Radioactivity

An accidental discovery

In 1896 the French physicist Henri Becquerel left some uranium salts on a wrapped photographic plate in a drawer. When he developed the plate, it was fogged, as if it had been exposed to light. The uranium was giving out invisible rays on its own, without any energy being put in. Marie Curie named this radioactivity. With Pierre Curie she showed it came from the atoms themselves, not from any chemical reaction, and she discovered two new radioactive elements, polonium and radium. She was the first woman to win a Nobel Prize, and is still the only person to win Nobel Prizes in two different sciences (Physics in 1903, Chemistry in 1911).

Nobody then knew radiation was dangerous. Curie carried radium in her pockets and kept it in her desk drawer, and she died in 1934 of a blood disease almost certainly caused by her long exposure. Her notebooks are still radioactive and are stored in lead-lined boxes.

Why some nuclei are unstable

A nucleus is held together by the strong nuclear force, which is pulling against the electric repulsion between protons. Only certain combinations of protons and neutrons are stable. A nucleus with too many neutrons, too few neutrons, or simply too many particles altogether is unstable. Sooner or later it changes into a more stable nucleus by throwing out a particle, energy, or both. This is called radioactive decay, and the particles and energy given out are nuclear radiation.

Decay happens inside the nucleus, so it is not affected by temperature, pressure or chemical reactions. Heating uranium or dissolving it in acid doesn't change how fast it decays. Isotopes that decay are called radioisotopes.

Alpha, beta and gamma

Rutherford found there were different kinds of radiation by seeing how far they travelled through materials and how they bent between charged plates. He named them after the first three letters of the Greek alphabet.

• Alpha (α) particles are 2 protons and 2 neutrons stuck together, the same as a helium nucleus. When a nucleus gives out an alpha particle, its atomic number goes down by 2 and its mass number by 4. Uranium-238 (92 protons) becomes thorium-234 (90 protons).
• Beta (β) particles are fast electrons made inside the nucleus when a neutron turns into a proton. The mass number stays the same but the atomic number goes up by 1. Carbon-14 (6 protons) becomes nitrogen-14 (7 protons).
• Gamma (γ) rays are not particles but high-energy electromagnetic waves, like X-rays but usually more energetic. A nucleus that is left with extra energy after alpha or beta decay gets rid of it as gamma. Gamma doesn't change the element.

So alpha and beta decay do something alchemists dreamed of: they turn one element into another. This is called transmutation.

Comparing the three types

All three are ionising radiation: they have enough energy to knock electrons out of atoms they hit, turning them into ions. In living cells this can break DNA. Alpha is the most ionising but can't get through skin, so alpha sources are most dangerous if they are swallowed or breathed in. Gamma is the least ionising but passes deep into the body.

Background radiation

Radiation is all around us. This background radiation comes mainly from:

• Radon gas, which seeps out of rocks and soil containing uranium and can build up in poorly ventilated rooms
• Rocks and building materials, especially granite
• Cosmic rays from space (pilots and frequent flyers get more, because there is less air above them)
• Food and our own bodies: potassium-40 and carbon-14
• Medical procedures such as X-rays and CT scans

Radiation dose is measured in sieverts (Sv). The average Australian receives about 1.5 millisieverts a year from natural sources, which is lower than in many countries. Radiation is detected with a Geiger–Müller counter, which clicks each time radiation ionises the gas inside its tube, and people who work with radiation wear dosimeter badges to record their dose. In Australia, radiation safety is overseen by ARPANSA, the Australian Radiation Protection and Nuclear Safety Agency.