Alpha, beta and gamma radiation
Key idea: Alpha particles ionise most strongly and are stopped by paper. Beta particles travel further and are stopped by a few millimetres of aluminium. Gamma rays ionise least and are only reduced by thick lead.
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Guided steps
- Separate a helium nucleus, emitted electron and uncharged photon.
- Check each particle’s nature and how uncharged gamma can ionise.
- Compare ionisation and penetration for the three radiation types.
- Read how paper changes the detector reading.
- Choose an absorber for beta and explain your choice.
- Explain how an uncharged gamma photon can cause ionisation.
- Interpret both track features before selecting the likely radiation.
- Compare penetration, exposure route and the conditions needed to judge risk.
- Compare alpha exposure outside and inside the body.
- Use plate signs for field direction, then charge sign for force direction.
- Identify the radiation and explicitly check its relative charge.
- Draw all three paths and explain their bending directions.
- Review radiation properties, then use Practise and check.
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Alpha, beta and gamma radiation all come from unstable nuclei, but they behave very differently. A sheet of paper stops alpha particles, while gamma rays pass through a person and need thick lead to cut them down. These differences come from what each radiation is made of, and they help explain applications such as smoke alarms and the different routes by which radiation reaches living tissue.
What alpha, beta and gamma radiation are
| Radiation | What it is | Written as | Relative charge |
|---|---|---|---|
| alpha (α) | helium nucleus: 2 protons, 2 neutrons | ⁴₂He | + 2 |
| beta-minus (β⁻) | fast-moving electron | ⁰₋₁e | -1 |
| gamma (γ) | electromagnetic wave | γ | 0 |
An alpha particle is relatively massive. A beta-minus particle is an electron, roughly 7000 times lighter than an alpha particle, made in the nucleus when a neutron changes into a proton. A gamma ray has no rest mass and no charge: it is a short-wavelength electromagnetic wave, like light but carrying far more energy.
Check your understanding 1 Separate charge from ionisation
Choose the nature of alpha and beta-minus, then decide whether uncharged gamma radiation can ionise material.
Show answer
Alpha is a helium nucleus, beta-minus is an emitted electron from a nuclear process, and gamma is an uncharged photon. Gamma can transfer energy to electrons in matter and cause ionisation. Charge, rest mass and ionising ability are separate properties.
Ionising effect and penetrating power
As radiation passes through a material, it can knock electrons out of the atoms it passes, turning them into ions. This is ionisation. Alpha and beta particles lose energy as they ionise, so dense ionisation gives short tracks. Gamma photons may pass through or interact and transfer energy. A shield reduces the gamma beam’s intensity rather than stopping every photon at one fixed distance.
- Alpha particles are massive, slow and have a charge of + 2. For typical school examples they produce dense ionisation and lose their energy over a short distance: a few centimetres in air, or a sheet of paper.
- Beta-minus particles are light, fast and have a charge of -1. Typical beta particles produce less dense ionisation and travel further than typical alpha particles. A few millimetres of aluminium absorbs beta radiation in the examples here; actual range depends on particle energy and the material.
- Gamma rays have no charge and ionise only weakly, so they are the most penetrating. Thick lead or concrete reduces their intensity, rather than providing a definite stopping thickness for all rays.
So the order of ionising effect is the reverse of the order of penetrating power:
| Property | Alpha | Beta-minus | Gamma |
|---|---|---|---|
| Ionising effect | strong | moderate | weak |
| Penetrating power | low | moderate | high |
| Stopped by | paper, a few cm of air | a few mm of aluminium | intensity reduced by thick lead or concrete |
Worked example 1
Identifying radiation with a sheet of paper
Problem
A detector near a source gives a count-rate well above background. When a sheet of paper is placed between the source and the detector, the count-rate falls to the background level. For this idealised identification question, the detector is sensitive to all three radiation types and the readings are repeated at fixed geometry. Which radiation best fits the paper test? Explain what a background-level reading tells you.
Show full solution
Use the absorber
Method
The detected source contribution is consistent with alpha radiation; no remaining source contribution is resolved after the paper.
Reason
Paper stops alpha particles. A detectable beta or gamma contribution would remain above background in the stated model. A weak beta or gamma contribution could be too small to detect above the background fluctuations.
Working
Answer
Stopped by paper → alpha.
Link to ionisation
Method
Alpha particles are stopped so easily because they ionise so strongly.
Reason
They give up their energy in many ionisations over a very short distance.
Working
Answer
Strong ionisation → rapid energy loss → short range.
Quick check
The count-rate falls to background, not to zero: the detector still records background radiation.
Exam-style question 1
Choosing an absorber for beta radiation
Examination question
A source emits beta-minus radiation only. Which of these would stop the radiation: a sheet of paper, a few millimetres of aluminium, or a few centimetres of air? Explain your answer. [2 marks]
Choose the absorber and its penetration reason
Show full solution
Choose and explain
2 marksMethod
A few millimetres of aluminium.
Reason
Beta-minus particles are more penetrating than alpha particles, so they pass through paper, but they are stopped by a few millimetres of aluminium.
Working
Answer
Paper: not enough. Aluminium, a few mm: stops beta.
Self-mark with the mark scheme
Tick each mark point that your answer includes.
Award one mark for each point.
Common mistake
Choosing the paper loses both marks: paper stops alpha particles, not beta.
Check your understanding 2 Can gamma rays ionise?
