Radioactive DecayCambridge IGCSE Physics: Revision notes
Section 1
What is radioactive decay?
Radioactive decay is a change in an unstable nucleus, resulting in the emission of an alpha particle, a beta particle and/or gamma radiation. These changes are spontaneous (they happen on their own, without any external trigger) and random (it is impossible to predict exactly when any one nucleus will decay, or in which direction it will emit radiation).
A nucleus is unstable, and therefore radioactive, if it has an excess of neutrons and/or is simply too heavy.
'Random' does not mean 'unaffected by anything' in a vague sense — it specifically means you cannot predict when a given nucleus will decay, only the overall pattern for a large sample.
Section 2
Alpha, beta and gamma radiation
The three main types of nuclear radiation differ in nature, ionising power and penetrating ability:
| Type | Nature | Ionising effect | Penetrating ability |
|---|---|---|---|
| Alpha (α) | Helium nucleus (2 protons + 2 neutrons) | Strongly ionising | Weakly penetrating — stopped by paper/skin |
| Beta (β) | Fast-moving electron | Moderately ionising | Moderately penetrating — stopped by a few mm of aluminium |
| Gamma (γ) | Electromagnetic radiation | Weakly ionising | Strongly penetrating — needs thick lead/concrete |
Ionising effects relate to the particle's charge and kinetic energy: alpha particles are large, slow-moving and highly charged, so they interact strongly and ionise many atoms over a short distance; gamma rays carry no charge and pass through matter with few interactions.
There's a trade-off: the more strongly ionising a radiation type is, the less penetrating it tends to be, because it loses its energy more quickly by interacting with matter.
Section 3
Deflection in electric and magnetic fields
Because alpha and beta particles carry charge, they are deflected by electric and magnetic fields, while gamma radiation (uncharged) is not:
- Alpha particles are positively charged and deflect one way, but only slightly (they are relatively heavy)
- Beta particles are negatively charged and deflect the opposite way, and more strongly (they are much lighter)
- Gamma radiation is undeflected, since it is electromagnetic radiation with no charge
Section 4
What happens to the nucleus during decay
During alpha decay or beta decay, the nucleus changes into that of a different element:
- Beta decay occurs when a neutron turns into a proton (releasing an electron, the beta particle) — this increases the proton number by 1 with no change in nucleon number
- Alpha decay removes 2 protons and 2 neutrons, decreasing both proton number and nucleon number
Each type of decay tends to move the nucleus towards greater stability and reduce any excess of neutrons.
During beta decay: neutron → proton + electron. The new proton stays in the nucleus (increasing proton number by 1); the electron is emitted as the beta particle.
Section 5
Representing decay with nuclide equations
Radioactive decay is represented using nuclide notation, balancing both the nucleon number (top) and proton number (bottom) on each side of the equation:
- Alpha decay: the nucleon number decreases by 4, the proton number decreases by 2
- Beta decay: the nucleon number is unchanged, the proton number increases by 1
- Gamma emission: no change to nucleon number or proton number (the nucleus loses energy, not mass or charge)
Must Know
- Radioactive decay is spontaneous and random, and can produce alpha particles, beta particles and/or gamma radiation
- Alpha: helium nucleus, strongly ionising, weakly penetrating; Beta: fast electron, moderately ionising and penetrating; Gamma: electromagnetic radiation, weakly ionising, strongly penetrating
- Charged alpha and beta particles are deflected by electric/magnetic fields; uncharged gamma radiation is not
- Beta decay: a neutron changes into a proton, releasing an electron (the beta particle)
- Alpha or beta decay changes the nucleus into a different element
- Nuclide equations must balance both nucleon number and proton number on each side
That's the notes covered.
Carry on to the next subtopic.