Alpha, beta and gamma are the three common types of radiation from radioactive material. They differ in what they are and, most usefully for exams, in how far they penetrate.
This lesson is part of nuclear energy. The next lesson, interpreting half-life data, looks at how a radioactive source decays with time.
How do the three types compare?
Use penetration as the thread. A less penetrating type is easier to stop.
| Type | What it is | Stopped by | Penetration |
|---|---|---|---|
| Alpha (α) | Helium nucleus, 2 protons and 2 neutrons | Paper or a few cm of air | Lowest |
| Beta (β) | A fast electron | A thin sheet of aluminium | Middle |
| Gamma (γ) | Electromagnetic wave, no mass | Thick lead or concrete | Highest |
Alpha is heavy and slow, so it loses energy quickly and stops easily. Gamma has no mass or charge, so it passes through most materials.
Worked example: a shielding question
Question: A source emits radiation. A detector reads 120 counts per minute with nothing in between. With paper in between it reads 118, with 3 mm aluminium it reads 45, and with 5 cm lead it reads 2. Identify the radiation emitted.
Step 1: read the paper result. Paper barely changes the count, 120 to 118. So alpha is not present in a large amount.
Step 2: read the aluminium result. Aluminium cuts the count from 118 to 45, so beta is stopped. Some radiation still passes.
Step 3: read the lead result. Only thick lead cuts the count to 2, which is typical of gamma.
The source emits beta and gamma. Write the answer as a conclusion and cite the readings.
The mistake that costs marks
The slip is to reverse the order of penetration. Students write that alpha is the most penetrating because it is the “largest” particle.
| Step | Wrong | Right |
|---|---|---|
| Alpha | Goes through aluminium | Stopped by paper |
| Beta | Stopped by paper | Stopped by thin aluminium |
| Gamma | Stopped by paper | Needs thick lead |
| Reasoning | Bigger means stronger | Heavier particles lose energy faster, so stop sooner |
Link the reason to the particle. Heavy and charged means short range, while no mass and no charge means long range.
Check yourself
A source is wrapped in paper and still gives a high count on a detector, but the count falls when a 3 mm aluminium sheet is added. Which type or types does this suggest?
Answer
Paper does not reduce the count, so the radiation is not alpha. Aluminium reduces the count, so beta is present.
If the count does not fall to zero behind the aluminium, some gamma may also be present.
What to study next
A source also weakens as it decays. Continue with interpreting half-life data. You can then test your understanding with the graph evidence and fair-comparison lab.
To practise shielding questions with a teacher, see online one-to-one Science tuition.