Particles and radiation

• Ionising radiation


  Any form of radiation that interacts with matter, resulting in ionisation of that matter
• Ionisation


  The formation of charged particles from neutral molecules or atoms by adding or removing electrons
• Types of ionising radiation

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


  • Helium nuclei
  • Relatively large mass
  • Highly ionising
  • Can travel 1-2 cm in air
  • Can be absorbed by a sheet of paper
• Beta radiation


  • Fast-moving electrons
  • Almost no mass
  • Less ionising than alpha
  • Can travel 15 cm in air
  • Can be absorbed by 5mm of aluminium
• Gamma radiation


  • Electromagnetic radiation
  • Passes through atoms rather than being absorbed
  • Weakly ionising
  • High penetrating power
  • Can travel many kilometres in air
  • Can be absorbed by thick lead or concrete
• Radioactive decay


  The random process that occurs when an unstable nucleus loses energy by giving out alpha or beta particles or gamma radiation
• Activity


  The rate at which a radioactive source decays, measured in becquerels (Bq)
• Count-rate


  The number of radioactive decays recorded each second
• Half-life


  The time taken for half the unstable nuclei in a sample to decay
• Radioactive decay is spontaneous and random
• Alpha decay


  1. Unstable nucleus loses 2 neutrons and 2 protons

  2. Becomes more stable

  3. Forms a different element
• Beta decay


  1. Neutron turns into a proton

  2. Electron is ejected at high speed

  3. Atomic number increases by 1

  4. Mass number remains the same
• Gamma radiation


  1. Excess energy is lost from the nucleus

  2. No change to atomic structure or element formed
• Radioactive sources with short half-lives have high activity and emit high levels of radiation, but become safe relatively quickly
• Radioactive sources with long half-lives have low activity but can still be dangerous for a long time
• Alpha radiation is the most dangerous via contamination, beta radiation is more dangerous via irradiation
• Ionisation


  Electrons are removed from atoms or molecules to produce positive ions
• Excitation


  Radiation transfers energy to atoms or molecules, moving electrons to higher energy levels
• Gamma radiation, X-rays and visible light can cause excitation of electrons
• Ionisation


  Occurs when electrons are removed from atoms or molecules to produce positive ions
• All forms of radioactive decay produce radiation that can cause ionisation - hence the term ionising radiation
• Types of radiation in order of ionising ability

  • Alpha (most)
  • Beta
  • Gamma (least)
• Excitation


  Occurs when radiation transfers energy to atoms or molecules, moving an electron to a higher energy level
• UV light can cause excitation of electrons in DNA molecules, leading to ionisation and cancer-causing mutations
• Removal or addition of an electron changes a molecule chemically, meaning it may behave differently in interactions with other molecules
• Alpha, beta and gamma radiation have energy millions of times greater than UV light, so they can be highly dangerous by changing the chemistry of the body
• Hasini's considerations for safe handling of radioactive sources

  1. Activity of the source

  2. Type of radiation emitted
• Radioactive sources in the college laboratory

  • Cobalt-60 (pure gamma)
  • Strontium-90 (pure beta)
  • Americium 241 (alpha and some gamma)
• Radioactive tracers


  Radioactive isotopes that can be injected or swallowed and their course around the body followed using an external detector
• Radioactive tracers are usually gamma emitters as these are weakly ionising but highly penetrating, less likely to cause damage but easily detected
• Radioisotopes used as tracers need to have short half-lives so their activity disappears soon after the procedure
• SPECT


  Single photon emission computerised tomography, uses gamma radiation detectors to detect gamma emission of a radioactive tracer from many angles and build up an image
• Iodine-123 is an example of a diagnostic radiopharmaceutical, various radioisotopes can be attached to biologically active substances and used with SPECT to examine a wide range of processes
• Food irradiation


  Sufficiently high doses of radiation will kill micro-organisms in food, including harmful pathogens and spoilage micro-organisms, helping to preserve food and extend shelf-life
• Isotopes


  Atoms of the same element with different numbers of neutrons
• Radioisotopes


  Unstable isotopes that undergo radioactive decay emitting radioactivity
• Radiopharmaceuticals


  Therapeutic drugs that incorporate radioisotopes, some used to treat disease and some used for diagnosis
• Irradiation


  The process of exposing an object to radiation, does not make the object radioactive
• Radiocarbon dating measures the age of any object that contains organic material by measuring the proportion of carbon-14, which has a half-life of about 5730 years