The production of aluminum-26 in the early solar system by the carbon-12 (oxygen-16,x) g.s. aluminum-26 reaction.

摘要

Because the half life of {dollar}sp{lcub}26{rcub}{dollar}Al (0.7 Myr) is comparable to the formation time of the solar system, its abundance in the early solar system probes the astrophysical environment of the proto-solar cloud. Gamma ray observations have found a Galactic isotopic abundance of {dollar}sp{lcub}26{rcub}{dollar}Al that is an order of magnitude less than the meteoritic abundance inferred from measurements of {dollar}sp{lcub}26{rcub}{dollar}Mg isotopic anomalies. It has been suggested (e.g., Cam77) that the unusually high abundance of {dollar}sp{lcub}26{rcub}{dollar}Al in the early solar system may be evidence that some sort of nucleosynthesis event (e.g., a supernova) injected radioisotopes into the proto-solar cloud. The COMPTEL observation of 4.4 and 6.1 MeV gamma rays from the direction of Orion led Clayton and Jin (Cla95a) to propose several alternatives to this explanation of the {dollar}sp{lcub}26{rcub}{dollar}Al abundance discrepancy. One of their proposals is that the proto-solar cloud was irradiated by oxygen rich cosmic rays that had energies of up to 10 MeV/u, and that the {dollar}rmsp{lcub}12{rcub}C(sp{lcub}16{rcub}O,x)sp{lcub}26{rcub}Alsb0{dollar} reaction produced the extinct {dollar}sp{lcub}26{rcub}{dollar}Al in the meteorites. Previous measurements of the cross section for this reaction differ by as much as a factor of 10 in absolute normalization, so I measured the yield using the activation method. Thick carbon samples were bombarded with beams of {dollar}sp{lcub}16{rcub}{dollar}O ions, and the resulting {dollar}sp{lcub}26{rcub}{dollar}Al beta-delayed gamma rays were counted off-line using a Compton suppressed HPGe detector. If {dollar}sp{lcub}26{rcub}{dollar}Al was uniformly distributed throughout the proto-solar cloud, then the ratio {dollar}rmlbracksp{lcub}26{rcub}Alrbrack/lbracksp{lcub}16{rcub}Orbrack{dollar} was {dollar}2times10sp{lcub}-7{rcub}{dollar} for the entire cloud. As the measured yield of {dollar}sp{lcub}26{rcub}{dollar}Al atoms is only {dollar}10sp{lcub}-7{rcub}{dollar} per stopped oxygen ion, if all the oxygen in the solar system was injected as cosmic rays the {dollar}rmsp{lcub}12{rcub}C(sp{lcub}16{rcub}O,x)sp{lcub}26{rcub}Alsb0{dollar} reaction could have produced a {dollar}rmlbracksp{lcub}26{rcub}Alrbrack/lbracksp{lcub}16{rcub}Orbrack{dollar} ratio of only {dollar}10sp{lcub}-7{rcub}{dollar} which is half of the {dollar}sp{lcub}26{rcub}{dollar}Al abundance in the proto-solar cloud. This is not plausible. Thus, it is likely that the meteoritic {dollar}sp{lcub}26{rcub}{dollar}Al was produced in a stellar environment and not by cosmic rays.