<prompt> Half- life of cobalt-60:53 years5.3 years0.53 years7.8 years  <response>: Ans. b 5.3 yearsRef: Basics radiological physics 2nd ed. 180 Iridium-192, emits gamma rays. It has energy of 0.4MeV. Tl2 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 