Nuclear

• Rutherford scattering


  Demonstrated the existence of a nucleus
• Thomson's plum pudding model

  • Atom made up of a sphere of positive charge, with small areas of negative charge evenly distributed throughout like plums in a plum pudding
• Nuclear model


  New model for the atom, produced after the plum pudding model was disproved
• Rutherford's apparatus


  1. Alpha source and gold foil in an evacuated chamber covered in a fluorescent coating

  2. Microscope which could be moved around the outside of the chamber to observe the path of the alpha particles
• If the plum pudding model was true, the expected results would be that the positively charged alpha particles would be deflected by a very small amount when passing through the foil
• Most alpha particles passed straight through the foil with no deflection
• A small amount of particles were deflected by a large angle
• Very few particles were deflected back by more than 90°
• Radiation


  Unstable nucleus emits energy in the form of EM waves or subatomic particles in order to become more stable
• Types of radiation


  • Alpha (α)
  • Beta (β)
  • Gamma (γ)
• Alpha radiation is highly ionising and can be incredibly dangerous if inhaled or ingested as it can ionise body tissue
• Beta particles are less ionising but can still cause damage to body tissue
• Prolonged exposure to gamma radiation can cause mutations and damage to cells
• Handling radioactive sources safely

  1. Using long handled tongs to move the source

  2. Storing the source in a lead-lined container when not in use

  3. Keeping the source as far away as possible from yourself and others

  4. Never pointing the source towards others
• Background radiation


  Radiation around us constantly, must be measured and subtracted to find the corrected count rate of a radioactive source
• Sources of background radiation

  • Radon gas
  • Artificial sources (nuclear weapons testing and nuclear meltdowns)
  • Cosmic rays
  • Rocks containing naturally occurring radioactive isotopes

  
• Radioactive decay


  Random process with a constant decay probability (decay constant)

  
• Half-life (T1/2)


  Time taken for the number of nuclei to halve
• Measuring half-life


  1. Graphically by plotting number of nuclei against time and measuring time taken to halve

  2. By plotting ln(N0) against time and finding the modulus of the gradient (decay constant)
• Activity


  Number of nuclei that decay per second, proportional to number of nuclei (N) and decay constant (λ)
• Decay constant can only be used to model decay when there is a large number of nuclei in the sample

  
• Uses of radioactive nuclei with different half-lives

  • Dating of objects (long half-life, e.g. carbon-14)
  • Medical diagnosis (short half-life, e.g. technetium-99m)

  
• Activity


  Easier to measure than the number of nuclei, often used to find the half-life of a sample
• Decay constant

  Can be used to model the decay of a nuclei only when there is a large number of nuclei in a sample, as it models the number of nuclei decayed by statistical means

  
• Uses of radioactive nuclei with different half-lives

  • Dating of objects - nuclei with long half-life like carbon-14 can be used to date organic objects
  • Medical diagnosis - nuclei with short half-lives like Technetium-99m are used as radioactive tracers

  
• Technetium-99m


  • Pure gamma emitter
  • Half life of 6 hours, short enough to limit exposure but long enough for tests
  • Can be easily prepared on site
• The activity and half-life of radioactive nuclei will affect the way they must be stored

  
• Strong nuclear force


  Holds nuclei together
• Electromagnetic force


  Causes protons to experience a force of repulsion
• Reasons why a nucleus might become unstable

  • Too many neutrons - decays through beta-minus emission
  • Too many protons - decays through beta-plus emission or electron capture
  • Too many nucleons - decays through alpha emission
  • Too much energy - decays through gamma emission
• Nuclei may decay through several types of emission before finally becoming stable
• As the number of neutrons and protons increases beyond around 20 each


  The electromagnetic force of repulsion becomes larger than the strong nuclear force keeping the nucleus together, so more neutrons are needed to increase the distance between protons and decrease the electromagnetic force to keep the nucleus stable
• Energy level diagrams


  • Alpha decay
  • Beta-minus decay forming Technetium-99m

  
• Distance of closest approach

  Point at which a charged particle fired at a nucleus stops and has no kinetic energy, its electrical potential energy is equal to its initial kinetic energy

  
• Electron diffraction


  Gives a more accurate estimate of nuclear radius than distance of closest approach, as electrons will not interact with nucleons through the strong nuclear force

  
• Measuring nuclear radius using electron diffraction


  1. Accelerate electrons to high speeds so De Broglie wavelength is around 1x10^-15 m

  2. Direct electrons at a thin film, causing them to diffract through gaps between nuclei and form a diffraction pattern

  3. Plot a graph of intensity against diffraction angle to find the diffraction angle of the first minimum

  4. Use the formula sin(theta) = 0.61*lambda/R to calculate the nuclear radius R

  
• Nuclear radius is around 1x10^-15 m, varying a bit as nucleon number increases
• Plotting a log graph of nuclear radius against nucleon number

  1. Find the relationship R = kA^(1/3)

  2. The gradient gives the relationship n=1/3

  3. The y-intercept gives the value of ln(k), where k is renamed R0 and the equation becomes R = R0*A^(1/3)
• Nuclear density is constant for all nuclei
• Mass defect / mass difference


  The difference between the mass of a nucleus and the mass of its constituents, this "lost" mass is converted into energy and released when the nucleons fuse to form the nucleus