Example 1: half-life calculation
A source has an activity of 640 Bq and a half-life of 8 days. What is its activity after 24 days?
- Number of half-lives = 24 ÷ 8 = 3.
- 640 → 320 → 160 → 80.
- Activity after 24 days = 80 Bq.
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IB DemystifiedMYP Sciences
Radioactivity
Some atomic nuclei are unstable. At a moment no one can predict, they break apart and throw out radiation. That randomness lets us date ancient bones, trace blood flow in a patient and stop smoke detectors from failing, but it also means radiation must be handled with care.
Recommended for MYP 5 · eAssessment priority · About 3 lessons · Criteria A, B, C and D
On this page
- Learning objectives
- Before you start
- Key vocabulary
- Understanding radioactivity
- Half-life and decay
- Radioactivity in the real world
- Worked examples
- In the eAssessment
- Check your understanding
- Practice questions
- Investigation
- Criterion-linked questions
- Challenge questions
- Topic check
- Review your mistakes
- Your progress
Learning objectives
By the end of this topic you should be able to:
- describe the nucleus, isotopes and what makes a nucleus radioactive
- compare alpha, beta and gamma radiation: what they are, how far they penetrate and how strongly they ionise
- use half-life to describe decay and calculate activities, times and ages
- write simple alpha and beta decay changes
- explain uses of radioactivity and how risks are reduced
- evaluate decisions about food irradiation and nuclear waste
Before you start
You will use these skills. If any feel shaky, review them first.
- atomic structure: protons, neutrons and electrons
- the electromagnetic spectrum and ionising radiation (see EM spectrum)
- reading curved graphs and halving repeatedly
Key vocabulary
- Isotope
- Atoms of the same element with different numbers of neutrons.
- Radioactive decay
- The random change of an unstable nucleus, giving out alpha, beta or gamma radiation.
- Activity
- The number of nuclei decaying per second, measured in becquerels (Bq).
- Half-life
- The time for the number of radioactive nuclei in a sample, or its activity, to halve.
- Background radiation
- Ionising radiation around us all the time, from natural and artificial sources.
- Irradiation / contamination
- Exposure to radiation from an outside source / radioactive material getting onto or into something.
Understanding radioactivity
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What is it?
Some nuclei have an unstable combination of protons and neutrons. They decay by emitting an alpha particle (two protons and two neutrons), a beta particle (a fast electron formed when a neutron turns into a proton) or gamma radiation (a high-energy electromagnetic wave).
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Why does it matter how it behaves?
Alpha is the most strongly ionising, so it loses its energy quickly and is stopped by paper or skin. Gamma is the least ionising and most penetrating, needing thick lead or concrete. Beta is in between. Ionising radiation can damage DNA in living cells, which is why it is both useful (killing cancer cells or bacteria) and hazardous.
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How do we know?
Henri Becquerel discovered radioactivity in 1896 when uranium salts fogged photographic plates. Marie and Pierre Curie discovered polonium and radium, and Ernest Rutherford identified alpha and beta radiation by how far they penetrated and how they were deflected. Today Geiger counters measure count rates directly.
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Why does it matter?
Radioactivity is used in medicine (tracers, radiotherapy, sterilising equipment), industry (thickness gauges, leak detection), dating rocks and ancient remains, smoke detectors and nuclear power.
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What does it connect to?
Radioactivity links to atomic structure and electromagnetic waves in physics; to isotopes and nuclear equations in chemistry; to mutations and cancer in biology; and to the age of rocks and the Earth.
Half-life and decay
We cannot predict when a single nucleus will decay, but in a large sample a fixed fraction decays in a given time. After each half-life, half of the remaining nuclei are left: 1 → ½ → ¼ → ⅛ → 1/16.
- Alpha decay: atomic number −2, mass number −4 (a new element).
- Beta decay: atomic number +1, mass number unchanged.
- Gamma emission: no change in either number; the nucleus loses energy.
Always subtract the background count rate from measured count rates before using them.
Radioactivity in the real world
Hospitals use technetium-99m, a gamma emitter with a half-life of about 6 hours, in millions of scans each year. Gamma rays sterilise syringes and surgical gloves after packaging. Radiocarbon dating, which relies on carbon-14's half-life of 5730 years, has dated ancient remains and artefacts around the world.
Worked examples
Example 2: choosing an isotope
Which isotope should be used to find a leak in an underground water pipe?
- The radiation must pass through soil to a detector above ground, so it must be gamma.
- The half-life should be long enough to do the search (hours to days) but short enough that it does not remain in the water supply.
- Choose a gamma emitter with a half-life of hours or a few days.
In the eAssessment
Radioactivity questions often combine data, a use and a safety decision. Expect:
- Read decay curves and data tables to find half-lives, after correcting for background.
- Deduce the type of radiation from absorber data.
- Calculate activities, times, remaining fractions and ages using half-life.
- Choose and justify isotopes for a use by type of radiation and half-life.
- Discuss risks and benefits, such as food irradiation or storing nuclear waste.
Common ways to lose marks: saying everything has decayed after two half-lives; saying irradiated objects become radioactive; confusing ionising with penetrating; and forgetting to subtract background.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Investigation: how much background radiation is around us?
Partially guided investigation · about 50 minutes · small groups with a teacher
- Research question
- How does the background count rate differ between five places in and around the school (for example a classroom, a basement or ground floor, near a granite or brick wall, outdoors on grass, and on an upper floor)?
- Scientific background
- Background radiation comes from rocks, building materials, radon gas in the air, cosmic rays and food. Because decay is random, count rates vary from one minute to the next.
- Hypothesis
- Write your own prediction about which location will have the highest count rate, with a justification.
- Variables
- Identify your independent, dependent and control variables, and explain how you will control them.
- Apparatus
- Geiger–Müller tube and counter (supplied and set up by your teacher), stopwatch, clamp stand, map of the school.
- Method
- At each location, hold the tube at the same height and orientation.
- Record the number of counts in 1 minute, five times, and calculate the mean count rate.
- Repeat for each location, then compare the means.
- Decide whether any differences are larger than the variation between repeat counts.
Safety. No radioactive sources are used; you are measuring only natural background radiation. Handle the Geiger–Müller tube carefully, as its window is fragile, and follow your teacher's instructions when moving around the school.
Then evaluate: are the differences between places real, or could they be caused by random variation?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
| Skill | Correct | Status |
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