Aluminum-26

Aluminum-26 can be used to determine the time elapsed since the fall of the meteorite to Earth. Since the decay of the parent body, a meteorite has been bombarded by cosmic rays, which create aluminum-26 nuclei in it. After the fall of the cosmic rays to Earth, the flux of cosmic rays sharply decreases, and the accumulation of ²⁶ Al stops, but its decay continues at the same speed. This means that the number of ²⁶ Al nuclei remaining in the sample can be used to calculate the date of the fall of the meteorite to Earth.

The gamma line with an energy of 1809 keV, appearing during the decay of ²⁶ Al, was the first observed gamma radiation from the galactic center ( NEAO-3 satellite, 1984 ^{[2] [3]} ).

The isotope in the Galaxy is created mainly in supernovae , which emit a lot of radioactive nuclides into the interstellar medium . It is believed that during the condensation of small planetary bodies, it provides sufficient heat for heating so that the gravitational differentiation of their subsoil begins, as happened in the early history of asteroids (1) Ceres and (4) West . ^{[4] [5] [6]} This isotope also plays a role in hypotheses regarding the origin of the equatorial convexity of Yapet , the satellite of Saturn ^[7] .

Until 1954, the measured half-life of aluminum-26 was considered to be 6.3 seconds ^[8] . After the publication of theoretical evidence that this decay actually relates to the metastable state ( isomer ) of aluminum-26, the nuclei of this isotope in the ground state were obtained by bombarding magnesium-26 and magnesium-25 with deuterons in the cyclotron of the University of Pittsburgh ^[9] . The first measurement gave the half-life of the ground state, estimated at ~ 10 ⁶ years.

The ground state of aluminum-26 with spin and parity J ^π = 5 ⁺ cannot directly decay to the ground state of the magnesium-26 core (which has spin 0) due to the significant difference in the spins; more precisely, beta transitions from the ground state to the ground state have a very high degree of prohibition and are not observed, despite the rather large available decay energy ( Q _ε = 4004.14 keV ). Decay (both electron capture and positron decay) almost always occurs (in 97.3% of cases) into the first excited state of magnesium-26 with an energy of 1808.7 keV and J ^π = 2 ⁺ . This level is immediately discharged to the ground state of ²⁶ Mg with emission of a gamma quantum with an energy of 1808.6 keV; A peak with this energy is the most characteristic feature of the ²⁶ Al gamma spectrum. In the remaining 2.7% of cases, the transition occurs to the second excited state of ²⁶ Mg with E = 2838.4 keV ( J ^π = 2 ⁺ ), which can decay directly to the main level, radiating a gamma quantum with an energy of 2938.3 keV , but more often (with respect to 0.27: 2.4) it decays through the already mentioned first excited state with emission of a cascade of gamma rays with energies of 1129.7 and 1808.7 keV . The lifetime of both excited levels is less than 1 ns . In addition to discharging excited levels with gamma-ray emission, in all cases it is possible to transfer the emitted energy E _{γ to the} orbital electron ( internal conversion effect ) with the emission of a conversion electron with the corresponding fixed energy E _γ - E _c , where E _c is the binding energy of an electron in an atom ²⁶ Mg. In this case, the excitation of the electron shell is removed by radiation of characteristic X-ray photons and Auger electrons with a total energy E _c .

The isomeric state of aluminum-26 ( ^26m Al) with isospin T = 1 has an energy of 228.305 keV above the ground state ( T = 0 ), but its spin (0+) is very different from the spin of the ground state (5+), therefore the isomeric transition to the ground state the condition is severely depressed. For 2015, this transition was not detected; Decay, as in the ground state, occurs through the emission of a positron or the capture of an orbital electron , but all decays occur in the ground (and not in the excited) state of magnesium-26.

Measuring the half-life in the Fermi beta decay channel of the metastable state of aluminum-26 is of interest for experimental verification of two components of the Standard Model , namely, the hypothesis of a persisting vector current and the required unitarity of the Cabibbo – Kobayashi – Maskawa matrix ^[10] . This decay is supersoluble, the initial and final ( ²⁶ Mg) states have the same spin and parity 0 ⁺ . Measurement in 2011 of the half-life of Al-26m gave a value of 6346.54 ± 0.46 (stat.) ± 0.60 (syst.) Milliseconds ^[11] . In addition, a value of ft = 3037.53 (61) ms was obtained. These half-lives and ft are the most accurately measured values ​​among all superresolved beta transitions ^[11] .

• Aluminum isotopes