radiometrically
adverb /ˌreɪdiəʊˈmetrɪkli/
  /ˌreɪdiəʊˈmetrɪkli/
- in a way that relates to a measurement of radioactivity辐射度测量的 - These rocks have been dated radiometrically at two billion years old.经辐射能探测,这些岩石形成于 20 亿年前。 
 
- These rocks have been dated radiometrically at two billion years old.