PhysicsICSE Class 10 · Radioactivity

Properties of Alpha, Beta and Gamma Radiation: Nature, Ionising and Penetrating Power (ICSE Class 10)

Q: What are the properties of alpha particles?

Alpha particles are helium nuclei with charge +2e and mass about 4 u. They travel at about one-tenth the speed of light, have the highest ionising power and the lowest penetrating power (stopped by paper), and are deflected by electric and magnetic fields.

Q: Which radiation is most penetrating and why?

Gamma rays, because they are uncharged electromagnetic waves that ionise very little, so they lose energy slowly and travel far through matter. Only thick lead or concrete reduces them appreciably.

Q: How are α, β and γ deflected in an electric field?

α bends slightly towards the negative plate, β bends strongly towards the positive plate, and γ is not deflected because it has no charge.

Updated 16 October 2026 · Fact-checked

Alpha particles are helium nuclei (charge +2e): most ionising, least penetrating, stopped by paper. Beta particles are fast electrons (charge −e): moderately ionising, stopped by a few mm of aluminium. Gamma rays are uncharged electromagnetic waves: least ionising, most penetrating, reduced only by thick lead, and not deflected by fields.

Notice that alpha, the most ionising, is stopped first by paper, while gamma, the least ionising, passes through the most material.

Illustration drawn for QuestionBuddy.

Understand Properties of Alpha, Beta and Gamma Radiation

The three radiations.

Property	Alpha (α)	Beta (β)	Gamma (γ)
Nature	Helium nucleus $_{2}^{4} He$ (2p + 2n)	Fast electron $_{- 1}^{0} e$ from the nucleus	Electromagnetic wave of very high frequency
Charge	$+ 2 e$	$- e$	None
Mass	About 4 u (heavy)	About $\frac{1}{1840}$ u (very light)	None (zero rest mass)
Speed	About one-tenth of $c$	Up to about 0.9 $c$	$c = 3 \times 1 0^{8}$ m s⁻¹
Ionising power	Highest	Medium	Lowest
Penetrating power	Lowest: stopped by a sheet of paper or a few cm of air	Medium: stopped by a few mm of aluminium	Highest: only reduced by several cm of lead or thick concrete
Electric and magnetic fields	Deflected (slightly; towards the − plate)	Deflected (strongly; towards the + plate)	Not deflected
Photographic plate, fluorescence	Affects	Affects	Affects

Ionising power. Radiation passing through a gas knocks electrons out of its atoms, producing ions. This is how radiation is detected, and how it damages living cells.

α particles are heavy, slow and doubly charged, so they interact strongly with every atom they pass, producing very many ions per centimetre and losing energy quickly.
β particles have a smaller charge and move much faster, so they produce fewer ions.
γ rays carry no charge and ionise least.

Penetrating power: the reverse order. A radiation that ionises strongly uses up its energy quickly and so cannot travel far. Hence

Ionising: α > β > γ. Penetrating: γ > β > α.

Deflection in electric and magnetic fields. Between two charged plates, α bends towards the negative plate, β towards the positive plate, and γ goes straight on. In a magnetic field, α and β are deflected in opposite directions (they carry opposite charges). Treat a beam of α as a current in its direction of motion, and a beam of β as a current opposite to its motion, then use Fleming's left-hand rule. β is deflected much more than α because it is thousands of times lighter (an α particle is roughly 7300 times as massive as an electron). γ, being uncharged, is never deflected.

Identifying radiation with absorbers. Place a detector in front of a source and insert absorbers in turn:

a drop in count when paper is inserted shows α;
a further drop when a few mm of aluminium is inserted shows β;
a count still above background after the aluminium shows γ, which is reduced further only by thick lead.

Always compare with the background count (taken with no source present).

Hazard depends on where the source is. Outside the body, α is stopped by the dead outer layer of skin and is the least dangerous; γ and β penetrate. Inside the body (if swallowed or breathed in), α is the most dangerous, because it deposits all its ionising energy in a small region of living tissue.

