The Irradiation Origin of Beryllium Radioisotopes and Other Short-lived Radionuclides

Two explanations exist for the short-lived radionuclides (T 1 2 ≤ 5 Ma) present in the solar system when the calcium-aluminum-rich inclusions (CAIs) first formed. They originated either from the ejecta of a supernova or by the in situ irradiation of nebular dust by energetic particles. With a half-life of only 53 days, Beryllium-7 is then the key discriminant, since it can be made only by irradiation. Using the same irradiation model developed earlier by our group, we calculate the yield of Be. Within model uncertainties associated mainly with nuclear cross sections, we obtain agreement with the experimental value. Moreover, if Be and Be have the same origin, the irradiation time must be short (a few to tens of years), and the proton flux must be of order F ∼ 2 × 10 cm s. The x-wind model provides a natural astrophysical setting that gives the requisite conditions. In the same irradiation environment, Al, Cl and Mn are also generated at the measured levels within model uncertainties, provided that irradiation occurs under conditions reminiscent of solar impulsive events (steep energy spectra and high He abundance). The decoupling of the Al and Be observed in some rare CAIs receives a quantitative explanation when rare Université Paris XI-Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse (CSNSM), Bâtiment 104, 91 405 Orsay Campus, France Also at Impact & Astromaterials Research Center (IARC), Department of Mineralogy, Natural History Museum, London, UK Present address: Laboratoire d’Étude la Matière Extraterrestre, Muséum National d’Histoire Naturelle, 57 rue Cuvier, 75 005 Paris National Tsing Hua University, Hsinchu, Taiwan Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan Department of Astronomy, University of California, Berkeley, Berkeley, CA 94720-3411, USA Physics Division, Argonne National Laboratory Argonne, IL 60439, USA Institute of Earth Science, Academia Sinica, Taipei 115, Taiwan