Aluminium-26

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.^[1] 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.^[2]

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 distribution of ²⁶Al in Milky Way

The gamma emission at 1809 keV was the first observed gamma emission from the galactic center. The observation was made by the HEAO-3 satellite in 1984.^[3][4]

The isotope is mainly produced in supernovae 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.^[5][6][7] This isotope also features in hypotheses regarding the equatorial bulge of Saturn's moon Iapetus.^[8]

History

Before 1954, the measured half-life of aluminium-26 was determined to be 6.3 seconds.^[9] 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.^[10] 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.^[11] 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.^[12] 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 & 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.^[13] 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.^[14]

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, Phys. Rev. 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.^[15] Then Black^[16] 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.^[17] 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.^[18][19] 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.^{[20] 26}Al 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.^[21] 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.^[22]

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.^[23] This represents a quasi steady state inventory corresponding to two solar masses of ²⁶Al was distributed . This discovery was greatly expanded on by observations from the Compton Gamma Ray Observatory using the COMPTEL telescope in the galaxy.^[24] 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.^[25]

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 ^[26][27] 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 is found in very early solar system debris.

See also

• Isotopes of aluminium
• Radiometric dating#The 26Al – 26Mg chronometer

References