Technetium-99 Is Injected into a Patient in Order to Monitor

Short Answer

Technetium-99 is injected into a patient in order to monitor heart function. Technetium-99 is a gamma emitter with a half-life of 6 hours. A patient receives an injection of 21 mg of Technetium-99 on a Tuesday at 9:00 a.m.
Calculate the number of hours and the number of half-lives that occur between 9:00 a.m. on Tuesday and 11:00 p.m. on Wednesday.