Alpha radiation has the greatest ionizing power but the lowest penetrating power. Gamma radiation has the lowest ionizing power but the greatest penetrating power, while beta radiation is intermediate in both properties.
This comparison is part of IB Physics E.3 Radioactive decay and applies to both SL and HL.
| Radiation | Ionizing power | Penetrating power and shielding |
|---|---|---|
| Alpha, | Very high | Very low, stopped by paper, skin, or a few centimetres of air |
| Beta, | Medium | Medium, stopped by a few millimetres of aluminium or plastic |
| Gamma, | Low | Very high, attenuated by thick lead or concrete |
An alpha particle is a helium nucleus containing two protons and two neutrons. Its charge of and relatively large mass cause frequent, strong interactions with atoms. It transfers energy rapidly, producing dense ionization, but therefore loses its kinetic energy over a short distance.
A beta particle is a fast electron or positron with charge magnitude and much lower mass. It interacts less strongly than an alpha particle, so it produces less ionization per unit distance and travels farther through matter.
Gamma radiation consists of high-energy electromagnetic photons with no charge and no rest mass. Gamma photons interact relatively infrequently, so they cause less ionization per unit distance and penetrate deeply. Shielding reduces gamma intensity rather than guaranteeing that every photon is stopped.
A common misconception is that gamma radiation is always the most dangerous because it penetrates furthest. External gamma radiation is a major hazard, but an alpha-emitting source inside the body can be extremely damaging because its energy is deposited through intense ionization in a small volume of tissue.
Exam technique: For an Explain question, connect interaction strength to energy loss: stronger interaction causes greater ionization and shorter range. Do not merely memorize the rankings; state the physical reason and identify suitable shielding.