Nuclear Physics

Table of Contents
1. Radioactive Split
2. The Nucleus
3. Nucleus Hypothesis
4. Artificial Radioactivity
5. The Radiations
6. The Coherences
7. The Laser
8. Periodic Table of Elements
9. Raman Spectrum
10. Zeeman Effect
11. Hydrogen Spectrum
12. Summary



Atom Bomb

An atom bomb, infact a nuclear fission bomb, is a device in which an uncontrolled chain reaction is built up in a fissionable material by means of fast neutrons. It then releases tremendous amount of energy in a very short time.
The bomb consists essentially of two pieces of U235 (or Pu239), which are kept separated in a massive cover of a high-density material (Fig.). The mass of each piece is below the critical value so that any stay neutron, either from cosmic rays or produced by spontaneous fission, is unable to start a chain reaction. Thus, so long the two pieces are kept separated, they are perfectly stable and safe.
When an explosion is required, the two pieces are brought rapidly together (in a time less than a microsecond) by means of some device so that the total mass becomes supercritical (greater than the critical value). As soon as this happens, the stray neutrons initiate chain reaction in which fast neutrons rapidly multiply and within a few millionths of a second the reaction acquires an explosive nature.
An idea of the rate of energy released in a nuclear bomb having 1 kg of uranium can be made in the following way: In a nuclear chain reaction the number of neutron N, available to produce fission at any time t is given by ...