You cannot say when one nucleus will decay, but you can say exactly how long it takes for half of a whole sample to go. That steady halving, the same for a given isotope every time, is the half-life.
The half-life is the time for half the radioactive nuclei in a sample to decay, which is the same as the time for the activity to fall to half. Decay is random, but each isotope has a fixed half-life.
The half-life of a radioactive isotope is the time taken for half the radioactive nuclei in a sample to decay, or for the activity (count rate) to fall to half.
Radioactive decay is random, but the half-life is constant for a given isotope.
Watch the activity fall to a half, then a quarter, then an eighth, one half-life at a time.
Four quick checks. Each correct answer earns XP and lights this skill on your star map.
The half-life is the time for...
After one half-life, the activity of a source is...
After two half-lives, the fraction of the original nuclei remaining is...
Radioactive decay of individual nuclei is...
One half-life leaves a half, two leave a quarter, three leave an eighth.
A source has an activity of 800 Bq and a half-life of 6 hours. Find its activity after 18 hours.
Each half-life halves what is left, it does not subtract a fixed amount. After two half-lives a quarter remains, after three an eighth, never reaching zero in a fixed number of steps. Decay is random but the half-life is constant.
Unlocks once the four checks above are done. Worth more XP, written in the style of Paper 2.
A source has an activity of 800 Bq and a half-life of 6 hours. After 18 hours its activity is...
After 3 half-lives, the fraction of the original radioactive nuclei remaining is...
The activity of a source falls from 240 Bq to 30 Bq. This took...
Half-life is mapped. Keep the chain going.