Alpha, beta and gamma radiation
Some atomic nuclei are unstable. To become more stable they throw out energy and particles, a process called radioactive decay. There are three kinds of radiation to know, and they differ in what they are made of and how far they can travel.
| Radiation | What it is | Stopped by |
|---|---|---|
| Alpha (\( \alpha \)) | A helium nucleus: 2 protons and 2 neutrons | A sheet of paper or a few cm of air |
| Beta (\( \beta \)) | A fast electron from the nucleus | A few mm of aluminium |
| Gamma (\( \gamma \)) | A high-energy electromagnetic wave | Several cm of lead or thick concrete |
Alpha is the most ionising but the least penetrating; gamma is the least ionising but the most penetrating; beta sits in between. When a nucleus decays, its mass number (top) and atomic number (bottom) must balance on both sides of the equation.
Alpha decay loses 2 protons and 2 neutrons, for example uranium decaying to thorium:
Beta decay turns a neutron into a proton and emits an electron, so the atomic number rises by 1 while the mass number stays the same, for example carbon decaying to nitrogen:
Where this is assessed
Balancing decay equations and describing penetration are core Criterion A skills. Interpreting a count-rate table or graph to find a half-life is strong Criterion C processing, and discussing the risks and uses of radiation reaches Criterion D.
Half-life
Radioactive decay is random: you cannot say when any single nucleus will decay. But across a large sample the behaviour is beautifully regular, and we measure it with the half-life.
- Half-life
- The average time taken for half of the unstable nuclei in a sample to decay, which is also the time for the activity (count rate) to fall to half its value.
Because it halves each time, the activity drops quickly at first and then more slowly, tracing a curve that never quite reaches zero. Counting how many halvings you need is usually the fastest way to solve these problems.
A radioactive source has an activity of 800 Bq and a half-life of 5 hours. How long does it take for the activity to fall to 100 Bq?
| Half-lives passed | Activity (Bq) |
|---|---|
| 0 | 800 |
| 1 | 400 |
| 2 | 200 |
| 3 | 100 |
Count the halvings
Rather than reaching for a formula, write the halving sequence in a little table. The number of arrows from start to finish is the number of half-lives; multiply by the half-life to get the time.
Nuclear fission and fusion
Both fission and fusion release enormous amounts of energy from the nucleus, but they are opposites. Fission splits a large nucleus apart; fusion joins small nuclei together.
| Feature | Fission | Fusion |
|---|---|---|
| What happens | A large nucleus (such as uranium-235) splits into smaller ones | Small nuclei (such as hydrogen) join into a larger one |
| Triggered by | Absorbing a slow neutron | Extremely high temperature and pressure |
| Where used | Today's nuclear power stations | The Sun and stars |
In fission the neutrons released can go on to split more nuclei, setting off a chain reaction that a reactor controls carefully. Fusion powers every star, but recreating it on Earth is hard because the nuclei must be squeezed together against their strong electrical repulsion.
Do not swap the definitions
Fission is splitting (think "fission = fracture"), fusion is joining (think "fusion = fuse together"). Muddling the two is one of the most common mistakes in this topic.
The life cycle of a star
A star is born from a huge cloud of gas and dust called a nebula. Gravity pulls the cloud together into a protostar, which heats up until fusion begins, and the star settles into a long, stable main sequence phase, like our Sun today. Here the outward push of fusion balances the inward pull of gravity.
What happens next depends on the star's mass.
| Stage | Star like the Sun | Much more massive star |
|---|---|---|
| After main sequence | Swells into a red giant | Swells into a red supergiant |
| Dramatic phase | Sheds a planetary nebula | Explodes as a supernova |
| Final remnant | Fades to a white dwarf | Neutron star or, if massive enough, a black hole |
Supernovae matter to us directly: the heavier elements they scatter across space, including the carbon and iron in our bodies, were forged inside dying stars. We really are made of star stuff.
Check yourself
Try each before you reveal it.
1. Which type of radiation is the most penetrating, and what is needed to stop it? +
Gamma radiation is the most penetrating. It needs several centimetres of lead or thick concrete to reduce it significantly.
2. A sample has a half-life of 2 days and a starting activity of 1600 Bq. What is its activity after 6 days? +
6 days is 3 half-lives. Halving each time: \( 1600 \rightarrow 800 \rightarrow 400 \rightarrow 200 \). So the activity is 200 Bq.
3. State the difference between nuclear fission and nuclear fusion, and give one example of where each occurs. +
Fission splits a large nucleus into smaller ones and happens in nuclear power stations. Fusion joins small nuclei into a larger one and happens in the Sun and other stars.
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