture to stablemagnesium-26. Thehalf-life of26Al is 717,000 years. This is far too short for the isotope to survive as aprimordial nuclide, but a small amount of it is produced by collisions of atoms withcosmic rayprotons.[3]
Decay of aluminium-26 also producesgamma rays andX-rays.[4] The x-rays andAuger electrons are emitted by the excited atomic shell of the daughter26Mg after the electron capture which typically leaves a hole in one of the lower sub-shells.
Because it is radioactive, it is typically stored behind at least 5 centimetres (2 in) of lead. Contact with26Al may result in radiological contamination. This necessitates special tools for transfer, use, and storage.[4]
Aluminium-26 can be used to calculate the terrestrial age ofmeteorites andcomets. It is produced in significant quantities in extraterrestrial objects viaspallation ofsilicon alongsideberyllium-10, though after falling to Earth,26Al production ceases and its abundance relative to othercosmogenic nuclides decreases. Absence of aluminium-26 sources onEarth is a consequence of Earth's atmosphere obstructing silicon on the surface and low troposphere from interaction with cosmic rays. Consequently, the amount of26Al in the sample can be used to calculate the date the meteorite fell to Earth.[3]
The gamma ray emission from the decay of aluminium-26 at 1809 keV was the first observed gamma emission from theGalactic Center. The observation was made by theHEAO-3 satellite in 1984.[5][6]
26Al is mainly produced insupernovae ejecting many radioactive nuclides in theinterstellar medium. The isotope is believed to be crucial for the evolution of planetary objects, providing enough heat to melt and differentiate accretingplanetesimals. This is known to have happened during the early history of the asteroids1 Ceres and4 Vesta.[7][8][9]26Al has been hypothesized to have played a role in the unusual shape ofSaturn's moonIapetus. Iapetus is noticeably flattened and oblate, indicating that it rotated significantly faster early in its history, with a rotation period possibly as short as 17 hours. Heating from26Al could have provided enough heat in Iapetus to allow it to conform to this rapid rotation period, before the moon cooled and became too rigid to relax back into hydrostatic equilibrium.[10]
The presence of thealuminium monofluoride molecule as the26Alisotopologue inCK Vulpeculae, which is an unknown type of nova, constitutes the first solid evidence of an extrasolar radioactive molecule.[11]
In considering the known melting of small bodies[12] in the early Solar System,H. C. Urey noted that the naturally occurring long-lived radioactive nuclei (40K,238U,235U and232Th) were insufficient heat sources. He proposed that the heat sources from short lived nuclei from newly formed stars might be the source and identified26Al as the most likely choice.[13][14] This proposal was made well before the general problems ofstellar nucleosynthesis of the nuclei were known or understood. This conjecture was based on the discovery of26Al in a Mg target by Simanton, Rightmire, Long & Kohman.[15]
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 long-lived26Al should exist - the life was not then known; it was only estimated to be between 104 and 106 years. The search for26Al took place over many years, long after the discovery of theextinct radionuclide129I which showed that nucleosynthesis not more than ~108 years before its formation had contributed to the Solar System. The asteroidal materials that provide meteorite samples were long known to be from the early Solar System.[16]
TheAllende meteorite, which fell in 1969, contained abundantcalcium–aluminium-rich inclusions (CAIs). These are very refractory materials and were interpreted as being condensates from a hotsolar nebula.[17][18] then discovered that the oxygen in these objects was enhanced in16O by ~5% while the17O/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 low87Sr/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 for26Al.[19]
To establish the presence of26Al in very ancient materials requires demonstrating that samples must contain clear excesses of26Mg/24Mg which correlates with the ratio of27Al/24Mg. The stable27Al is then a surrogate for extinct26Al. The different27Al/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 of26Al at an abundance ratio of 5×10−5 (relative to27Al, the standard way of quantifying this isotope) was shown by Lee et al.[20][21] The value (26Al/27Al ~ 5×10−5) 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 this26Al 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.[22] Other extinct radioactive nuclei, which clearly had a stellar origin, were then being discovered.[23]
That26Al is present in the interstellar medium as a majorgamma ray source was not explored until the development of the high-energy astronomical observatory program. TheHEAO-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 distributed26Al source.[5] This represents about twosolar masses of26Al. This discovery was greatly expanded on by observations from theCompton Gamma Ray Observatory using the COMPTEL telescope in the galaxy.[24] Subsequently, the60Fe lines (1.173 MeV and 1.333 Mev) were also detected showing the relative rates of decays from60Fe to26Al to be60Fe/26Al ~ 0.11.[25]
In pursuit of the carriers of22Ne 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 in26Mg/24Mg from the decay of26Al with26Al/27Al sometimes approaching 0.2.[26][27]
The production of26Al bycosmic 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.[citation needed]
Before 1954, the half-life of aluminium-26m was measured to be 6.3 seconds.[28] After it was theorized that this could be the half-life of a metastable state (isomer) of aluminium-26, the ground state was produced by bombardment ofmagnesium-26 andmagnesium-25 withdeuterons in thecyclotron of theUniversity of Pittsburgh.[15] The first half-life was determined to be in the range of 106 years.The Fermibeta decay half-life of the aluminium-26 metastable state is of interest in the experimental testing of two components of theStandard Model, namely, theconserved-vector-current hypothesis and the required unitarity of theCabibbo–Kobayashi–Maskawa matrix.[29] The decay issuperallowed. The 2011 measurement of the half-life of26mAl is6346.54 ± 0.46(statistical) ± 0.60(system) milliseconds.[30]
