A count rate meter shows a count of 240 per min from a given radioactive source. One hour later, the meter shows a count rate of 30 per min. The half-life of the source is

Step-by-step solution

In radioactive decay, the decay rate is proportional to the amount of substance. So, for change of count rate from 240 per min to 30 per min, let N be the number of half-life passed in one hour. Then, 240 2^ N =30 2^ N =8 N=3 Half-life, t_ 1 / 2 = 1 h 3 = 60 3 ~min =20 ~min