Measurements of iridium concentration in the oldest rock sample, ISUA.

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Non-destructive ultra-sensitive elemental analyses method using gamma-gamma coincidence detection technique combined with neutron activation analysis was developed by JAEA group. In this study, iridium concentrations in the geological samples were measured using the multiple gamma detection with neutron activation analysis method. The ISUA samples from Greenland were analyzed. The ISUA is recognized as which was formed at 3.8 billion years ago, almost same age with the Late Heavy Bombardments. By the iridium measurements of the ISUA samples, we proved that Late Heavy Bombardments were caused by impacts of comets, not asteroids.