Aluminium-26

Aluminium-26 (Al, Al-26) is a radioactive isotope of the chemical elementaluminium, decaying by either positron emission or electron capture to stable magnesium-26. The half-life of Al is 717,000 years. 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 rayprotons.

Decay of aluminium-26 also produces gamma rays and X-rays. The x-rays and Auger electrons are emitted by the excited atomic shell of the daughter Mg 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 with Al may result in radiological contamination. This necessitates special tools for transfer, use, and storage.