Nothing decides to decay

It is tempting to learn alpha, beta and gamma as three lists of facts to memorise. There is a single idea underneath them, and it turns the memorising into reasoning. ⭐⭐ A nucleus does not choose to emit anything. It is UNSTABLE, and it settles by throwing something out. What it throws out is the clue: alpha, beta and gamma are three different things leaving a nucleus. Because they are three different things, they differ in everything at once. What each one IS made of, how far it gets through matter, how much damage it does on the way, and what is LEFT BEHIND in the nucleus. ⭐ And the two facts people try hardest to memorise, penetration and ionising power, are really the SAME fact seen twice. A big, slow, charged particle bumps into everything it passes, so it does a lot of damage and stops almost at once. A small light one slips further before it is stopped. Something with no mass and no charge barely interacts with anything, so it goes a long way. How much it interacts decides BOTH how far it gets AND how much it ionises. So there are just two questions to carry through the whole module: WHAT LEFT THE NUCLEUS, and WHAT IS LEFT BEHIND? The first tells you the radiation and its behaviour; the second is all a nuclear equation ever asks. ⚠️ One more thing this answers: the difference between being irradiated and being contaminated, which comes down to whether the unstable nucleus is outside you or on and in you. Carry the two questions through: what left the nucleus, and what is left behind?

Three things that can leave

Each column is one of the three radiations. Read across and notice that penetration and ionising power always move together, because they are the same story about how much it interacts.

Tap the two true of alpha

Tap the TWO statements that are true of ALPHA radiation.

It is a helium nucleus, two protons and two neutrons.
It is stopped by a sheet of paper or by skin.
It passes through everything but thick lead.
It is an electromagnetic wave with no charge.

Why alpha stops so soon

Alpha radiation is the most strongly ionising but the least penetrating. Why are those two facts really one fact?

Because it is large, slow and charged, it collides with almost everything it passes, which both ionises heavily and uses up its energy quickly, so it stops after a short distance
They are unrelated facts that simply both happen to be true of alpha
Because alpha is the smallest and lightest of the three
Because it has no charge, so it slips through easily

Words for radioactivity

Six terms this topic turns on. The three radiations are named as words, never as symbols.

Match each clue to what it shows

Two protons and two neutrons leave together
A fast electron leaves as a neutron becomes a proton
A wave with no mass and no charge leaves
A sheet of paper stops it completely
Only thick lead stops it

This is alpha radiation, a helium nucleus
This is beta radiation, a fast electron from the nucleus
This is gamma radiation, an electromagnetic wave
It must be large and strongly ionising, so it stops almost at once
It must interact very little, so it travels far and is hard to stop

Build the alpha decay change

Assemble the sentence that describes what alpha decay does to a nucleus.

Bookkeeping, and inside versus outside

Two things make the marks on this topic, and both come straight from the one idea. ⭐ NUCLEAR EQUATIONS ARE JUST BOOKKEEPING. The mass numbers on each side must add up to the same total, and so must the atomic numbers. Alpha carries away four of mass and two of charge, so both fall. Beta carries away almost no mass and a charge of minus one, so the mass number stays the same and the atomic number RISES by one, because a neutron has turned into a proton. If you know what left, you can always work out what is left. ⭐⭐ CONTAMINATION IS NOT THE SAME AS IRRADIATION, and the difference is simply where the unstable nucleus is. IRRADIATION is being hit by radiation that left the nucleus of somebody else, from a source outside you; move away or remove the source and it stops, and you do not become radioactive. CONTAMINATION is getting the radioactive material itself onto or into you; now the nucleus is with you and keeps emitting until it is removed or decays. That is why an alpha source, almost harmless outside the skin because paper stops it, is dangerous inside, where there is no skin to stop it and it ionises everything nearby. ⚠️ A word on tone. Radiation worries people, and some will have family experience of illness. State the physics plainly and leave it there. The examinable point is only that the hazard depends on WHICH radiation and on whether the source is OUTSIDE or INSIDE you. And keep penetration and ionising joined: the more a radiation interacts, the more it ionises and the sooner it stops. Say one and you have said the other. Keep the two questions to hand: what left the nucleus, and what is left behind?

Two that are true of beta

Select the TWO statements that are true of BETA decay.

A neutron in the nucleus turns into a proton, and a fast electron is thrown out
The atomic number rises by one, while the mass number stays the same
The mass number falls by four
A helium nucleus is emitted

Work out what is left

A nucleus has a mass number of 226. It emits an alpha particle. Work out the mass number of the nucleus that is left behind.

Order them by how far they get

Put the three radiations in order of how far they penetrate, from the one stopped most easily to the one that travels furthest.

Alpha, stopped by a sheet of paper or by skin
Beta, stopped by a few millimetres of aluminium
Gamma, stopped only by thick lead or concrete

The radiation run

Five quick decisions about the three radiations. Three lives.

Crossing a crowded hall

Leave physics aside for a screen. Picture three very different people trying to cross a hall packed shoulder to shoulder with a crowd. The first is large and barges straight in. They collide with almost everyone, knock people aside at every step, and are brought to a halt only a little way in. They caused a lot of disturbance and got nowhere. ⭐ The second is small and nimble and slips between people. They knock into far fewer, cause much less fuss, and so get a good deal further before the crush finally stops them. ⭐⭐ The third is not really in the crowd at all, more like a beam of light shining across the room. It touches almost nobody, disturbs almost nothing, and passes clean through to the far wall. Notice the single rule behind all three. How much each one BUMPS INTO the crowd decides BOTH how much disturbance they cause AND how far they get. The one that hits everything causes the most trouble and travels the least; the one that hits nothing causes no trouble and travels furthest. It is one fact, not two. ⚠️ Hold that picture. Three things leaving a nucleus behave exactly like the three people: how much each interacts with the matter it passes through decides both the damage it does and the distance it covers.

Complete the radiation facts

A nucleus emits radiation because it is _____. Radiation made of a helium nucleus, large and slow and stopped by paper, is _____ radiation. Radiation made of a fast electron, stopped by a few millimetres of aluminium, is _____ radiation. Radiation that is an electromagnetic wave with no mass or charge, stopped only by thick lead, is _____ radiation. When radiation knocks electrons off the atoms it passes, it is described as _____.

unstable alpha beta gamma ionising penetrating electron neutron

Handle three radiation situations

Three situations come up in a lab. For each, reason from what the radiation is and where the source is, keeping to the physics.

A source that emits only alpha sits in a sealed case on a bench, a short distance from the people working nearby.
The same alpha material is instead accidentally swallowed, so it is now inside the body.
A nucleus of mass number 218 and atomic number 84 emits an alpha particle, and you must state what is left.

Explain the three radiations

Explain how alpha, beta and gamma radiation differ, using the idea that how much a radiation interacts with matter decides both how far it gets and how much it ionises. Then explain the difference between irradiation and contamination.

Say what each of alpha, beta and gamma is made of
Explain how far each penetrates and what stops it
Explain why penetration and ionising power are really the same fact seen twice
Explain what happens to the mass number and atomic number in alpha decay and in beta decay
Finish by explaining the difference between being irradiated and being contaminated, and why it matters

Decay Detective

Get the method right under pressure

What you'll cover