Properties of Alpha, Beta and Gamma Radiation: formulas and key facts

Ionising and penetrating order: $I o ni s in g : α > β > γ ∣ P e n e t r a t in g : γ > β > α$; Strong ionisation means short range.
Charges: $q_{α} = + 2 e, q_{β} = - e, q_{γ} = 0, e = 1.6 \times 1 0^{- 19} C$; α: 3.2 × 10⁻¹⁹ C.
Stopped by: α: paper (few cm of air)
β: few mm of aluminium
γ: reduced by several cm of lead; γ is reduced, not completely stopped.
Deflection: $α : s l i g h t, t o w a r d s - pl a t e ∣ β : l a r g e, t o w a r d s + pl a t e ∣ γ : n o n e$; Magnetic field: α and β opposite ways.

How to solve Properties of Alpha, Beta and Gamma Radiation questions

Most questions test one property at a time; match the property to the cause (charge, mass, speed).

1Identify which property is asked: nature, charge, ionising, penetrating, or deflection.
2For ionising and penetrating power, remember they run in opposite orders.
3For deflection between plates, unlike charges attract: α to −, β to +, γ straight.
4For a magnetic field, use Fleming's left-hand rule: current along α's motion, opposite to β's motion.
5For absorber data, subtract the background count, then look at the drop caused by each absorber.

Properties of Alpha, Beta and Gamma Radiation in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Why a smoke detector's source is safe on the ceiling

Suppose you worry about the small α source inside the smoke detector on your ceiling.

Explain why it poses little risk to people in the room.

Try it first, then show the solutionSolution

α particles are stopped by a few centimetres of air and by the detector's casing.
Even if some reached you, they cannot get through the dead outer layer of the skin.

Answer: α has very low penetrating power; it cannot reach people in the room.

Memory hook: Alpha: big hitter, short reach.

Lead aprons in an X-ray and radiotherapy unit

Imagine visiting a hospital where staff near a γ-ray source stand behind thick concrete walls and lead screens, not just thin plastic sheets.

Why are such heavy shields needed?

Try it first, then show the solutionSolution

γ rays have the highest penetrating power of the three radiations.
They pass through paper, plastic and aluminium; only thick, dense materials such as lead and concrete reduce them enough.

Answer: γ rays are very penetrating, so only thick lead or concrete shields the staff.

Memory hook: Gamma: gentle ioniser, long reach.

Tricks to remember Properties of Alpha, Beta and Gamma Radiation

Short memory aids. Say each one aloud once, then use it on a question the same day.

Paper, aluminium, lead
'Papa Always Leads': Paper stops α, Aluminium stops β, Lead reduces γ.
Use it when: Penetrating power.
The seesaw rule
The more it ionises, the less it penetrates: α top for one, bottom for the other.
Use it when: Ordering ionising and penetrating power.
Opposites attract
α (+) bends to the − plate; β (−) bends to the + plate; γ (0) goes straight.
Use it when: Electric-field deflection.
Light ones swerve
β is thousands of times lighter than α, so it swerves far more.
Use it when: Comparing amount of deflection.

Properties of Alpha, Beta and Gamma Radiation: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: A stream of electrons · 1 mark · MCQBoard level

Which radiation emitted by radioactive nuclei is a stream of fast-moving electrons?
(a) α
(b) β
(c) γ
(d) X-rays

Try it first, then show the solutionSolution

β particles are electrons created when a neutron changes into a proton.

Answer: (b) β

Example 2: Alpha and gamma: opposites · 3 marks · Short answerBoard level

(a) State two properties in which α particles and γ rays are at opposite extremes. (b) Why do α particles produce so many ions?

Try it first, then show the solutionSolution

(a) Any two: α has the highest ionising power, γ the lowest; α has the lowest penetrating power, γ the highest; α is charged and deflected by fields, γ is uncharged and not deflected. (2)
(b) They are heavy, relatively slow and carry a charge of +2e, so they interact strongly with the atoms they pass. (1)

Answer: (a) Ionising power, penetrating power, charge/deflection (any two) (b) Large charge, large mass, low speed

Example 3: Beams in a magnetic field · 3 marks · Short answerHarder board level

Beams of α and β particles travel northwards, horizontally, through a magnetic field that points vertically downwards. In which direction is each beam deflected? What happens to a γ beam?

