Measurement of the 24Mg(3He,p)26Al cross section: Implication for 26Al production in the early solar system
Résumé
The nucleosynthetic origin of 26Al (t1/2=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 26Al is produced by 3He-induced reactions on 24Mg. Little experimental data exist on 3He 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 26Al 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 26Al production of a factor of 2. We calculated the relative contribution of the different 26Al production channels depending on the irradiation scenario. We show that extremely large particles fluxes would be necessary to reach the canonical 26Al/27Al =5×10-5 in solids that were present in the early solar system. An in situ origin of this important isotopic chronometer by irradiation is unlikely.