A-level Physics (Advancing Physics)/Radioactive Emissions

Table of the number of neutrons and protons in the nuclei of isotopes, showing their most common mode of decay.

'Radioactivity' is a catch-all term for several different emissions from the nuclei of 'radioactive' atoms. There are three main types of radiation: alpha (α), beta (β) and gamma (γ). When radiation occurs, four things must be conserved:

• Mass
• Charge
• Lepton Number
• Baryon Number

In formulae, mass and charge are shown next to the symbol of the particle. For example, a neutron with mass 1u and no charge is $_{0}^{1}n$. The charge on a nucleus is equal to the number of protons in the nucleus (electrons can be ignored). The lepton and baryon numbers may be obtained by counting the number of leptons and baryons on either side of the equation, remembering that antiparticles have negative lepton and baryon numbers.

α Radiation

Unstable nuclei with a mass greater than 82u emit α radiation. This consists of an Helium nucleus ($_{2}^{4}He$). The alpha particle simply splits off from the nucleus. Since the particle has no electrons, it has a charge of +2e. This, combined with its relatively large mass, means that it reacts easily with other particles, ionising them, meaning that it cannot penetrate more than a few centimetres of air.

β Radiation

Unstable nuclei with a mass below 82u emit β radiation. There are two types of β radiation. β^- radiation consists of an electron ($_{-1}^{0}e$). This is produced by nuclei with many more neutrons than protons. A neutron changes into a proton, emitting an electron and an antineutrino in order to balance the lepton number. β⁺ radiation consists of an positron ($_{1}^{0}e$). This is produced by nuclei with roughly the same number of neutrons as protons. A proton changes into a neutron, emitting a positron and a neutrino.

β particles also ionise particles, but since they have less charge and mass, they do this less easily, and so they travel further (on average). Both α and β radiation result in the nucleus which emitted them being changed into another element (Transmutation).

γ Radiation

The binding energies of nuclei are quantized - they can only take on certain values. When an electron jumps down an energy level, this energy has to go somewhere - it takes the form of a γ photon. The structure of the nucleus is not changed by γ radiation. γ radiation is ionising, but only at the right frequency - the resonant frequency of the things it ionises. γ radiation travels very far, and only a good thick layer of lead can stop it.

Questions

You will need a periodic table.

1. Americium-241 is an α emitter. What element, and what isotope, is produced by this decay?

2. Iodine-129 is a β^- emitter. What element, and what isotope, is produced by this decay?

3. Gamma rays are used to kill microbes in food. Why doesn't the food become radioactive?

4. Plutonium-244 decays by emitting an α particle. It does this twice, emits a β^- particle, and then emits a further two α particles. The nucleus becomes a different element each time. What element is produced at the end?

5. Carbon-11 changes into Boron-11 by a radioactive emission. What was emitted?

6. Uranium-236 decays, following the equation:

$_{92}^{236}U\to \;_{90}^{232}Th+\;X$

Identify the particle X in this equation.

Worked Solutions