Alpha, Beta and Gamma

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 7.4 — Alpha, beta and gamma as ionising radiations emitted at random from unstable nuclei
• 7.5 — The nature of alpha, beta and gamma, and how they differ in penetration and ionisation
• 7.6 — Practical: investigate the penetrating power of different types of radiation (required practical)

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Some nuclei are UNSTABLE, meaning that the balance of protons and neutrons in them cannot hold together indefinitely. Such a nucleus will sooner or later break up, throwing out something in the process. That emission is RADIOACTIVE DECAY, and there are three kinds on this specification: ALPHA, BETA and GAMMA.

Before what they are, the word that matters most: RANDOM. It is impossible to predict when any particular nucleus will decay. Not difficult — impossible. A nucleus that has sat unchanged for a thousand years is no more likely to decay in the next second than one made a moment ago; the nucleus has no memory and nothing builds up inside it. Nothing you can do to a sample changes it either: heating it, freezing it, crushing it, or reacting it chemically leaves the decay entirely unaffected, because decay happens in the nucleus and all of those act on the electrons.

Think of it like a hall full of people tossing coins

Put a million people in a hall, give each a coin, and ask everyone to toss once a minute and sit down if they get heads. You cannot predict who will sit down next, or when any particular person will — each toss is genuinely unconnected to every other and to how long that person has been standing. Yet you can say with great confidence that after one minute about half will be sitting, after two minutes about three quarters, and so on. That is radioactivity exactly. The individual event is unpredictable and the behaviour of the crowd is highly predictable, and it is only because the crowd is so enormous — even a speck of material contains far more than a million nuclei — that the pattern comes out smooth.

Now the three emissions. An ALPHA particle is two protons and two neutrons bound together — which is a helium nucleus — with a charge of +2 and a relative mass of 4. It is by far the biggest and most heavily charged of the three. A BETA particle is a fast-moving ELECTRON, charge −1, with a mass about one two-thousandth of a proton's. GAMMA is not a particle at all: it is an electromagnetic wave, at the very short-wavelength end of the spectrum, with no charge and no mass.

All three are IONISING, which means they knock electrons off the atoms they pass, leaving charged ions behind. Ionisation is the reason these radiations are useful and the reason they are dangerous — it is what a detector registers, and it is what damages living cells.

Why the most ionising radiation is the least penetrating

1. Ionising an atom costs energy, so every ionisation an emission causes takes a little of its energy away.
2. An alpha particle is large and doubly charged, so it interacts strongly with almost every atom it passes and ionises very heavily.
3. Ionising heavily means losing energy quickly, so an alpha particle runs out of energy within a few centimetres of air and is stopped by a sheet of paper.
4. A beta particle is far smaller and singly charged, so it ionises much less per centimetre and therefore travels further — a few millimetres of aluminium will stop it.
5. Gamma has no charge at all, so it interacts only weakly and ionises very little, which is precisely why it penetrates so far that thick lead only reduces it rather than stopping it.
6. So the ordering is not a coincidence: MOST IONISING IS LEAST PENETRATING, and it is the same property viewed from two ends.

Charge also decides how they behave in a field. An alpha particle is positive and a beta particle negative, so a magnetic field deflects them in OPPOSITE directions; gamma has no charge and is not deflected at all. Beta is deflected far more than alpha for the same field, because it is so much lighter — a small mass is easier to push off course.

One consequence catches most learners out and is worth stating now, because it looks backwards. Alpha is the easiest to stop, so OUTSIDE the body it is the least dangerous — your skin stops it and it never reaches anything vital. But if an alpha source is swallowed or inhaled, it is by far the MOST dangerous, because all that heavy ionisation is now delivered directly into living tissue at point-blank range with nothing in between. Gamma is the reverse: dangerous from outside because it reaches your organs, less damaging inside because most of it passes straight through without depositing much.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

The three radiations

Learn this definition · Ionising radiation

Radiation that knocks electrons off the atoms it passes through, leaving charged ions behind. It is this ionisation that detectors register and that damages living cells.

Learn this definition · Radioactive decay

The breaking up of an unstable nucleus, which emits alpha, beta or gamma radiation. The process is entirely random: it is impossible to predict when any individual nucleus will decay.

Required practical 7.6 — investigating penetrating power

1. Measure the count rate with no source present, over a long count, to find the background rate.
2. Place the sealed source in a holder a fixed short distance from a Geiger–Müller tube, handling it only with long tongs.
3. Record the count over a measured time with nothing between source and detector, and subtract the background to get the corrected count rate.
4. Place a sheet of paper between the source and the detector and repeat the measurement.
5. Replace the paper with a few millimetres of aluminium and repeat.
6. Replace the aluminium with a thick sheet of lead and repeat.
7. Identify the radiation: stopped by paper means alpha; passing paper but stopped by aluminium means beta; still detected through lead means gamma.
8. Return the source to its lead-lined store as soon as the readings are finished.

Variables

Independent (changed) — The absorbing material placed between the source and the detector
Dependent (measured) — The corrected count rate registered by the Geiger–Müller tube

Sources of error

What randomness means — and what it rules out

• It is IMPOSSIBLE to predict when a particular nucleus will decay
• A nucleus has no memory: age does not make decay more likely
• HEATING, cooling, crushing or reacting the material chemically changes nothing, because decay happens in the NUCLEUS
• Only the vast number of nuclei present makes the behaviour of the sample predictable

Danger from outside the body against danger from inside

A. OUTSIDE, gamma is the most dangerous because it penetrates to the organs, and alpha the least because skin stops it.

B. INSIDE, alpha is by far the most dangerous because its heavy ionisation is delivered straight into tissue, and gamma mostly passes through.

