Measurement of the 24Mg(3He,p)26Al cross section: Implication for Al26 production in the early solar system

The nucleosynthetic origin of Al-26 (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 Al-26 is produced by He-3-induced reactions on Mg-24. Little experimental data exist on He-3 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 Al-26 production on Mg target by means of 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 Al-26 production of a factor of 2. We calculated the relative contribution of the different Al-26 production channels depending on the irradiation scenario. We show that extremely large particles fluxes would be necessary to reach the canonical Al-26/Al-27 =5x10(-5) in solids that were present in the early solar system. An in situ origin of this important isotopic chronometer by irradiation is unlikely.