Ionising Power and Deflection of Nuclear Radiation

Radioactive nuclei give out radiation spontaneously and in random directions. The three types are alpha (α), beta (β), and gamma (γ). This topic explains how strongly they ionise matter and how they are deflected by electric and magnetic fields.

What each radiation is

• Alpha (α): a helium nucleus (2 protons + 2 neutrons), charge $+2e$, relatively heavy and slow.
• Beta (β−): a fast electron, charge $-e$, very light and fast.
• Gamma (γ): high-energy electromagnetic wave, no mass, no charge.

Ionising power

Ionisation means knocking electrons off atoms to make ions. Think of radiation moving through air like objects moving through a crowd: larger or more strongly charged “objects” bump into more people.

• Alpha: very strong ioniser. Reasons: large charge $+2e$ pulls on electrons strongly; moves more slowly so spends longer near atoms; heavy, so it does not get pushed off course easily.
• Beta: moderate ioniser. Charge is only $-e$ and the particle is very light and fast, so it makes fewer ions than alpha.
• Gamma: weak ioniser. No charge, so it usually passes through without removing many electrons.

Link to penetration: strong ionisers lose energy quickly and do not travel far. Alpha is stopped by paper or skin, beta by thin aluminium, gamma needs thick lead or concrete.

Deflection in electric fields

Electric fields push charged particles. The force size depends on charge; direction depends on the sign of the charge.

• Alpha ($+2e$): attracted to the negative plate; bends a little (heavy).
• Beta ($-e$): attracted to the positive plate; bends a lot (very light).
• Gamma (0 charge): no deflection.

In symbols, the electric force is bigger for bigger charge: $$F = qE$$

Deflection in magnetic fields

Moving charged particles are pushed sideways in magnetic fields. Alpha and beta bend in opposite directions because their charges are opposite. Beta bends more than alpha because it is much lighter. Gamma is not deflected.

The size of the magnetic force depends on charge and speed: $$F = qvB$$ (direction is at right angles to motion and field).