A radium sample weighs 1 gram at time t = 0 . at time t = 10 years it has diminished to 0.997 grams. how long will it take to diminish to 0.5 grams?

A radium sample weighs 1 gram at time t = 0 . at time t = 10 years it has diminished to 0.997 grams. how long will it take to diminish to 0.5 grams?