Alpha, beta-minus and gamma radiation

Alpha, beta-minus and gamma are three forms of nuclear radiation. All three are ionising: they carry enough energy to knock electrons out of the atoms they pass. What each one is, especially its charge and mass, explains how strongly it ionises, how far it travels through matter and how it moves in electric and magnetic fields.

What each radiation is

Alpha emission is common in heavy unstable nuclei. A beta-minus particle is created at the moment of decay, when a neutron in the nucleus changes into a proton; it is not one of the atom’s orbiting electrons. Gamma radiation is emitted when a nucleus left in an excited state drops to a lower energy, often just after an alpha or beta decay. It carries away energy but changes neither the proton number nor the nucleon number.

Ionisation and penetration

Charged particles lose energy as they ionise atoms along their paths. At comparable initial energies, greater energy loss per centimetre usually gives a shorter range. Gamma photons instead have a chance of interacting at any depth, so their intensity decreases through an absorber without a fixed stopping distance.

• An alpha particle has a large charge and moves relatively slowly, so it pulls hard on the electrons of every atom it passes. A typical alpha particle produces tens of thousands of ion pairs in each centimetre of air and stops within a few centimetres.
• A beta particle has half the charge and moves much faster, so it spends less time near each atom and ionises far fewer atoms per centimetre. It travels much further: a metre or more in air, and it needs a few millimetres of aluminium to stop it.
• A gamma photon has no charge. It interacts only occasionally, so it has no fixed range. A thickness of lead reduces the mean transmitted intensity, and more lead reduces it further. Absorption is a matter of probability, rather than every photon stopping at one fixed depth.

These ranges compare typical emissions. The exact range depends on the energy of the radiation and on the absorber.

Deflection in electric and magnetic fields

A charged particle in an electric field feels a force F = qE, so the direction of deflection follows the sign of the charge. Between charged plates, alpha particles curve towards the negative plate and beta-minus particles towards the positive plate. Gamma radiation has no charge, so it passes straight through.

A magnetic field at right angles to a beam also deflects charged particles, and again alpha and beta-minus curve in opposite directions. Use Fleming’s left-hand rule, remembering that a beam of beta-minus particles is a current flowing opposite to their motion. Gamma is not deflected.

How sharply each beam curves depends on more than the sign of the charge. For motion perpendicular to a magnetic field, the radius is r = p/B|q|, where p is the momentum. The magnitude |q| sets the radius; the sign sets the direction. For a non-relativistic particle, p = mv, but a fast beta particle may need relativistic momentum. A typical alpha particle has a far larger momentum than a typical beta particle, even though it is slower, so its path curves much less. In a weak field, alpha particles can look almost undeflected while beta particles bend strongly.

Comparing the three radiations

Common mistakes

• Saying a beta particle is an orbiting electron knocked out of the atom. It is created in the nucleus when a neutron changes into a proton.
• Assuming a more penetrating radiation must ionise more strongly. Compare energy deposited per centimetre, rather than range alone.
• Saying gamma is completely stopped by lead. Lead reduces its intensity; it does not give gamma a fixed range.
• Judging how much a beam bends from its charge alone. Mass and speed matter too, which is why alpha curves so little in a magnetic field.