Radioactivity & half-life

At the dentist's X-ray, in smoke detector ads, in news about nuclear power: the word radiation is everywhere, often with a spooky undertone. Yet you have been surrounded by natural radiation since birth, from the ground, from the air, even from space. Not every kind of radiation is the same: X-rays are produced in a machine and can be switched off, while this topic is about radiation from decaying atomic nuclei. If you understand what is going on, you can weigh risks instead of just fearing them.

Unstable atomic nuclei

The vast majority of atomic nuclei in the materials around you are stable, they stay as they are forever. Some, however, are unstable: at some point they transform on their own and send out energetic radiation. This is called radioactive decay. Nobody can predict when a single nucleus will decay, it is pure chance. But with billions of nuclei, chance turns into a reliable rule, much like rolling dice: one roll is unpredictable, but over a thousand rolls you know pretty well how often a six comes up.

Half-life: halving on the clock

This rule is called the half-life: the time in which half of all unstable nuclei have decayed. After one half-life, half remain; after two, a quarter; after three, an eighth. It is exactly the doubling from the widget further below, only played backwards: instead of 1, 2, 4, 8 it goes 8, 4, 2, 1. Every radioactive substance has its own half-life, from fractions of a second to billions of years.

Useful: medicine and dating the past

Used properly, radioactivity saves lives. In medicine, weakly radiating substances with short half-lives make organs visible on images so diseases are found early, and targeted radiation delivered by specialists destroys tumour cells. And the half-life is a clock for the past: living things absorb the carbon C-14 throughout their lives, and after death it decays with a half-life of about 5,730 years. From the C-14 still left in a find, researchers can read off its age. That is how the glacier mummy Ötzi was found to be about 5,300 years old.

Radiation protection: distance, shielding, time

Because too much radiation can damage body cells, three simple protection rules apply. Distance: from a small source the radiation spreads out in all directions, so it weakens quickly with distance, and at twice the distance only a quarter arrives. Shielding: materials like concrete or lead absorb radiation, which is why you sometimes wear a protective apron during an X-ray. Time: the shorter the exposure, the lower the dose. Professionals in clinics and labs work exactly by these rules, and measuring devices worn on the body, called dosimeters, record the dose received so that limits are reliably kept.