Half Lives
Physics: AQA GCSE Higher, topic 4.2.16
Half-Lives
The half-life is the average time taken for the number of radioactive nuclei to halve. The activity of a radioactive substance is measured in Becquerels (Bq).
Predicting half-lives
- The radioactive decay of an individual atom is random and unpredictable.
- However, large groups of nuclei do normally behave in predictable ways.
Constant half-lives
- The half-life is the average time taken for the number of radioactive nuclei to halve.
- The half-life for any isotope of an element is constant.
- The half-life of unstable isotopes can be seconds or millions of years.
Measuring half-lives
- To find the half-life of an isotope from a graph or table,
- Measure the time that it takes for the radioactive count rate or the number of unstable nuclei to halve.
Background radiation
- To find the half-life of a radioactive isotope, we must strip out the background radiation of the environment around us.
- The equation for calculating substance radiation is:
- Substance radiation = total radiation − background radiation
Measuring background radiation
- We can measure the level of background radiation using a Geiger counter.
Test yourself on Half Lives
A substance's radioactivity is measured over several hours. What is its half-life?
What is the half-life of the substance in this graph?
Kendall measures the count rate of Plutonium at 9:00 am at 50 Hz. She then measures it again at 1:00 pm and finds the count rate is now 25 Hz. What is Plutonium's half-life?
Kendall measures the count rate of Plutonium at 10:00 am at 50 Hz. She then measures it again at 1:00 pm and finds the count rate is now 12.5 Hz. What is Plutonium's half-life?