A learner says, “Gamma rays have no charge, so they cannot ionise anything.” Explain the energy transfer that makes this possible, on paper or here. Compare your full explanation below; a typed note is optional.
Show answer
Gamma photons carry energy and can transfer enough to remove an electron from an atom. They ionise less densely than typical alpha or beta particles because their interactions are less frequent; many photons can pass through substantial thicknesses of material.
Try it yourself 1
Reading a cloud chamber track
Problem
In a cloud chamber, tiny droplets form on the ions left behind by a particle, so the particle’s path shows up as a track. One source produces short, thick, straight tracks. Which radiation is it most likely to be? Explain both features of the tracks.
Interpret thickness, then length, then likely identity
Hints
Hint 1
thickness
A thick track means many droplets, so many ions were made along the path.
Hint 2
length
A radiation that ionises strongly uses up its energy over a short distance.
Show solution step by step
Thick tracks
Method
The radiation ionises strongly.
Reason
Many ions along the path give many droplets, so the track is thick.
Working
Answer
Thick track → strong ionisation.
Short tracks
Method
The radiation is alpha.
Reason
Strong ionisation uses up the particle’s energy within a few centimetres of air, so the track is short.
Working
Answer
Short, thick track → alpha.
Quick check
Beta-minus particles would leave thinner, longer tracks, because they ionise less strongly and travel further.
Which radiation is most dangerous?
Penetration and ionisation matter together with the exposure route. Alpha radiation is largely stopped by intact outer skin, while penetrating gamma radiation can reach internal tissue from outside. Alpha-emitting material inside the body can deposit energy densely in nearby living tissue. This does not rank every source: risk also depends on activity, radiation energy, exposure time and where radioactive material goes in the body. See the EPA explanation of exposure pathways and radiation risk.
Spot the mistake 1
Is alpha always the most dangerous?
Learner response
A learner says, “Alpha is the most ionising, so an alpha source on a laboratory bench is always more dangerous to stand near than a gamma source.” Correct the learner.
Compare access to living tissue before ranking danger
Show solution step by step
The alpha source outside the body
Method
Alpha particles from the bench are stopped by a few centimetres of air or by the outer layer of skin.
Reason
Their strong ionisation gives them a very short range, so they do not generally reach internal tissue through intact skin.
Working
Answer
External alpha through intact skin: strongly ionising but limited penetration.
The gamma source outside the body
Method
Gamma rays can reach internal organs from outside the body.
Reason
They are penetrating enough to pass through the skin and reach internal organs, where they can ionise.
Working
Answer
External gamma: weakly ionising but reaches the organs.
Common mistake
How dangerous a radiation is depends on whether it can reach living cells as well as on how strongly it ionises. Internal alpha emission can be very harmful, but a universal ranking still requires information about the sources and doses.
Deflection in an electric field
Because alpha and beta-minus particles are charged, they are deflected when they pass between charged plates. Alpha particles, being positive, bend towards the negative plate. Beta-minus particles, being negative, bend towards the positive plate. Gamma rays have no charge and go straight on. The direction follows charge sign. The amount of bending also depends on speed, field strength and time in the field; the figure illustrates directions rather than giving a universal curvature ratio.
Guided practice 1
Identifying radiation by its deflection
Problem
A narrow beam of radiation passes between two charged plates and bends towards the positive plate. Identify the radiation and state its charge.
Work out the sign of the charge first
Hints
Hint 1
attraction
Opposite charges attract. What sign of charge is pulled towards a positive plate?
Hint 2
identity
Which of the three radiations has a negative charge?
Show solution step by step
Find the sign of the charge
Method
The radiation is negatively charged.
Reason
It is attracted towards the positive plate.
Working
Answer
Bends towards + → negative charge.
Identify it
Method
Beta-minus radiation, with a charge of -1.
Reason
It is the only negatively charged one of the three.
Working
Answer
β⁻ = ⁰₋₁e
Quick check
Alpha particles would bend the other way, towards the negative plate, and gamma rays would not bend at all.
Check your understanding 3 Reverse the field and reconstruct the paths
On paper, draw three beams entering from left to right between an upper negative plate and lower positive plate. Show the electric field and the initial bending direction of alpha and beta-minus, then gamma’s path. Explain the charge-to-force link for each; no numerical curvature is required. Compare the complete labelled sketch below; a typed note is optional.
Show answer
The electric field points upwards, from the lower positive plate to the upper negative plate. Alpha has +2 charge, so its force and initial bending are upwards towards the negative plate. Beta-minus has −1 charge, so its force and initial bending are downwards towards the positive plate. Gamma is uncharged and continues straight in this electric field. Check both plate signs, beam-motion arrows, the field arrow and all three paths with their charge reasons. The sketch does not assign measured bending angles or uniquely identify a source from one track.
Common mistakes
- Mixing up the two orders. Alpha ionises most and penetrates least; gamma ionises least and penetrates most.
- Saying gamma rays are charged, or that they cannot ionise at all.
- Saying alpha radiation is harmless. It is very dangerous inside the body.
- Saying a sheet of lead stops all gamma rays. Lead reduces their intensity; thicker lead reduces it more.
- Saying a beta-minus particle is an electron from the atom’s outer shells. It is made in the nucleus.
- Expecting a detector reading to fall to zero when the source contribution is absorbed. It approaches the background reading, which fluctuates.
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