Ionizing Radiation From The Nucleus

Put a radiation counter on a bench in an empty room, switch it on and walk away. It clicks. Not steadily, and not because anything in the room is broken: a click now, two more a second later, then a pause, then another. Those clicks have been arriving since long before anyone built a counter to hear them, and they are coming from the bricks in the wall, from the food in your bag, from the air you are breathing and from the depths of space. Somewhere inside each of those sources, a nucleus that had held together for a million years chose this particular second to fall apart, and nothing that has ever been done to it changed that choice by a fraction of a second.

This lesson is about that behaviour and about how physicists learned to predict it without ever being able to predict any single event. You will find out what an unstable nucleus throws out and why, how to balance a decay equation using nothing harder than subtraction, how a sheet of paper stops one kind of radiation while several centimetres of lead only weaken another, and how a quantity called half-life turns a completely random process into something a hospital can schedule around. By the end you will be able to look at a table of isotopes and say which one belongs in a smoke alarm, which one belongs in a patient and which one nobody should let near either.