Label the components of a nuclear power plant A 462.5 ng sample of an unknown radioactive substance was placed in storage and its mass measured periodically. After 47 days the amount of radioactive substance had decreased to 57.81 ng. How many half-lives of the unknown radioactive substance have occurred? Sodium-24, which is used to locate blood clots in the human circulatory system, has a half-life of 15.0 h. A sample of sodium-24 with an inital mass of 24.5 g was stored for...