Question

The radioisotope cesium-137 has a half-life of 30 years. a sample decayed at the rate of 544 counts per minute in the year 1985. in what year will the decay rate be 17 counts per minute?

Answer 1

544 divided by 17 yields a ratio of 32. 32 is 2 to the fifth power. Therefore it will take five half-life period to decay from 544 counts per minute to 17 counts per minute. Since the half-life is given as 30 years, it will take 150 years (5 half-lives*30 years) and the goal decay rate will be achieved in the year 2135.