Topic 6 ~ Radioactivity

• Atom


  A positively charged nucleus made of positive protons and neutral neutrons, surrounded by negatively charged electrons that orbit the nucleus at different fixed distances
• Atom


  • The nuclear radius is a lot smaller than the radius of the atom
  • Almost all the mass of the atoms lies in the nucleus
• Subatomic Particles


  • Proton (Relative Mass: 1, Relative Charge: +1)
  • Neutron (Relative Mass: 1, Relative Charge: 0)
  • Electron (Relative Mass: 0.0005, Relative Charge: -1)
  • Positron (Relative Mass: 0.0005, Relative Charge: +1)
• Size of atom: ~0.1 nanometres, 10^-10
• Isotopes


  Atoms of the same element with the same number of protons but different numbers of neutrons
• Atomic Notation


  X is the letter of the element, A is the mass number, Z is the proton number, N is the charge
• Atoms and EM Radiation


  1. When electrons move to a higher orbit, the atom has absorbed EM radiation

  2. When electrons fall to a lower orbit, the atom has emitted EM radiation

  3. If an electron gains enough energy, it can leave the atom to form an ion
• Forms of Radioactive Decay

  • Alpha (helium nucleus, highly ionising, weakly penetrating)
  • Beta Minus (electron, medium ionising, medium penetration)
  • Beta Plus (positron, medium ionising, medium penetration)
  • Gamma (radiation, low ionising, highly penetrating)
  • Neutrons
• Background Radiation


  Weak radiation that can be detected from natural / external sources
• Examples of Background Radiation

  • Cosmic rays
  • Radiation from underground rocks
  • Nuclear fallout
  • Medical rays
• Photographic Film


  Film goes darker when it absorbs radiation - the more radiation absorbed, the darker it gets
• Geiger-Muller Tube


  A tube which can detect radiation, each time it absorbs radiation it transmits an electrical pulse to the machine, producing a clicking sound
• Atomic Structure Model Changes

  1. 1800 - Dalton said everything was made of atoms

  2. 1897 - JJ Thomson discovered the electron, Plum Pudding Model formed

  3. 1911 - Rutherford realised most of the atom was empty space, Gold Foil Experiment

  4. 1913 - Rutherford Model, positive nucleus at the centre with negative electrons in a cloud

  5. 1913 - Bohr produced the final model of the atom
• Beta-Minus Decay


  Neutron becomes a proton, and releases an electron
• Beta-Plus Decay


  Proton becomes a neutron, and releases a positron
• Alpha Decay


  Nucleus emits an alpha particle (helium nucleus)
• Gamma Decay


  Nucleus emits a gamma ray (electromagnetic radiation)
• Activity


  The number of decays in a sample per second, initially very high and decreasing exponentially over time
• Half-Life


  The time taken for half the nuclei in a sample to decay, or the time taken for the activity of a sample to decay by half
• Net Decline of Radioactive Nuclei

  Work out the ratio of net decline after X half-lives by halving the initial number of nuclei X times
• Uses of Radioactivity


  • Smoke Alarms
  • Irradiating Food
  • Sterilisation of Equipment
  • Tracing and Gauging Thickness
  • Diagnosis and Treatment of Cancer
• Dangers of Ionising Radiation


  • Short half-life presents less long-term risk as it quickly dies down
  • Long half-life presents more long-term risk as it remains strongly radioactive
• Paper thickness measurement


  • Detector placed on either side of the paper during its production
  • If there is a drop or rise in received electrons, then that means the thickness of the paper has changed – i.e. a defect in the production
  • Also used inside pipes, with a detector placed externally, to measure the thickness of walls of the pipe
• Gamma emitter


  Radioactive tracer that is consumed/injected and passes through the body, allowing an external detector to picture where it has collected and reveal tumours
• Diagnosis and treatment of cancer

  1. Consuming/injecting a gamma emitter

  2. Gamma rays used on the tumour, killing the cancer cells

  3. Exposing healthy cells to rays can cause them to mutate or be damaged
• Short half-life radioactive source

  • Presents less of a risk as it does not remain strongly radioactive
  • Initially very radioactive, but quickly dies down
  • Presents less of a long-term risk
• Long half-life radioactive source

  • Remains weakly radioactive for a long period of time
  • Americium is suitable in smoke alarms because it will not need to be replenished, and its weak activity means it won't be harmful to anyone
• Limiting patient dose


  1. Only use radioactive tracers with a short enough half life to be quickly removed over a day or so, but long enough to still be detectable after the time taken for it to pass through the body

  2. Common medical tracers used have a half-life of 6hrs
• Limiting risks to medical personnel

  They leave the room during radioactive tests, as their everyday close proximity to the radioactive sources puts their health at risk in the long-term
• Contamination


  • Lasts for a long period of time
  • The source of the radiation is transferred to an object
• Irradiation


  • Lasts only for a short period of time
  • The source emits radiation, which reaches the object
  • Medical items are irradiated sometimes to kill bacteria on its surface, but not to make the medical tools themselves radioactive
• External treatment of tumours

  1. A beam of gamma radiation (usually a wide beam) rotates around the body, continually focusing on the tumour while only passing momentarily across healthy cells surrounding the tumour

  2. Takes a long time to fully treat the tumour, taking multiple visits for around 5 weeks

  3. Greater risk of long-term side effects as the radiation passes through healthy tissues
• Internal treatment of tumours

  1. Radioactive material is held within a needle, and is injected directly into the tumour

  2. Longer period of time needs to be spent in hospital, as some radioactive implants are of high radioactivity (so you emit radiation, requiring you to have very limited contact with visitors until the source's activity has decreased)
• PET scanners


  • Positron emission tomography
  • Radioactive tracer is inserted into the body, tagged to the desired chemical, and the scanner records where the tracer emits radiation
  • Produces a live 3D visualisation of the body
  • Used to show how effective current treatment is or to diagnose cancer, epilepsy, Alzheimer's
• The isotope used in PET scanners is made locally just before insertion because the tracer has a half-life of 110mins, so it cannot be stored for long before it decays
• Nuclear fission in nuclear power plants

  1. Uranium fuel splits releasing neutrons, which are absorbed by further uranium nuclei, which split (this is fission, in a chain reaction)

  2. No carbon dioxide is produced

  3. Safety risk of radiation leaking, or the chain reaction become uncontrollable and causing a meltdown

  4. Security risk as terrorists can try and obtain the radioactive material

  5. Public perception of nuclear power is negative, due to the fatal disasters caused by nuclear power plants

  6. Waste disposal is difficult – initially extremely hot, the waste needs to be placed deep in lakes, 'cooling ponds' to cool down, before being stored deep underground (it can be used for nuclear warheads so is a terrorist risk) for centuries
• Nuclear fusion


  • The process of small nuclei being forced together (under immense temperature and pressure) to form a heavier nucleus
  • The energy source for stars
  • The electrostatic repulsion of the protons in the two different nuclei means a lot of energy is required to bring the nuclei close enough to fuse, so fusion cannot happen at low temperatures and pressures, making it very difficult to make a practical and economic fusion power station
• Nuclear fission


  The process of a nucleus splitting into two smaller nuclei after absorbing neutrons, which releases more neutrons
• Radioactive decay


  When an unstable nucleus decays into two smaller nuclei
• All these nuclear processes (fusion, fission, radioactive decay) release energy and can be a source of energy