Half-life

• What happens when an unstable nucleus gives out an alpha or beta particle?

  The nucleus turns into the nucleus of a new element
• What happens in radioactive decay?

  When the nucleus of an unstable atom gives out an alpha or beta particle, it turns into the nucleus of a new element
• Is radioactive decay a random process?

  Yes.
• What happens even though radioactive decay is a random process?

  Over a time called the half-life, half of the parent radioactive nuclei will have decayed
• What does working out the half-life allow us to calculate?

  When a radioactive element will become stable
• What is a radioactive element?

  A chemical element whose atoms have unstable nuclei than can break apart or change, releasing radiation.
• What is the daughter nucleus?

  The nucleus of a new element formed from the decay of the starting parent nucleus
• What is the parent nucleus?

  The initial unstable nucleus
• All radioactive isotopes have a decay curve, however the half-life times can vary from seconds to millions of years
• The half life of a radioactive element is the time taken for:

  • the activity to halve
  • the number of parent nuclei to halve
  • the mass of the parent nuclei to halve
• What is activity?


  The number of decays of a radioactive element per second. Measured in Becquerels (Bq)
• The activity of a radioactive isotope is the number of atoms that disintegrate or decay in one second
• Each atom decaying is registered as a ‘click’ or a count on a Geiger counter.
• What is a decay curve?


  A graph that shows how the activity of a radioactive isotope changes with time
• How do you find the half-life on a decay curve?

  Half the highest value on the y-axis, draw a line to the curve and draw a line down to the x-axis to find the amount of time
• What are the axis on a decay curve?

  Counts per minute and time
• Does a radioactive nucleus decay randomly?

  Yes.
• What can we not predict when it will happen?

  When a radioactive nucleus decays
• Lots of nuclei of the same isotope will decay following a pattern called a radioactive decay curve
• What pattern will nuclei of the same isotope follow?

  A radioactive decay curve
• What can the decay of radioactivity in a radioactive element be modelled by using?

  Cubes, dice or coins
• What happens in decay?

  A radioactive parent nucleus randomly emits an alpha or beta particle and turns into a new daughter element
• Is the daughter nucleus more or less stable than a parent nucleus?

  More stable
• How do we model radioactive decay using coins?

  1. Collect coins and count them
  2. Put the coins into a container, shake them and then throw them into a tray
  3. Remove coins showing heads, they represent decayed atoms
  4. Count the remaining coins and record the number
  5. Repeat steps 2-4
  6. Plot results on a graph
• What is useful to collect class results of a radioactive decay experiment?

  Spreadsheets
• What does using a spreadsheet do?

  Reduces the risk of anomalous results and causes the decay graph to be smoother
• What is a graph produced by the results of a radioactive decay experiment?

  A model for radioactive decay
• What are the uses of radioactivity?

  • Measuring thickness of paper
  • Sterilising medical instruments
  • Prolonging the life of fruit
  • Smoke alarms
  • Blood and fluid tracers
  • Cancer treatment
  • Carbon-14 dating
• How does radioactivity measure the thickness of paper?

  In a rolling mill, the thickness of the paper is monitored by how much beta radiation is received at the detector
• How does radioactivity sterilise medical equipment and prolong the life of fruit?

  Gamma radiation kills microbes which sterilises equipment and kills the bacteria on fruit and vegetable so they stay fresh longer
• How is radioactivity useful in smoke alarms?

  Americium, which emits alpha particles, is used. The alpha radiation ionises the air and allows a small current to flow between two electrodes. Alpha is weakly penetrating so smoke stops it, the current drops and the alarm goes off
• Why is Americium used in smoke alarms?

  The isotope used should be an alpha emitter with a long half-life, so the smoke alarm doesn’t need to be changed daily.
• How is radioactivity used in fluid tracers?

  Radioactive tracers are added to liquids to show if they are flowing correctly. They can show the movement of pollution e.g sewage
• How is radioactivity used in blood tracers?

  Technetium-99m, gamma source with short half-life, is injected to monitor blood flow.
• What causes a build up of radioactivity that can be easily detected outside the body?

  Arterial blockages or internal bleeds
• What causes a build up of radioactivity that can be detected above the ground?

  Leaks
• What are radioactive tracers?

  A radioisotope used to trace, e.g the flow of blood through an organ
• How is radioactivity used in cancer treatment inside the body?

  An alpha source with a short half-life can be injected directly into a tumour. This is called alpha therapy as alpha is strongly ionising so will kill the cancer cells but is weakly penetrating so won‘t harm healthy cells
• How is radioactivity used in cancer treatment outside the body?

  Gamma radiation can be beamed at cancer cells to kill them. It has a long half life to maintain the dose of radioactivity and so the equipment doesn’t need to be changed regularly
• How is radioactivity used in Carbon-14 dating?

  It can be used to date organic object as living things, such as plants or animals absorb Carbon-14 daily