The Physics of the Atomic Bomb (Level 1)

Level 1: Half-life

Radioactive decay is the process in which unstable nuclei try to become more stable by ejecting particles or energy. Radioactivity is directly proportional to the number of undecayed radioactive nuclei present. Since the number of undecayed nuclei of a sample is proportional to the mass of the sample, the radioactive half-life of a given radioisotope (radioactive isotopes of an element) is the time needed for half of the radioactive nucleus in any sample to undergo radioactive decay. After two half-lives, there will be one-fourth the original sample, so on and so forth. 

Q: Do all radioisotopes have the SAME half-life?

Ans: NO. Polonium-215 has a half-life of 0.0018 seconds while Uranium-235 has a half-life of 4.5 billion years.

Q: So why is knowing the half-life of a radioisotope IMPORTANT?

Ans:  In the case of the atomic bomb blast, it will help to determine whether the location has a level of radioactivity within safe limits.