actinium       
noun [ U ] uk/ækˈtɪn.i.əm/ us/ækˈtɪn.i.əm/
 (symbol Ac)锕 
a chemical element that is a soft, silver-white metal and is radioactive
  Researchers are testing the actinium. 研究者正在测试锕。 
 - More examples
 - Thorium decays into a radium isotope, and then into actinium, which has a 10-day half-life.
 - Projected future needs for actinium and bismuth are high.
 - Various alternative processes have been proposed for making actinium.