Cobolt-60 has a half-line of 30 years.

Question

Cobalt-60 has a half-line of 30 years. A cobalt-60 sample has an initial activity of 100mCi. What is the activity after 120 years?

Answer 1

120 years is 4 times the half life.

 

So the amount of cobalt-60 remaining after 120 years is (1/2)⁴ of the original amount, which is 1/16.

 

So the amount of radioactivity from the cobalt-60 would go to 1/16 of the original. 1/16 of 100 is 6.25 mCi.