Half life of cobalt-60: September 2011: (A) 3.3 years, (B) 4.3 years, (C) 5.3 years, (D) 6.3 years
Answer is C. 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