Nuclear Physics

• The plum pudding model refers to the idea that the atom is a sphere of positive charge with negative electrons in it.
• The discovery of the atomic nucleus came about as a result of the Rutherford Scattering Experiment.
• Name 4 conclusions of the Rutherford Scattering Experiment.

  The atom must be mostly empty space as most alpha particles pass through the gold foil.

  The nucleus must have a large positive charge as some alpha particles are repelled and deflected by a large angle.

  Nucleus very tiny as few alpha particles deflected by an angle >90°.

  Mass is concentrated in the nucleus as fast alpha particles are deflected.
• Radioactive decay is when an unstable nucleus becomes stable by emitting particles or energy.
• Radioactive decay is random and unpredictable.
• α particles consist of 2 protons and 2 neutrons. They have a relative charge of +2 and mass of 4u.
• β− particles consist of 1 electron and have a relative charge of -1.
• β+ decay is also known as positron emission.
• β+ particles consist of 1 positron and have a relative charge of +1.
• γ radiation is in the form EM waves with high frequencies & short wavelengths.
• Alpha particles can be stopped by a few cm of air or a thin sheet of paper.
• Beta-minus particles are stopped by ~3mm of Aluminium.
• Beta-plus particles are annihilated by electrons, and so have a virtually zero range.
• Gamma radiation can be stopped by many cm of lead, or several m of concrete.
• Alpha particles have the highest ionising power as they have a large mass, are slow moving and have a double positive charge.
• Beta minus particles have a weak ionising power as they are fast moving and not very massive.
• Gamma radiation has a very weak ionising power as it is very fast (3×10^8 m/s) and has no charge.
• Alpha and beta particles are affected by magnetic fields as they have a charge.
• Sources of background radiation:

  • CMBR
  • Radioactive rocks
  • Man-made e.g. from nuclear power plants
  • Living things, they contain C-14
• The intensity of gamma rays varies with the inverse square of distance.

  I=k/x^2
• Inverse square law is not usually used for alpha or beta radiation as the particles don't travel large distances, therefore the distance doesn't vary much unless in a vacuum.
• The decay constant, λ, is the probability of a specific nucleus decaying per unit of time.
• The number of unstable nuclei decreases exponentially with time.
• Derivation of half life equation:

  
• Living organisms contain C-14, which is naturally replenished by cosmic rays interacting with nitrogen. When living, the carbon is replaced but when dead, the isotope, with a half life of 5730 years, begins to decay.
• Conditions for fusion to take place:

  • Extremely high temperature and pressure to overcome electrostatic repulsion between nuclei
  • Extremely high density to ensure that the rate of collision between nuclei is high enough.