Alpha, beta and gamma radiation are three kinds of emission from unstable nuclei. They differ in what they are made of, their charge, how far they travel and what stops them. Exam answers usually ask you to compare them, so a clear table in your head is worth more than three separate paragraphs.
This lesson builds on balancing nuclear equations and feeds the ionisation lesson in atomic and nuclear models.
How do the three radiations compare?
| Property | Alpha (α) | Beta-minus (β) | Gamma (γ) |
|---|---|---|---|
| What it is | Helium nucleus: 2 protons and 2 neutrons | Fast electron from the nucleus | Electromagnetic wave |
| Charge | Positive (+2) | Negative (−1) | None |
| Mass | Relatively large | Very small | None |
| Typically stopped by | Paper, or a few centimetres of air | A few millimetres of aluminium | Not fully stopped; reduced by thick lead or concrete |
| Ionising effect | Strong | Weaker | Weak |
| Deflection in electric or magnetic field | Yes, slightly | Yes, strongly, opposite way to alpha | No |
The stopping materials and air ranges are guide values, and the exact figures depend on the source energy. Your exam answer should use the order: alpha, then beta, then gamma.
How do you answer a “compare” question?
Name the property, then state the difference in the same sentence. “Alpha is stopped by paper, but gamma is only reduced by thick lead” is a comparison.
“Alpha is stopped by paper. Gamma is reduced by lead.” leaves the work to the examiner.
Use words that connect: “whereas”, “but”, “more than”, “less than”.
Worked example
A radiation source is tested. The results, from an invented experiment, are:
- A sheet of paper placed in front of the detector does not reduce the count rate.
- A 3 mm sheet of aluminium reduces the count rate to background level.
- A magnetic field deflects the radiation.
Step 1, penetration. Paper does not stop it, so it is not alpha.
Step 2, absorber. Aluminium stops it fully, so it is not gamma. Gamma would only be reduced by thick lead.
Step 3, field. It is deflected, so it is charged. This fits a beta particle.
Conclusion: the radiation is beta. The evidence that rules out each alternative is the stronger part of the answer.
The mistake to watch for
Mistaken answer: “Gamma is deflected in a magnetic field because it is very energetic.”
Energy is not the same as charge. Gamma rays are electromagnetic waves with no charge, so a magnetic or electric field does not deflect them. A deflection needs a charged particle.
A second slip is to write that alpha is “the most powerful” because it is large and heavy. It has the strongest ionising effect, but the shortest range. “Powerful” is too vague to earn marks, so always name the property.
Check yourself
1. State two differences between alpha particles and beta particles.
Show answer
Alpha particles are positive and relatively massive (helium nuclei), whereas beta particles are negative and have a very small mass (fast electrons). Alpha is stopped by paper, whereas beta passes through paper and needs a few millimetres of aluminium.
2. A radiation is not deflected by a magnetic field and is reduced only slightly by 3 mm of aluminium. Which radiation could it be, and why?
Show answer
Gamma. It is not deflected, so it has no charge, and it is barely affected by a thin sheet of aluminium, which fits a highly penetrating electromagnetic wave.
3. True or false: “A thick sheet of lead stops all gamma rays completely.” Explain.
Show answer
False. Lead reduces the intensity of gamma rays, and thicker lead reduces it more, but some gamma rays can still pass through. We say gamma is reduced, not stopped.
Where this leads next
Properties are the tools you use to read experiments like the worked example. The next step is to interpret random count-rate data. The mixed practice set then tests comparisons together with calculations.
If your comparisons are correct in your head but vague on paper, a teacher can work through your sentences with you in online one-to-one Physics tuition.