Chapter 2 of 8

All 8 chapters
  1. 1Unstable nuclei, readunderstood
  2. 2Alpha, beta and gamma, readunderstood
  3. 3Half-life, readunderstood
  4. 4Background radiation, readunderstood
  5. 5Radiation and safety, readunderstood
  6. 6Uses of radiation, readunderstood
  7. 7Fission and fusion, readunderstood
  8. 8Weighing up nuclear power, readunderstood

S4.11.2 Chapter 2

Alpha, beta and gamma

Radiation comes in three kinds. What are they, and how can we tell them apart?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Compare alpha, beta and gamma by charge, range and what stops them
  • Write nuclear equations for alpha and beta decay

An unstable nucleus can decay in more than one way. It gives out one of three kinds of radiation: alpha, beta or gamma. The names are the first three letters of the Greek alphabet, α, β and γ.

An alpha particle is 2 protons and 2 neutrons stuck together. That is the same as the nucleus of a helium atom. It has a charge of +2 and is heavy for a particle.

An alpha particle drawn as touching circles: 2 hatched circles marked with a plus sign, the protons, and 2 plain circles, the neutrons. The label says alpha particle. A key names proton and neutron.alpha particleprotonneutron
An alpha particle has the same make-up as a helium nucleus.

A beta particle is a fast electron. It comes from the nucleus, not from the shells. Inside the nucleus a neutron changes into a proton and an electron. The proton stays, and the electron flies out. It has a charge of −1.

A tiny neutral particle called an antineutrino also leaves in beta decay. It does not change the numbers we balance, so this book leaves it out. This is called beta minus decay. Some nuclei with too few neutrons decay the other way: a proton changes into a neutron, and a positron leaves. A positron is like an electron with a + charge. This is beta plus decay. In this book, beta means beta minus.

A gamma ray is not a particle. It is an electromagnetic wave, like light, but with far more energy. It has no mass and no charge. After an alpha or beta decay the new nucleus often holds spare energy. It gets rid of it as a gamma ray.

Each kind knocks electrons off the atoms it passes. This is called ionisation, and the atoms that are left become ions. The more ionisation, the more the radiation can harm living cells. It also uses up the energy of the radiation, so it stops.

A table with the columns Type, Charge and Ionising. Alpha has a charge of +2 and is strongly ionising. Beta has a charge of −1 and is medium. Gamma has a charge of 0 and is weak.TypeChargeIonisingalpha+2strongbeta−1mediumgamma0weak
Alpha ionises the most and gamma the least.

Now see how far each one goes. Alpha loses its energy fast, because it ionises so much. Gamma loses it slowly. So alpha is the most ionising and the least penetrating, and gamma is the least ionising and the most penetrating.

A source on the left sends three rays to the right through three barriers. The barriers are paper, aluminium 3 mm and lead 5 cm. The alpha ray stops at the paper. The beta ray goes through the paper and stops inside the aluminium. The gamma ray goes through the paper and the aluminium. The lead takes in most of it, and a thin dashed ray labelled some pass comes out of the far side.sourcepaperaluminium3 mmlead5 cmalphabetagammasome pass
Typical stopping. The exact thickness depends on the energy of the radiation.

In air, alpha goes only a few centimetres. Beta goes from tens of centimetres to a few metres, depending on its energy. Gamma goes very far, and gets weaker with distance. No thickness of lead stops all the gamma rays. Each layer takes away some, and thick lead takes most.

Alpha and beta carry charge, so a magnet or an electric field bends their paths. They bend in opposite ways, because their charges are opposite. Gamma has no charge and goes straight on.

Now the equations. A nuclear equation shows a decay with the mass number on top and the atomic number below each symbol. Both of these must add up to the same total on each side of the arrow. Nothing is lost.

A nuclear equation for alpha decay. Uranium, symbol U with 238 at the top and 92 at the bottom, gives thorium, symbol Th with 234 and 90, plus an alpha particle, symbol He with 4 and 2.23892U23490Th42He
238 = 234 + 4 and 92 = 90 + 2. Both totals match.

Alpha decay: the mass number goes down by 4 and the atomic number goes down by 2. The new nucleus is a different element, two places back in the periodic table. The alpha particle takes 2 protons and 2 neutrons away.

Worked example

A nucleus of radium 226 has atomic number 88. It decays by giving out an alpha particle. Find the mass number and atomic number of the new nucleus.

  1. Alpha decay lowers the mass number by 4 and the atomic number by 2.
  2. New mass number = 226 − 4 = 222.
  3. New atomic number = 88 − 2 = 86.
  4. The element with atomic number 86 is radon, so the new nucleus is radon 222.

