A new approach to nucleocosmochronology

Abstract

A new model of nucleocosmochronology based on discrete addition of r-process elements to the presolar cloud is proposed. It is suggested that the solar system accreted its r-process (and presumably all other) elements during its passage through the galactic arms, which has been treated as a discrete event. The number of times the presolar cloud passed through the galactic arm before contraction is calculated to be 50 +- 10.If one uses only the /sup 238/U//sup 232/Th ratio, the accretion (nucleosynthesis) period is calculated to be (8(+2, -1)) x 10/sup 9/ yr. The free decay period (also the transit time between the galactic arms) has been calculated to be (140 +- 10) x 10/sup 6/ yr, if one uses the /sup 129/I//sup 127/I ratio only.The model has been applied to predict the other two isotopic ratios, viz., /sup 244/Pu//sup 238/U and /sup 235/U//sup 238/U, in meteorites. The calculated values (0.0128 for /sup 244/Pu//sup 238/U and 0.292 for /sup 235/U//sup 238/U) are in remarkably good agreement with the measured values (0.0127 +- 0.0026 and 0.015 +- 0.0014 for /sup 244/Pu//sup 238/U, and 0.313 +- 0.026 for /sup 235/U//sup 238/U) in meteorites. This is the first time that a nucleochronology model has beenmore » put to such a test and has been so successful.The cosmoradiogenic fraction of /sup 1/87Os and /sup 87/Sr has been calculated to be 6.6% and 5.7%, respectively.« less