4. In a few instances, gamma rays are the only form of radiation coming from a radioactive material. This can happen when a nucleus which is energized and unstable returns to its energy ground state without losing nuclear particles or changing to another element. For example, technetium-99m, a ‘metastable’ form of technetium, emits gamma radiation as it changes to technetium-99, with a half-life of 6.03 hours. Technetium-99 is itself radioactive, emitting beta and gamma radiation as it changes to ruthenium-99 with a half-life of 212,000 years.

12. Uranium-235 Plutonium-239 Bare sphere:56 kg 11 kg Thick Tamper:15 kg 5 kg