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

日本原子能机构（JAEA）开发了γ-γ符合探测技术与中子活化分析相结合的无损超灵敏元素分析方法。本研究采用中子活化多重伽马探测分析方法，对地质样品中铱的含量进行了测量。对国际职工会协会从格陵兰采集的样本进行了分析。ISUA被公认为形成于38亿年前，几乎与晚期重轰炸时代相同。通过对ISUA样品的铱测量，我们证明了晚期重撞击事件是由彗星而不是小行星撞击造成的。

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.