Sec 4 Combined Science (Physics) Radioactivity notes

An atom has a tiny, dense nucleus of protons (charge +1) and neutrons (charge 0), around which electrons (charge −1) orbit in shells. Almost all the mass sits in the nucleus; almost all the volume is the electron space (an atom is mostly emptiness). Two numbers label every nucleus: the proton number Z — the number of protons, which fixes what element it is — and the nucleon number A — the total of protons + neutrons.

Nuclide notation stacks them: ^A_Z X. Read off: protons = Z; neutrons = A − Z; electrons (in a neutral atom) = Z. So carbon-14, ¹⁴₆C, holds 6 protons, 8 neutrons, 6 electrons. Isotopes share a proton number but differ in nucleon number — ¹²₆C and ¹⁴₆C are both carbon, both chemistry-identical, just two neutrons apart.

Unstable nuclei rearrange themselves by decaying — spitting out radiation — and the process has two defining properties. It is random: no one can say which nucleus will decay next, or when — every nucleus has the same chance per second, and a counter next to a source visibly flickers. It is spontaneous: decay happens no matter what — heating, cooling, dissolving or bonding the atoms does not hurry or delay it.

But with billions of nuclei the randomness averages out into a law: in any fixed time interval, the same fraction decays. That fraction-clock is the half-life.

Alpha (α) is a particle — 2 protons + 2 neutrons, a helium nucleus. It is the most ionising (it rips electrons off atoms it passes) and the least penetrating: a sheet of paper or a few centimetres of air stop it. Beta (β) is a fast electron shot out of the nucleus — moderately ionising, stopped by a few millimetres of aluminium. Gamma (γ) is not a particle but an electromagnetic wave — the least ionising and most penetrating: air barely touches it and only thick lead reduces it meaningfully.

Even with no source in the room, a radiation counter keeps clicking: background radiation. Its sources include radon gas seeping from the ground, cosmic rays from space, rocks and building materials (traces of uranium and thorium), food and drink (potassium-40), and medical exposures. The level varies place to place but never falls to zero.

For accurate source measurements, measure the background count rate first and subtract it from every reading — the residue is the source’s own activity. Every careful half-life calculation in this chapter uses corrected counts.

The half-life is the time for half the unstable nuclei in a sample to decay — equivalently, for the activity to halve. Each half-life cuts the count by half again: after n half-lives, activity = starting value ÷ 2ⁿ. Half-lives run from microseconds to millennia, but the arithmetic is identical. Read them from tables (successive halvings) or off a decay curve (find the time for the count to fall to half its corrected start).

The penetration and ionising power of radiation make it a tool. Medical tracers: a gamma- or beta-emitting isotope injected in tiny amounts follows blood or organs and is detected outside the body. Cancer treatment: tightly focused gamma beams kill tumour cells. Sterilisation: gamma passes through sealed syringes and food packaging, killing microbes without opening them. Thickness gauges: beta counts falling on a detector reveal foil thickness. Dating: carbon-14's half-life times ancient remains.

The same ionising power is the hazard: radiation damages or kills living cells, and damaged DNA can produce mutations and cancers. Precautions follow from the physics: shielding (lead), short exposure times, large distances — handle sources with tongs, store them in lead-lined boxes, point them away from people.