Half- life of cobalt-60:

Ans. (b) 5.3 yearsRef: Basics radiological physics 2nd ed. /180* Iridium-192, emits gamma rays. It has energy of 0.4MeV.* Tl/2 of iridium 192: 74 daysImportant Half Life of Radionuclides* Gold-198: 2.7 days* Iodine- 123: 13.3 hours* Iodine-131: 8 days* Phosphorus-32: 14.3 days* Yttrium 90: 64 hours* Cobalt-60: 5.26 years* Caesium-137: 30 years* Radium-226: 1620 years