Radioactive Decay

Radioactive decay is random and spontaneous. Random means it is impossible to predict which nucleus decays or when. Spontaneous means it is unaffected by temperature, pressure or chemical state — you cannot slow it down or speed it up by any ordinary means.

Alpha (α) — a helium nucleus, two protons and two neutrons. Charge +2, heavily ionising, stopped by paper or a few centimetres of air, and deflected by fields.

²³⁸U → ²³⁴Th + ⁴He

The nucleon number falls by 4 and the proton number by 2.

Beta-minus (β⁻) — an electron emitted when a neutron becomes a proton. Charge −1, moderately ionising, stopped by a few millimetres of aluminium.

¹⁴C → ¹⁴N + e⁻ + ν̄_e

The nucleon number is unchanged and the proton number rises by 1.

Gamma (γ) — a high-energy photon emitted when a nucleus loses excess energy. No charge, no mass, weakly ionising, and reduced but never fully stopped by thick lead.

Example: the ordering is not a coincidence. Ionising power and penetrating power are inversely related, and for the same reason. Alpha particles are large and highly charged, so they interact strongly, deposit their energy quickly and stop almost immediately. Gamma rays barely interact, so they travel far but do little along the way.

The hazard depends on where the source is — outside the body, gamma is the most dangerous because alpha cannot penetrate skin. Inside the body, alpha is by far the most dangerous, since all that ionising energy is deposited in a small volume of living tissue. A source that is harmless in the hand can be lethal if swallowed.

Background radiation must be subtracted from any measurement. Its main sources are radon gas from rocks, cosmic rays, food and drink, building materials and medical procedures — the majority of it natural.

Checking a decay equation — nucleon number and proton number must each balance on the two sides. Charge and baryon number conservation are what those balances express.