What half-life means, how to read it off a decay graph, and how to work out what is left after a number of half-lives, correcting for background.
What you will learn
- Read a half-life from a decay graph
- Work out the activity, count rate or mass left after a whole number of half-lives
- Find a half-life or a time from two readings
- Express the net decline as a ratio (Higher)
Random, but predictable in bulk
One nucleus could decay in the next second or in a thousand years.
But a sample holds billions of nuclei, and in a fixed time a fixed fraction of them decay. So the sample behaves predictably.
The half-life of an isotope is the time for the number of unstable nuclei in a sample to halve, or, the same thing measured, the time for the count rate (or activity) to fall to half. Nothing you do to the sample, heating or squeezing it, changes the half-life.
The rest of this lesson and its 11 practice questions are locked.
In this lesson
- Random, but predictable in bulk
- Reading a half-life from a graph
- Counting half-lives
- Background, and net decline
- Try it: Test the half-life in the simulation
- Try it: Check an answer with the calculator
11 practice questions, marked as you go; about 25 minutes in all.
Key terms
Half-life
In this unit: Atomic structure and radioactivity
The nuclear atom and how its model developed, isotopes, alpha, beta and gamma radiation, nuclear equations, half-life, the hazards and uses of radiation, and fission and fusion.