When an unstable nucleus decays, it throws out one of three types of radiation: alpha, beta, or gamma. They’re wildly different — one is a chunky helium nucleus, one is a fast electron (or positron), and one is a burst of pure electromagnetic energy. Each has its own charge, its own ionising power, and its own penetrating power, and there’s a beautiful rule of thumb linking them all: the more strongly a radiation ionises, the less far it travels.
📚 What you need to know
An alpha (α) particle is a helium nucleus: 2 protons + 2 neutrons, charge +2e, mass 4u
A beta-minus (β−) particle is a fast electron, charge −1e; a beta-plus (β+) is a positron, charge +1e
A gamma (γ) ray is high-energy electromagnetic radiation, no charge, no mass
Stopped by: alpha → paper; beta → a few mm of aluminium; gamma → thick lead / concrete
In fields: alpha bends to the negative plate, beta to the positive plate, gamma is not deflected
The greater the ionising ability, the lower the penetrating power
The three types
Let’s meet each one. The key facts to lock in are the charge and what the particle actually is — everything else follows from those.
Radiation
What it is
Charge
Mass
Alpha (α)
Helium nucleus (2p + 2n)
+2e
4u
Beta-minus (β−)
Fast electron
−1e
≈ 0 (0.0005u)
Beta-plus (β+)
Positron (anti-electron)
+1e
≈ 0 (0.0005u)
Gamma (γ)
High-energy EM wave (photon)
0
0
A memory hook: alpha is the heavyweight (big, slow, +2 charge), beta is the sprinter (tiny, fast, ±1 charge), and gamma is the ghost (no charge, no mass, just energy). Their charge decides how they behave in electric and magnetic fields, and their size decides how far they travel. Get those two properties and you can reason out everything else.
Ionising and penetrating power
Two properties define how each radiation behaves:
Ionising ability — how many ion pairs it knocks out as it passes through matter. More ionising = more damage to cells.
Penetrating power — how far it travels before losing all its energy.
The crucial rule: these two are inversely linked. A strongly-ionising radiation dumps its energy quickly, so it can’t travel far. A weakly-ionising radiation slips through matter, so it goes a long way.
The key trade-offgreater ionising ability → lower penetrating power
Alpha is stopped by paper, beta by a few mm of aluminium, and gamma only reduced by thick lead or concrete. Most penetrating (γ) = least ionising, and vice versa.
Properties at a glance
Radiation
Ionising
Penetrating
Range in air
Stopped by
Alpha
Highly
Weakly
3–7 cm
Paper
Beta
Moderately
Moderately
20 cm – 3 m
~3 mm aluminium
Gamma
Weakly
Highly
Infinite (inverse-square)
Thick lead / concrete
Notice how the two middle columns run in opposite directions: as you go alpha → beta → gamma, ionising power drops but penetrating power climbs. If you remember just one of the two orders, you automatically know the other — they’re always reversed.
Behaviour in electric and magnetic fields
Because alpha and beta are charged, they get deflected by fields; gamma, being neutral, sails straight through. In an electric field between two plates:
Alpha (+2e) is pulled towards the negative plate.
Beta-minus (−1e) is pulled towards the positive plate — the opposite way.
Gamma (0) passes straight through, undeflected.
Alpha bends towards the negative plate (small bend — it’s heavy), beta bends the other way towards the positive plate (big bend — it’s light), and gamma goes straight (no charge).
Why does alpha bend less than beta even though it has more charge? Because it’s roughly 8000 times heavier. That huge mass gives it big momentum, so the field barely nudges it. Beta is featherlight, so the same field whips it round sharply. Charge sets the direction of the bend; mass sets how much it bends.
Deflection in a magnetic field
A charged particle moving through a magnetic field also curves, following a circular path. The radius depends on the particle’s speed, mass and charge:
Radius of a charged particle in a magnetic fieldBqv = mv2 / r ⇒ r = mv / BqB = field strength • q = charge • v = speed • m = mass
A larger circular path means a greater deflection. The amount of bending depends on the particle’s speed, mass and charge — the same three things that mattered in the electric field.
WE 1
A radioactive nucleus 23792W undergoes three successive decays: first a beta-minus, then an alpha, then a beta-plus. Determine the proton number and nucleon number of the final nucleus Z.
Step 1 — beta-minus (Z +1, A same)Z: 92 → 93, A: 237Step 2 — alpha (Z −2, A −4)Z: 93 → 91, A: 237 → 233Step 3 — beta-plus (Z −1, A same)Z: 91 → 90, A: 233Z = 90, A = 233 → 23390ZTrack A and Z one decay at a time. Beta-minus pushes Z up, alpha pulls both down, beta-plus pulls Z down. The nucleon number only changes at the alpha step.
WE 2
A source is placed near a detector. Paper does not reduce the count, but a few millimetres of aluminium reduces it to background level. Identify the radiation, and explain your reasoning.
Step 1 — paper has no effect
Paper would stop alpha, so there’s no alpha present.
Step 2 — aluminium stops it completely
A few mm of aluminium stops beta, but gamma would pass through.
The source is a beta emitterAbsorber tests work by elimination: paper rules out alpha, thin aluminium rules in beta (and rules out gamma, which needs thick lead). Match what stops it to the property table.
⚛ Working an alpha/beta/gamma question
Which radiation from an absorber test? Paper stops α; thin aluminium stops β; thick lead needed for γ.
Alpha deflects less than beta despite more charge, because it’s much heavier.
Absorber test: paper (α), thin aluminium (β), thick lead/concrete (γ).
Gamma is the only one with no charge and no mass.
⚠ Common mistakes
Getting the deflection direction backwards — alpha to negative, beta to positive
Thinking alpha deflects more than beta — it deflects less (heavier)
Mixing up the two orders — ionising and penetrating are reversed
Saying gamma is deflected in fields — it has no charge, so it isn’t
Forgetting gamma follows an inverse-square law with infinite range
Confusing beta-minus (electron) with beta-plus (positron)
Quick recap:Alpha is a helium nucleus (+2e, most ionising, least penetrating, stopped by paper), beta is a fast electron or positron (±1e, moderate on both, stopped by thin aluminium), and gamma is a chargeless EM wave (least ionising, most penetrating, needs thick lead). Ionising and penetrating powers run in opposite orders. In fields, alpha bends to the negative plate, beta to the positive plate, and gamma not at all.
You now know what each radiation is and how it behaves. The next step is to write down exactly how the nucleus changes when it emits one — balancing the numbers on each side of a nuclear equation. That’s where we can predict the daughter nucleus every time. Next page: Decay Equations.
Alpha, beta and gamma properties not sticking?
Book a free meeting and we’ll drill the ionising-vs-penetrating trade-off, deflection directions, and absorber tests until they’re automatic.
