Half-Life Flashcards

All 6 cards in this deck

What is the half-life of a radioactive isotope?

The time taken for half the undecayed nuclei to decay, or for the activity of a source to fall by half.

What fraction of the original nuclei remains undecayed after $n$ half-lives?

$\left(\frac{1}{2}\right)^n$

e.g. after 4 half-lives, $\frac{1}{16}$ remains.

What are the steps to find the half-life from a graph of activity against time?

e.g. a source starts at 80 Bq

1. Halve the starting activity: $80 \div 2 = 40$ Bq.
2. Read across from 40 Bq to the curve, then down to the time axis.
3. That time is the half-life.

What are the steps to find the activity remaining after a given number of half-lives?

e.g. 800 Bq after 3 half-lives

1. Start with the initial activity: 800 Bq.
2. Halve it once per half-life: 400 Bq, 200 Bq, 100 Bq.
3. Activity after 3 half-lives = 100 Bq.

What are the steps to give the net decline of a source as a ratio?

e.g. after 3 half-lives

1. Find the fraction remaining: $\left(\frac{1}{2}\right)^3 = \frac{1}{8}$.
2. Write it as remaining : original.
3. Net decline $= 1 : 8$.

True or false? The half-life of a particular isotope is always the same, whatever the size of the sample.

True. Half-life is a fixed property of the isotope.