radioactivity

• Rutherford scattering


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

  
• Thomson's plum pudding model

  Atom made up of a sphere of positive charge, with small areas of negative charge evenly distributed throughout

  
• Nuclear model


  New model for the atom after the plum pudding model was disproved

  
• Rutherford's experiment


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

  2. Microscope moved around the outside of the chamber to observe the path of the alpha particles

  
• 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°
• Types of radiation


  • Alpha (α)
  • Beta (β)
  • Gamma (γ)

  
• Alpha (α) radiation


  • Range in air: 2 - 10 cm
  • Highly ionising
  • Deflected by electric and magnetic fields
  • Absorbed by paper

  
• Beta (β) radiation


  • Range in air: Around 1 m
  • Weakly ionising
  • Deflected by electric and magnetic fields
  • Absorbed by aluminium foil (around 3 mm)

  
• Gamma (γ) radiation


  • Infinite range: follows inverse square law
  • Very weakly ionising
  • Not deflected by electric and magnetic fields
  • Absorbed by several metres of concrete or several inches of lead

  
• Identifying radiation types


  1. Measure background count

  2. Measure count rate with source

  3. Place paper between source and GM tube to check for alpha

  4. Place aluminium foil to check for beta

  5. Place lead block to check for gamma

  
• Gamma radiation used in medicine


  • As a detector to help diagnose patients
  • To sterilise surgical equipment
  • In radiation therapy to kill cancerous cells
• Intensity of gamma radiation

  Follows inverse square law: I = k/x^2
• Alpha radiation is highly ionising and can be incredibly dangerous if inhaled or ingested
• Beta particles and gamma radiation can also cause damage to body tissue
• Handling radioactive sources safely

  1. Use long handled tongs

  2. Store in lead-lined container

  3. Keep as far away as possible

  4. Never point towards others
• Background radiation


  Needs to be measured and subtracted to find corrected count rate

  
• Sources of background radiation

  • Radon gas
  • Artificial sources from 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 λ)

  
• Half-life (T1/2)


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


  1. Plot graph of ln(N) vs time to find decay constant λ

  2. Use formula T1/2 = ln(2)/λ
• Activity


  Number of nuclei that decay per second, proportional to number of nuclei (A = λN)

  
• Decay constant can only model decay when there is a large number of nuclei

  
• Uses of radioactive nuclei based on half-life

  • Dating of objects using long half-life nuclei like carbon-14
  • Medical diagnosis using short half-life nuclei like 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 (e.g. carbon-14 with half-life of 5730 years used to date organic objects)
  • Medical diagnosis (e.g. Technetium-99m with half-life of 6 hours used as radioactive tracer)

  
• Nuclei with extremely long half-life must be suitably stored (e.g. in steel casks underground) to prevent them from damaging the environment and people living around them hundreds of years into the future
• Strong nuclear force


  Holds nuclei together
• Electromagnetic force


  Causes protons to experience a force of repulsion, leading to nuclear instability
• Reasons for nuclear instability

  • Too many neutrons (leads to beta-minus emission)
  • Too many protons (leads to beta-plus emission or electron capture)
  • Too many nucleons (leads to alpha emission)
  • Too much energy (leads to gamma emission)

  
• Nuclei may decay through several types of emission before finally becoming stable

  
• As the number of neutrons and protons in a nucleus 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 diagram


  Shows the differences in energy of particles in a nuclear decay process

  
• Energy level diagrams for

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

  
• Technetium-99m is ideal for medical diagnosis as it is a pure gamma emitter, has a half-life of 6 hours, and can be easily prepared on site

  
• Distance of closest approach

  The point at which a charged particle fired at a nucleus stops and has no kinetic energy, due to the electrostatic force of repulsion

  
• Electron diffraction


  A more accurate method for calculating nuclear radius, as electrons will not interact with nucleons through the strong nuclear force

  
• Calculating nuclear radius using electron diffraction


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

  2. Direct electrons at a thin film of material, causing them to diffract through the gaps between nuclei

  3. Measure the diffraction angle of the first minimum in the diffraction pattern

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