This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Radiology
Half life of cobalt-60: September 2011 
Select and explain. 
A. 3.3 years, B. 4.3 years, C. 5.3 years, D. 6.3 years
C: 5.3 years
Answer: C: 5.3 years 
Explanation: Ans. C: 5.3 years Half life of cobalt-60 is 5.3 years, therefore in practice have to replaced every 3-4 years Half-lives of impoant isotopes: 1-131: 8 days Rn 222: 3-6 days Co 60: 5.2 years