Physics › Electromagnetism

Radiation

Unstable nuclei don't sit still — they fling out particles and energy until they settle down. Three kinds, three different ways to stop them.

What “radiation” actually means

Radiation is just energy on the move — either as waves (light, radio, X-rays) or as fast-moving particles. The kind people worry about is ionizing radiation: it carries enough punch to knock electrons off atoms, which is what damages cells. Visible light and radio waves don't — they're non-ionizing.

The three flavours from a nucleus

Alpha (α) — a clump of 2 protons + 2 neutrons (a helium nucleus). Heavy and slow, so it dumps its energy fast. A sheet of paper — or your skin — stops it. Dangerous only if you breathe or swallow the source.
Beta (β) — a high-speed electron (or positron) shot out when a neutron flips to a proton. Lighter, faster, more penetrating. A few millimetres of aluminium or plastic blocks it.
Gamma (γ) — pure electromagnetic energy, the highest-frequency light there is. No charge, no mass, very penetrating. It takes thick lead or concrete to cut it down.

Stopping power, in one line

Paper stops α · thin metal stops β · thick lead/concrete is needed for γ. Penetration goes up, ionizing punch per collision goes down.

Half-life: the maths of decay

You can't predict when one atom decays — but a huge pile of them follows a clean rule. After one half-life, half are left. After two, a quarter. After n half-lives:

N = N₀ · (1/2)ⁿ

Carbon-14's half-life is about 5,730 years, which is why it dates things tens of thousands of years old. Iodine-131's is 8 days — handy in medicine because it's gone within weeks.

Background radiation

You're bathed in low-level radiation all the time: cosmic rays, radon seeping from rock, potassium-40 in bananas and in your own body. Dose is measured in sieverts (Sv); a typical year gives you a few millisieverts. The numbers only matter when they pile up far past that.