IB Physics SLTopic 5 — The Atomic & Nuclear WorldPaper 1 & 2α · β · γ~8 min read
Alpha, Beta & Gamma Radiation
An unstable nucleus has three ways to shed its excess: it can spit out a chunky alpha particle, fling out a fast beta particle, or release a burst of gamma radiation. Each one carries a different charge, a different mass, and a different reach — and knowing which is which is the key to almost every radioactivity question you’ll meet.
📘 What you need to know
There are three types of nuclear radiation: alpha (α), beta (β), and gamma (γ)
Alpha = a helium nucleus (2 protons + 2 neutrons), charge +2e, relatively heavy
Beta-minus = a fast electron, charge −1e, very light; beta-plus = a positron, charge +1e
Gamma = a high-energy electromagnetic photon, no charge, no mass
Ionising power: α > β > γ. Penetrating power: γ > β > α (they run in opposite order)
Stopped by: alpha → paper; beta → a few mm of aluminium; gamma → thick lead or concrete
In an electric or magnetic field, alpha and beta deflect in opposite directions (opposite charges) and gamma is undeflected
The Three Types
Alpha (α)
An alpha particle is a helium nucleus — two protons and two neutrons bound together. It’s the heavyweight of the three, with a charge of +2e. Because it’s big and highly charged, it bulldozes into atoms and rips off electrons very easily, making it the most ionising type. But that same trait means it loses energy fast and doesn’t get far. Its symbol is α or 42He.
Beta (β)
A beta-minus particle is a fast-moving electron flung out of the nucleus, with charge −1e and almost no mass. It’s lighter and less charged than alpha, so it ionises moderately and penetrates further. (There’s also beta-plus, a positron, the electron’s antimatter twin, with charge +1e.) Its symbol is 0−1β.
Gamma (γ)
Gamma radiation isn’t a particle of matter at all — it’s a burst of high-energy electromagnetic waves (a photon), released when a nucleus has too much energy. It has no charge and no mass, so it barely interacts with atoms: least ionising, but most penetrating. Its symbol is 00γ.
Ionising vs Penetrating Power
Two properties matter, and they run in opposite order. Ionising power is how easily the radiation knocks electrons off atoms as it passes. Penetrating power is how far it travels before being stopped. The rule of thumb: the more strongly something ionises, the faster it uses up its energy, so the less it penetrates.
most ionising: α
— then —
β
— then —
least ionising: γ
most penetrating: γ
— then —
β
— then —
least penetrating: α
The neat way to remember penetration is by what stops each type: a sheet of paper stops alpha, a few millimetres of aluminium stops beta, and it takes thick lead (or several metres of concrete) to cut gamma down.
Alpha is stopped by paper, beta by a few mm of aluminium, and gamma passes through both — only thick lead or concrete cuts it down. Penetration runs γ > β > α.
Deflection in an Electric Field
Because alpha and beta particles are charged, they bend when they pass between charged plates — and because their charges are opposite (α is +, β− is −), they curve in opposite directions. Gamma has no charge, so it sails straight through, undeflected.
There’s a subtlety worth knowing: alpha carries the bigger charge, but it’s also far more massive, so its high momentum means it actually deflects less than the light, nimble beta particle in the same field.
Alpha (+) bends toward the negative plate and beta (−) toward the positive plate — opposite ways. Gamma is uncharged and goes straight. Alpha’s greater mass means it deflects less than beta.
Quick recap: α is a helium nucleus (+2e, most ionising, least penetrating), β is a fast electron (−1e, moderate), and γ is an uncharged photon (least ionising, most penetrating). Paper stops α, aluminium stops β, thick lead stops γ.
🧭 Identifying an unknown radiation
Try a sheet of paper — if it stops the radiation, it’s alpha
Try a few mm of aluminium — if that stops what got through, it’s beta
Try thick lead — if only lead reduces it, it’s gamma
Or use a field — deflects one way → one charge; the opposite way → opposite charge; no deflection → gamma
Remember to correct for background when reading the count rate at each stage
WE 1
A beam of radiation is passed between two charged plates. It is deflected slightly toward the negative plate. (a) State the type of radiation and explain your reasoning. (b) Explain why a beta particle in the same field would deflect more.
Part (a) — identify the radiation
it deflects toward the negative plate, so it must be positively charged
gamma has no charge (would go straight); beta-minus is negative (would go the other way)
→ it is an alpha particle (charge +2e)Part (b) — why beta bends more
a beta particle has far less mass than an alpha particle
so for a similar force it has much lower momentum and is deflected more easily
→ beta deflects more than alpha in the same field
WE 2
A radioactive source is tested with absorbers. Paper does not reduce the count rate, but a 3 mm aluminium sheet stops it completely. (a) Identify the radiation. (b) State one property that makes this type more ionising than gamma.
Part (a) — identify the radiation
paper doesn’t stop it → it is not alpha
but a few mm of aluminium stops it completely
→ it is beta radiationPart (b) — why beta ionises more than gamma
a beta particle is charged (−1e), whereas gamma has no charge
a charged particle interacts strongly with atoms, knocking off electrons
→ beta ionises more than uncharged gamma
💡 Top tips
Ionising and penetrating run in opposite orders. α > β > γ for ionising; γ > β > α for penetrating — learn both directions
Match the absorber to the type:Paper → alpha, Aluminium → beta, Lead → gamma (think “PAL”)
Opposite charges, opposite bends. In a field, α and β curve opposite ways; γ doesn’t bend at all
Alpha bends less than beta despite its bigger charge — because it’s so much heavier (higher momentum)
⚠ Common mistakes
Swapping the orders — saying gamma is the most ionising, or alpha the most penetrating. It’s the reverse
Thinking gamma bends in a field — it’s uncharged, so it never deflects
Assuming alpha deflects most because it has the biggest charge — its large mass means it actually bends less than beta
Calling gamma a particle of matter — it’s an electromagnetic photon with no mass and no charge
Up next: Radioactive Decay Equations. Now you know what each particle is, we’ll balance the books — writing decay equations so the nucleon and proton numbers add up correctly on both sides, and working out the daughter nucleus.
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