The irradiation origin of beryllium radioisotopes and other short-lived radionuclides

Two explanations exist for the short-lived radionuclides (T-1/2 <= 5 Myr) 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, Be-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-7. Within model uncertainties associated mainly with nuclear cross sections, we obtain agreement with the experimental value. Moreover, if Be-7 and Be-10 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 similar to 2 x 10(10) cm(-2) s(-1). The X-wind model provides a natural astrophysical setting that gives the requisite conditions. In the same irradiation environment, Al-26, Cl-36, and Mn-53 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-3 abundance). The decoupling of the Al-26 and Be-10 observed in some rare CAIs receives a quantitative explanation when rare gradual events (shallow energy spectra and low He-3 abundance) are considered. The yields of Ca-41 are compatible with an initial solar system value inferred from the measured initial Ca-41/Ca-40 ratio and an estimate of the thermal metamorphism time ( from Young et al.), alleviating the need for two-layer proto-CAIs. Finally, we show that the presence of supernova-produced Fe-60 in the solar accretion disk does not necessarily mean that other short-lived radionuclides have a stellar origin.