Try it first, then show the solutionSolution

α (positive): current northwards; left hand with forefinger down, middle finger north gives the thumb pointing west. α is deflected to the west. (1)
β (negative): current southwards, so the force is reversed: β is deflected to the east (and more sharply). (1)
γ is uncharged and passes straight through. (1)

Answer: α: west; β: east (more sharply); γ: undeflected

Example 4: Why β bends more · 2 marks · Short answerHarder board level

In the same electric field, β particles are deflected much more than α particles. Give two reasons connected with their nature.

Try it first, then show the solutionSolution

A β particle is an electron, roughly $4 \times 1836 \approx 7300$ times lighter than an α particle, so the same size of force changes its path far more. (1)
Although α has twice the charge, its huge mass outweighs this; α also moves more slowly but is still much harder to deflect. Hence β bends more. (1)

Answer: β is far lighter (about 1/7300 of α's mass); its smaller charge does not make up the difference

Example 5: Gamma rays · 3 marks · Short answerHarder board level

(a) What is the nature of γ rays? (b) Why are they not deflected by electric or magnetic fields? (c) Why are they the most penetrating?

Try it first, then show the solutionSolution

(a) Electromagnetic waves of very high frequency (shorter wavelength than X-rays), travelling at the speed of light. (1)
(b) They carry no charge. (1)
(c) Being uncharged, they ionise very little, so they lose energy slowly and travel far through matter. (1)

Answer: (a) High-frequency EM waves (b) No charge (c) Weak ionisation, slow energy loss

Example 6: Reading absorber data · 3 marks · Short answerHOTS / competency

A detector placed near a source gives the following counts per minute: no absorber 480; with paper 300; with 5 mm of aluminium 120; with 5 cm of lead 30. With the source removed, the count is 30 per minute. Which radiations does the source emit? Justify each.

Try it first, then show the solutionSolution

α: the paper reduces the count by $480 - 300 = 180$ per minute. (1)
β: the aluminium reduces it by a further $300 - 120 = 180$ per minute. (1)
γ: after aluminium the count (120) is still $90$ above background; only the lead brings it down to background (30). So the source emits α, β and γ. (1)

Answer: α, β and γ

Example 7: Case study: identifying an unknown source · 4 marks · Case-basedHOTS / competency

A narrow beam from a source in a lead block passes between two charged plates. A detector screen shows two spots: one deflected slightly towards the negative plate, one undeflected. When a sheet of paper is placed in the beam, the deflected spot disappears.
(a) Identify the two radiations.
(b) Why is there no spot deflected towards the positive plate?
(c) Which of the two radiations is more dangerous if the source is swallowed? Explain.
(d) Which is more dangerous to a person standing a metre away? Explain.

Try it first, then show the solutionSolution

(a) Slightly deflected towards −, stopped by paper: α. Undeflected: γ. (1)
(b) The source emits no β particles (which would bend strongly towards +). (1)
(c) α: inside the body it deposits all its strong ionising energy in nearby living cells. (1)
(d) γ: α is stopped by the air and the skin, but γ penetrates the body. (1)

Answer: (a) α and γ (b) No β emitted (c) α (d) γ

Where students lose marks in Properties of Alpha, Beta and Gamma Radiation

The slips examiners see most often, and what to write instead.

Mistake: Wrong radiation for lowest ionising power
Ionising power is greatest for α and least for γ.
Wrong working
'The radiation with the lowest ionising power is α.'
Right working
'γ has the lowest ionising power.'
Do this: Remember: ionising and penetrating orders are opposite.
Usually costs: Reported by CISCE examiners for 2025 Q2(i)(f).
Mistake: Saying γ rays are 'stopped' by lead
Lead greatly reduces γ rays but does not stop them completely.
Wrong working
'γ rays are stopped by a thin lead sheet.'
Right working
'γ rays are reduced (weakened) by several cm of lead.'
Do this: Use 'reduced' for γ.
Usually costs: Usually costs the mark.
Mistake: β deflected less than α
β is far lighter, so it is deflected more.
Wrong working
'α is deflected more because its charge is larger.'
Right working
'β is deflected more; its tiny mass outweighs α's larger charge.'
Do this: Think of mass first when comparing deflection.
Usually costs: Usually costs the reasoning mark.
Mistake: Ignoring the background count
Some radiation is always present; counts must be compared with background.
Wrong working
'The lead does not stop all the radiation, since 30 counts remain.'
Right working
'30 per minute is the background; the lead has cut the source's γ to (nearly) zero.'
Do this: Subtract background before drawing conclusions.
Usually costs: Usually costs the conclusion mark.

