Aluminium-26, ²⁶Al, is a radioactive isotope

Isotope

Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

 of the chemical element

Chemical element

A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

 aluminium

Aluminium

Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

, decaying by either of the modes beta-plus

Positron emission

Positron emission or beta plus decay is a type of beta decay in which a proton is converted, via the weak force, to a neutron, releasing a positron and a neutrino....

 or electron capture

Electron capture

Electron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron and simultaneously emits a neutrino...

, both resulting in the stable nuclide

Nuclide

A nuclide is an atomic species characterized by the specific constitution of its nucleus, i.e., by its number of protons Z, its number of neutrons N, and its nuclear energy state....

 magnesium

Magnesium

Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

-26. The half-life

Half-life

Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...

 of ²⁶Al is 7.17 years. This is far too short for any but a very minute amount of the primordial atoms of the isotope to survive to the present, but small amounts of the nuclide is produced by collisions of argon

Argon

Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...

 atoms by cosmic ray

Cosmic ray

Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...

 proton

Proton

The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

s.

Aluminium-26 also emits gamma rays and x-rays , and is one of the only radionuclides to emit x-rays. Because it is radioactive it should be stored behind at least 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

Meteorite

A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...

 fell to earth.

Occurrence in the interstellar medium

The gamma emission at 1809 keV was the first observed gamma emission from the galactic center. The observation was made by the HEAO-3

High Energy Astronomy Observatory 3

The last of NASA's three High Energy Astronomy Observatories, HEAO 3 was launched 20 September 1979 on an Atlas-Centaur launch vehicle, into a nearly circular, 43.6 degree inclination low-Earth orbit with an initial perigee of 486.4 km....

 satellite in 1984.

History

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 the metastable state 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

University of Pittsburgh

The University of Pittsburgh, commonly referred to as Pitt, is a state-related research university located in Pittsburgh, Pennsylvania, United States. Founded as Pittsburgh Academy in 1787 on what was then the American frontier, Pitt is one of the oldest continuously chartered institutions of...

. The first half-life was determined to be in the range of 10⁶ years.