Stable and unstable nuclei

• The strong nucleur force:

  • to hold the nucleus together, it must be an attractive force that is stronger than the electrostatic force
  • it has a very short range - only a few femtometres (the size of a nucleus)
  • below 0.5fm, it is repulsive so that it doesn't crush the nucleus
  • beyond 0.5fm, it is attractive, and it reaches its maximum attractive value at around 1fm. after this, its strength decreases rapidly
  • its strength is negligible past 3fm
• alpha emission only happens in very big nuclei, as they are too massive for the strong nuclear force to keep them stable. when an alpha particle is emitted, the proton number decreases by two, and the mass number decreases by four.
• alpha particles have a very short range - only a few cm in air. this can be seen by observing the tracks left by alpha particles in a cloud chamber. you could also use a geiger muller counter to observe their short range, if you bring it close to the alpha source and then move it away slowly and observe how the count rate drops.
• during beta minus decay, a neutron changes into a proton and emits an electron and an antineutrino - the antineutrino carries away some energy and momentum.

  beta decay happens in isotopes that are unstable due to being 'neutron rich' (ie they have too many more neutrons than protons in their nucleus). during beta minus decay, the proton number increases by one, and the nucleon number stays the same.
• scientists originally thought that the only particle emitted from the nucleus during beta decay was an electron. however, observations showed that the energy of the particles after beta decay was less that it was before, which didn't fit with the principle of conservation of energy.
• In 1930, Wolfgang Pauli suggested another particle was being emitted during beta decay, and that it carried away the missing energy. This particle had to be neutral (or charge wouldn't be conserved during beta decay) and have zero or almost zero mass (as it had never been detected). The neutrino was eventually discovered, providing evidence for Pauli's hypothesis.