Chapter 25- Radioactivity

• Radioactivity is the process by which unstable nuclei decay emitting ionising radiation
• Ionising radiation is any form of radiation that can ionise atoms by removing an electron to leave a positive ion
• A cloud chamber can be used to detect the presence of ionising radiation. It contains air saturated with vapour at a very low temperature. When air molecules are ionised, liquid condenses into the ions to leave tracks of droplets marking the path of the radiation
• Alpha radiation is a helium nuclei, it has a charge of +2$e$
• Beta - radiation consists of fast moving electrons, it has a charge of -$e$
• Beta + radiation consists of fast moving positrons, it has a charge of +$e$
• Gamma radiation consists of high-energy photos that travel at the speed of light and carry no charge.
• When travelling through an electric field;

  • Beta (-) is deflected towards the positive plate
  • Gamma is unchanged
  • Alpha radiation is slightly deflected towards the negative plate
  • Beta (+) is deflected more towards the negative plate

  
• When travelling through a uniform magnetic field;

  • Beta (+) is deflected upwards
  • Alpha radiation is deflected slightly upwards
  • Gamma is unaffected
  • Beta (-) is deflected downwards

  
• The large mass and charge of alpha particles mean that they interact with surrounding particles to produce strong ionisation, and therefore only have a very short range in air. A couple of cm of air and one sheet of paper is enough to stop it
• The small mass and charge of beta particles means that they are much less ionising than alpha particles and therefore has a much longer range in air (~1m) and it takes 1-3mm of aluminium to stop it
• Gamma rays have no charge meaning they are even less ionising than beta particles . A few centimetres of lead absorbs a significant proportion of gamma rays
• Beta minus decay results in an increase in the proton number
• Beta positive decay results in a decrease in the proton number
• Radioactive decay is random because we cannot predict when a particular nucleus will decay and each nucleus within a sample has the same chance of decaying per unit time
• Radioactive decay is spontaneous because the decay of the nuclei is not effected by the presence of other nuclei in the sample or external factors such as pressure
• The half life of an isotope is the average time it takes for half the number of active nuclei in the sample to decay 


  $\lambda t_{\frac{1}{2}}=$$\ \ln2$
• The activity on a source is the rate at which nuclei decay or disintegrate, measured in decays per second or becquerels


  $A=$$\lambda N$
• $\lambda$ is the decay constant of the isotope
• All living things contain carbon atoms which comes from the atmosphere. Atmospheric carbon is mainly the stable isotope of carbon 12 but also contains a small amount of carbon-14. The ratio of carbon-14 to carbon-12 in the atmosphere is almost constant and the ratio is the same in all living things. Once an organism dies, it stops taking in carbon whist the total amount of carbon-14 continues to decay, so the ratio decreases over time. The activity from c-14 in a sample is proportional to the number of undecayed c-14 nuclei therefore the time since the organism died can be determined by comparing activities or ratios of c-12 to c-14 with a similar living material. This process is called carbon dating
• Carbon-dating has a few limitations including the assumption that the ratio of c-12 to c-14 has remained constant over time as there are many different reasons that could change the ratio; burning fossil fuels, volcanic eruptions, solar flares or nuclear bombs
• Rocks can be dated using rubidium-87 as it has a very high half life so can be used to date ancient rocks