Understanding Radioactivity

  • Radioactivity is the spontaneous emitting of particles and energy from unstable atomic nuclei. This process is random and cannot be influenced by chemical reactions, physical conditions, or other atoms.

  • There are three types of radioactive emissions: alpha particles, beta particles, and gamma rays.

  • Alpha particles consist of two protons and two neutrons (equivalent to a helium-4 nucleus). They are positively charged and relatively heavy. They cannot penetrate far into matter and can be stopped by a sheet of paper or a few centimetres of air.

  • Beta particles are fast-moving electrons or positrons. They have a moderate penetrating power, and require a few millimetres of aluminium to stop them.

  • Gamma rays are high-energy photons (packets of electromagnetic energy). They have no mass and no charge, and thus are not deflected by electric or magnetic fields. They can penetrate deeply into matter.

Properties and Detection of Radioactive Emissions

  • Radioactive substances emit ionising radiation that can ionise atoms and molecules. This is harmful to living tissues, and can cause medical conditions such as radiation sickness or cancer.

  • Ionisation refers to the process in which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons.

  • Radioactivity can be detected and measured using instruments like Geiger-Muller counters, cloud chambers, or photographic film.

  • The rate of decay of a radioactive source is measured in decays per second, often counted in Becquerels.

Nuclear Transformations

  • Radioactivity leads to nuclear transformations, where one element can change into a different element.

  • Alpha emissions result in the atomic number decreasing by two and the mass number decreasing by four.

  • In beta decay, a neutron changes into a proton and an electron. The proton remains in the nucleus, increasing the atomic number by one, whilst the electron is emitted as a beta particle.

  • Gamma decay occurs after alpha or beta decay when the nucleus is left in an excited state. It releases excess energy as a gamma photon. This does not change the atomic number or the mass number.

Radioactive Decay and Half-Lives

  • Radioactive decay is a random process. You can’t predict when a specific atom will decay, but you can predict how many atoms in a sample will decay over time.

  • The half-life of a radioactive isotope is the time taken for half the radioactive atoms to decay. This value is the same for given isotope, regardless of the quantity of atoms present.

  • This means that after one half-life, there will be 50% of the original sample remaining. After two half-lives, there will be 25%, and after three half-lives, there will be 12.5%.

  • Half-life measurements are used to determine the age of archaeological samples, in a process called radioactive dating.