Model answer [4 marks]

Explain why alpha radiation is the most ionising but the least penetrating of the three radiations. [4]

An alpha particle has a charge of +2 and a relatively large mass, so it interacts strongly with the atoms it passes and knocks electrons off a great many of them. Each ionisation takes energy from the alpha particle. Because it ionises so heavily, it loses its energy over a very short distance and is brought to a stop after only a few centimetres of air or by a sheet of paper. The two properties are therefore linked: it is precisely because alpha ionises so strongly that it cannot penetrate far.

Model answer [5 marks]

Describe how absorbers are used to identify whether a sealed source emits alpha, beta or gamma radiation. [5]

Measure the background rate first and subtract it from every reading. With the source a fixed distance from the tube, record the corrected count rate with nothing between them, then with paper, then with a few millimetres of aluminium, then with thick lead. A fall to background behind paper means alpha; a fall only behind aluminium means beta; a significant count still recorded through lead means gamma. The distance and counting time must be kept the same throughout.

Not this: Lead stops gamma radiation.

This: Lead REDUCES gamma radiation; it does not stop it. Gamma has no definite range — a thicker absorber always cuts the intensity further but never quite to zero. Alpha and beta genuinely are stopped; gamma is only ever attenuated.

Mark-losing trap. Decay is RANDOM: no heating, cooling or chemical reaction changes it in any way.

Mark-losing trap. Alpha is stopped by PAPER, beta by a few mm of ALUMINIUM, gamma only REDUCED by thick lead.

Mark-losing trap. Alpha and beta are deflected OPPOSITE ways by a field; gamma is not deflected at all.

Mark-losing trap. Alpha is safest outside the body and most dangerous inside it. Gamma is the other way round.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — What is an alpha particle?
2. Grade 7 · Explain [3 marks] — A student heats a radioactive source strongly in a flame, expecting it to decay faster. The count rate is unchanged. Explain why.
3. Grade 8 · Explain [4 marks] — Explain why alpha radiation is the most ionising of the three but travels the shortest distance.
4. Grade 9 · Describe [6 marks] — Select every statement that belongs in a full-mark description of how to identify which radiation an unknown sealed source emits, using a Geiger–Müller tube and absorbers.
5. 9+ · Evaluate [6 marks] — A laboratory must choose which sealed source to store in an open room where people work nearby, and which to seal into a sample of soil that will be handled outdoors. A technician says: 'Alpha is the most ionising, so alpha is the more dangerous in both cases.' Select every statement that belongs in a full-mark evaluation.

The people behind this science

Two ways into the same idea — the one who found it by developing an experiment the weather had ruined, and the one who proved the emission comes from inside the atom. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Henri Becquerel — the one who found it by developing an experiment the weather had ruined

Radioactivity was discovered by accident in 1896, and the accident is instructive. Becquerel expected uranium salts to emit something only after being made to phosphoresce in sunlight, so he wrapped photographic plates in black paper, laid the salts on top, and put them out in the sun. Late February in Paris was overcast, so the whole arrangement went into a drawer. He developed the plates anyway, days later, and found strong images — stronger than any sunlight had produced. The uranium had needed nothing done to it at all. He is the right person to ask why a spoiled experiment is still worth finishing, and how honest a scientist should be about not understanding their own result.

• “Why did you develop a plate from an experiment the weather had ruined?”
• “What did you expect to find, and what did you actually find?”
• “How did you know the uranium needed no sunlight?”
• “What did Marie Curie understand that you did not?”
• “What happened when you carried radium in your pocket?”

Marie Curie — the one who proved the emission comes from inside the atom

Becquerel found the effect; Curie established what it meant, named it, and showed that it is a property of the ATOM itself rather than of any chemical arrangement it happens to be in. She proved this by measuring the ionisation each compound produced and finding it depended only on how much uranium was present, whatever the compound. That was the decisive step: if no chemistry can alter it, the source must lie deeper than chemistry — in the nucleus. She also found that pitchblende was more active than its uranium content could explain, and chased that discrepancy through tonnes of ore to polonium and radium.

• “How did you show radioactivity comes from the atom itself?”
• “Why does no chemical reaction change how radioactive something is?”
• “What made you think there was another element in the pitchblende?”
• “What does it mean to say a nucleus is unstable?”
• “Did you understand the danger you were working in?”

Then defend it

On Incandio a lesson is not finished when the questions come out right. You teach the idea back to Ember, an AI apprentice who asks the awkward question, and then you argue it against Ernest Rutherford in a structured debate marked against descriptors you can read before you enter. Learn it, teach it, then defend it — all three happen on this page once the app loads.

Carry on through the course

• Previous lesson: Atomic Structure and Isotopes
• Next lesson: Nuclear Equations
• Atomic Structure and Isotopes — A nucleus ten thousand times smaller than the atom around it, and why two atoms of the same element can weigh different amounts.
• Nuclear Equations — What each kind of decay does to the numbers — and why losing a negative particle makes the charge go up.
• Detecting Radiation and Background — How you measure something invisible, and why the counter clicks even with no source in the room.
• Half-Life — Half of what is left, every time — which is why a radioactive sample never quite runs out.
• Uses and Dangers of Radiation — Choosing a source by its half-life and its penetration — and the difference between being irradiated and being contaminated.
• All of Physics · Incandio Science