Aluminium-26

Aluminium-26 (26Al, Al-26) is a radioactive isotope of the chemical element aluminium, decaying by either positron emission or electron capture to stable magnesium-26.

This is far too short for the isotope to survive as a primordial nuclide, but a small amount of it is produced by collisions of atoms with cosmic ray protons.

It is produced in significant quantities in extraterrestrial objects via spallation of silicon alongside beryllium-10, though after falling to Earth, 26Al production ceases and its abundance relative to other cosmogenic nuclides decreases.

The isotope is believed to be crucial for the evolution of planetary objects, providing enough heat to melt and differentiate accreting planetesimals.

[10] 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.

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.

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.

[21] 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 26Al source.

[clarification needed] This discovery was greatly expanded on by observations from the Compton Gamma Ray Observatory using the COMPTEL telescope in the galaxy.

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 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.