Exam tips

Use a table when asked to compare α, β and γ; give one entry per property for each.
In deflection diagrams, show α bending slightly to −, β bending sharply to +, γ straight.
Write 'helium nucleus' (not 'helium atom') for an α particle.
For γ, say 'electromagnetic waves', not 'particles'.

Practice questions on Properties of Alpha, Beta and Gamma Radiation

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The radiation stopped by a sheet of paper is:
(a) α
(b) β
(c) γ
(d) X-rays
Hint
Least penetrating.
Answer
(a) α
Q2 · 1 mark · Very short answerBoard level
Name the radiation that travels at the speed of light.
Hint
Electromagnetic wave.
Answer
γ rays.
Q3 · 1 mark · Very short answerBoard level
State the charge on (a) an α particle, (b) a β particle.
Hint
In units of e.
Answer
(a) $+ 2 e$ . (b) $- e$ .
Q4 · 2 marks · Short answerHarder board level
Arrange α, β and γ in increasing order of (a) penetrating power, (b) ionising power.
Hint
Opposite orders.
Answer
(a) α < β < γ. (b) γ < β < α.
Q5 · 2 marks · Short answerHarder board level
Why do α particles have the lowest penetrating power?
Hint
Ionisation uses up energy.
Answer
They ionise very strongly (heavy, slow, charge +2e), so they lose their energy in a short distance and are stopped by paper or a few cm of air.
Q6 · 3 marks · Short answerHarder board level
A source emitting α, β and γ is placed so that its beam passes between two charged plates. Describe the path of each radiation.
Hint
Charge and mass.
Answer
α bends slightly towards the negative plate; β bends sharply towards the positive plate; γ goes straight through.
Q7 · 1 mark · Very short answerHarder board level
Express the charge on an α particle in coulombs ( $e = 1.6 \times 1 0^{- 19}$ C).
Hint
Two units of e.
Answer
$2 \times 1.6 \times 1 0^{- 19} = 3.2 \times 1 0^{- 19}$ C.
Q8 · 1 mark · Very short answerHarder board level
α particles leave a nucleus at about one-tenth of the speed of light. Find this speed in m s⁻¹ ( $c = 3 \times 1 0^{8}$ m s⁻¹).
Hint
0.1c.
Answer
$0.1 \times 3 \times 1 0^{8} = 3 \times 1 0^{7}$ m s⁻¹.
Q9 · 2 marks · Short answerHOTS / competency
A beam of β particles travels eastwards through a magnetic field pointing north. In which direction is it deflected?
Hint
Current is opposite to electron motion.
Answer
The equivalent current is westwards. Left hand: forefinger north, middle finger west, thumb points down. The β beam is deflected downwards.
Q10 · 2 marks · Short answerHOTS / competency
Explain why an α source is relatively safe outside the body but very dangerous if swallowed.
Hint
Penetration versus ionisation.
Answer
Outside, α cannot get through the dead outer skin layer, so it does no harm. Inside, there is no protective layer: α ionises living cells very strongly over a short distance, causing severe damage.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Nature of an α particle?Show answer
Helium nucleus (2 protons + 2 neutrons).
2.Most penetrating radiation?Show answer
γ.
3.Lowest ionising power?Show answer
γ.
4.What stops β particles?Show answer
A few mm of aluminium.
5.Which radiation is not deflected by fields?Show answer
γ.

Properties of Alpha, Beta and Gamma Radiation: frequently asked questions

What are the properties of alpha particles?

Alpha particles are helium nuclei with charge +2e and mass about 4 u. They travel at about one-tenth the speed of light, have the highest ionising power and the lowest penetrating power (stopped by paper), and are deflected by electric and magnetic fields.

Which radiation is most penetrating and why?

Gamma rays, because they are uncharged electromagnetic waves that ionise very little, so they lose energy slowly and travel far through matter. Only thick lead or concrete reduces them appreciably.

How are α, β and γ deflected in an electric field?

α bends slightly towards the negative plate, β bends strongly towards the positive plate, and γ is not deflected because it has no charge.