Measurement of the 24 Mg ( 3 He , p ) 26 Al cross section: Implication for Al 26 production in the early solar system

The nucleosynthetic origin of $^{26}\mathrm{Al}$ (${t}_{1/2}=0.72$ Myr) in the early solar system is still an open question. Several models predict that short-lived radionuclides could be produced by irradiation of circumsolar material by light charged particles emitted by the young sun. Within some models, most of the $^{26}\mathrm{Al}$ is produced by $^{3}\mathrm{He}$-induced reactions on $^{24}\mathrm{Mg}$. Little experimental data exist on $^{3}\mathrm{He}$ reactions so that irradiation models have had to rely on theoretical cross sections deduced from statistical nuclear reaction codes. We performed a direct measurement of the $^{26}\mathrm{Al}$ production on Mg target by means of $\ensuremath{\gamma}$ ray spectroscopy and accelerator mass spectrometry (AMS). The data indicate that the theoretical cross section used in previous approaches was overestimated by a factor of 3. Taking the particle spectra considered in theoretical approaches these data lead to a net reduction of the $^{26}\mathrm{Al}$ production of a factor of 2. We calculated the relative contribution of the different $^{26}\mathrm{Al}$ production channels depending on the irradiation scenario. We show that extremely large particles fluxes would be necessary to reach the canonical ${}^{26}\mathrm{Al}{/}^{27}\mathrm{Al}$ $=5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ in solids that were present in the early solar system. An in situ origin of this important isotopic chronometer by irradiation is unlikely.