An isotope of thorium (thorium-227) which is produced by beta decay of actinium and itself decays by alpha emission with a half-life of 18.7 days.
Early 20th century.
英汉双解词典包含464360条英汉词条，基本涵盖了全部常用单词的翻译及用法，是英语学习的有利工具。