Radioactivity

• Atoms are made of 3 subatomic particles:

  Particle Relative charge Relative mass Location

  Proton +1 1 Nucleus

  Neutron 0 1 Nucleus

  Electron -1 Negligible Shells around nucleus
• Atomic number = small number = number of protons
• Mass number = large number = number of protons + neutrons
• Number of neutrons = mass number - atomic number
• All nuclei of an element have the same positive charge. This is shown by the atomic number.
• Isotopes are elements with same number of protons but a different number of neutrons, so the mass number is different.
• Alpha radiation is a helium nucleus emitted from the nucleus. A helium nucleus means 2 protons and 2 neutrons.
• Beta radiation is an electron emitted from the nucleus.
• Gamma radiation is a high energy electromagnetic wave.
• A nucleus can be stable or unstable. If it is unstable it tries to become more stable, by either ejecting mass or energy. We call these atoms radioactive.
• The process of ejecting radiation is called decay. Decay is random, you cannot predict when it will happen.
• The Geiger-Muller (GM) tube works by detecting when the gas inside the chamber is ionised by radiation. Every click/beep is one incident of radiation - it could be gamma, beta or alpha!
• When radiation comes into contact with photographic film, it darkens, showing the presence of radiation.
• Alpha:

  • Very ionising
  • Low penetration (stopped by paper)
• Beta:

  • Moderate ionisation
  • Moderate penetration (stopped by aluminium)
• Gamma:

  • Not very ionising
  • Very penetrating (stopped by lead)
• An ion is an atom that has gained or lost electrons.
• Ionisation means "to make ions"
• Origins of background radiation:

  • Food and drink: Radioactive isotopes (like K-40) decay over time
  • Nuclear power: From power stations and bombs
  • Medical: X-Rays, gamma ray scans, cancer treatments, sterilisation
  • Radon gas: Produced by rocks containing small amounts of uranium
  • Cosmic rays: Radiation from space, produced in stars (inc. the sun)
• The activity of a sample of radioactive isotopes is a measure of how many radioactive decays happen over a period of time. It is measured in Becquerels (Bq).
• The activity of a radioactive source decreases over time.
• Half-life = time taken for half the undecayed nuclei to decay
• Half-life = time taken for activity to half its original value.
• (Large) samples of the same radioactive substance have the same half-life.
• (Large) samples of different radioactive substances have different half-lives.
• To find the number of undecayed nuclei in a sample:

  • state the start number
  • go forward a half-life, halving the sample size
  • repeat until the target time has passed
• To find the age of an isotope:

  • find the total number in the current sample
  • go back a half-life, doubling the sample size
  • repeat until all of the sample is the radioactive isotope
• Ions in DNA can cause mutations. This can result in cell damage and death (e.g. radiation sickness and radiation burns) or cancer (in the long term).
• Precautions are taken to reduce the risk of harm when using radioactive sources. These include:

  • keeping radioactive sources shielded when not in use, for example in a lead-lined box
  • wearing protective clothing to prevent the body becoming contaminated
  • limiting exposure time so less time is spent with radioactive materials
  • handling radioactive materials with tongs to increase distance from them
  • monitoring exposure using detector badges
• Irradiation:

  • when someone is exposed to alpha/beta/gamma from a nearby source
  • once the person moves away the irradiation stops
• Contamination

  • when someone gets particles of a radioactive source on their person or inside their body
  • they will continue to be exposed to radiation until the material has all decayed or it is removed
• Radioactive waste must be disposed of:

  • in strong containers that do not rust
  • in a special facility, not landfill
  • with security to prevent public access
  • with a low earthquake risk
  • far from the water table
• Heavy nuclei (like Uranium-235) can be fissioned:

  • a parent nucleus absorbs an incident neutron, making it unstable
  • the nucleus splits (fission)
  • energy is released as kinetic energy of the fission products
  • the products of the fission of U-235 are:

  1. 2 radioactive daughter nuclei, smaller nuclei that are not always identical to each other
  2. a small number of neutrons
  3. the combined atomic mass of daughters and neutrons must equal the parent

  • having extra neutrons at the end means that they can be absorbed by other nuclei and induce more fission - this creates a chain reaction
• The energy produced from fission can be used to make electricity

  • when a coil of wire is rotated inside a magnetic field it induces a voltage
• Neutrons are released in a chain reaction. Slow neutrons are needed for fission.
• Graphite core around the reactor is a moderator:

  • neutrons in the moderator are slowed down
  • increasing the rate of fission
• Control rods:

  • absorb neutrons
  • can be moved in and out of the reactor
  • increases or decreases the rate of fission

  This makes more of fewer neutrons available.
• Reactor vessels are made of steel and surrounded by a layer of concrete about 5 metres thick. This prevents any radiation escaping, even neutrons.
• Fusion is when 2 smaller particles join to form a larger one. More massive particles are less massive than the sum of the smaller parts. This mass becomes energy, which is released in fusion. Fusion is the energy source for stars.
• Positive charges will repel each other - this is called electrostatic repulsion. The faster the particles are, the closer they will get to each other before repelling.