Short-lived nuclides in hibonite grains from Murchison: Evidence for solar system evolution

Records of now-extinct short-lived nuclides in meteorites provide information about the formation and evolution of the solar system. We have found excess B-10 that we attribute to the decay of short-lived Be-10 (half-life 1.5 million years) in hibonite grains from the Murchison meteorite. The grains show no evidence of decay of two other short-lived nuclides - Al-26 (half-life 700,000 years) and Ca-41 (half-life 100,000 years)-that may be present in early solar system solids. One plausible source of the observed Be-10 is energetic particle irradiation of material in the solar nebula. An effective irradiation dose of similar to2 x 10(18) protons per square centimeter with a kinetic energy of greater than or equal to10 megaelectronvolts per atomic mass unit can explain our measurements. The presence of Be-10, coupled with the absence of Ca-41 and Al-26, may rule out energetic particle irradiation as the primary source of Ca-41 and Al-26 present in some early solar system solids and strengthens the case of a stellar source for Ca-41 and Al-26.