Answer: mass number 222, atomic number 86

A nuclear equation. Radium, symbol Ra with 226 at the top and 88 at the bottom, gives radon, symbol Rn with 222 and 86, plus an alpha particle, symbol He with 4 and 2.22688Ra22286Rn42He
The worked example, written as an equation.
A nuclear equation for beta decay. Carbon, symbol C with 14 at the top and 6 at the bottom, gives nitrogen, symbol N with 14 and 7, plus a beta particle, symbol e with 0 at the top and −1 at the bottom.146C147N0−1e
Beta decay of carbon 14. 14 = 14 + 0 and 6 = 7 + (−1).

Beta decay: the mass number stays the same and the atomic number goes up by 1. An electron has almost no mass, so its mass number is 0. Its charge is −1, so a nucleus that loses it and gains a proton has one more unit of positive charge.

Worked example

A nucleus of strontium 90 has atomic number 38. It decays by giving out a beta particle. Find the mass number and atomic number of the new nucleus.

  1. Beta decay leaves the mass number the same and raises the atomic number by 1.
  2. New mass number = 90.
  3. New atomic number = 38 + 1 = 39.
  4. The element with atomic number 39 is yttrium, so the new nucleus is yttrium 90.

Answer: mass number 90, atomic number 39

Gamma decay changes neither number. The nucleus is the same nucleus, with less energy. Cobalt 60 shows all of this. It gives out a beta particle and becomes nickel 60. The nickel nucleus then gives out gamma rays and settles down.

Try it

Check an equation

You need: paper, a pencil.

  1. Write this decay. A thorium nucleus has mass number 230 and atomic number 90. It gives out an alpha particle.
  2. The new nucleus is radium, with mass number 226 and atomic number 88. Write that too.
  3. Add the mass numbers on the right: the radium and the alpha particle.
  4. Add the atomic numbers on the right in the same way.
  5. Compare each total with the number on the left.
  6. Change the radium mass number to 227 and add again. What goes wrong?

What you will see: With the right numbers, 226 + 4 = 230 and 88 + 2 = 90, so both sides match. Change one number and the totals differ. A real decay never does that, so an equation that does not balance is wrong.

If you cannot try it, this is what happens.
A nuclear equation. Thorium, symbol Th with 230 at the top and 90 at the bottom, gives radium, symbol Ra with 226 and 88, plus an alpha particle, symbol He with 4 and 2.23090Th22688Ra42He

A wrong idea many people have

A beta particle is one of the atom's electrons, knocked out of its shell.

Why it feels right: A beta particle is an electron, and we know atoms have electrons in shells.

The test that breaks it: A decay changes the nucleus. The nucleus of carbon 14 becomes nitrogen. That needs a neutron to change into a proton, and the electron is made in that change.

What is true: A beta particle comes from the nucleus. A neutron changes into a proton and an electron, and the electron leaves. The shell electrons stay where they were.

A wrong idea many people have

Gamma rays change the mass number, because they carry energy away.

Why it feels right: Alpha and beta change the nucleus, so gamma seems to change it too.

The test that breaks it: Look at a gamma ray as a wave. It has no mass and no charge. Take it away and the number of protons and neutrons is still the same.

What is true: Gamma changes neither the mass number nor the atomic number. It only carries away spare energy, so the nucleus stays the same element.

Key words

alpha particle
2 protons and 2 neutrons, the same as a helium nucleus. It stops in paper.
beta particle
a fast electron from the nucleus. A few millimetres of aluminium stop it.
gamma ray
an electromagnetic wave from the nucleus. Thick lead reduces it a lot.
ionisation
knocking electrons off atoms to make ions.
nuclear equation
a decay written with mass numbers and atomic numbers that add up on both sides.

Back to your guess

The answer is B: They differ in what they are made of, and in what stops them.

Alpha is a helium nucleus. Beta is a fast electron. Gamma is a wave, like light but far more energetic. Paper, aluminium and lead stop them in different ways.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Which kind of radiation is the same as a helium nucleus?

    Tap your answer.

    1. Not this one. Gamma is an electromagnetic wave. It has no particles in it, and no charge. Try another.

    2. Yes, that is right. An alpha particle is 2 protons and 2 neutrons, which is the nucleus of a helium atom.

    3. Not this one. A beta particle is a fast electron. It is far lighter than a helium nucleus. Try another.

    4. Not this one. A proton alone is a hydrogen nucleus. An alpha particle is 2 protons and 2 neutrons together. Go to S4.6.1. Try another.

    See the answer

    Alpha

    An alpha particle is 2 protons and 2 neutrons, which is the nucleus of a helium atom.

    • If you chose A: Gamma is an electromagnetic wave. It has no particles in it, and no charge.
    • If you chose C: A beta particle is a fast electron. It is far lighter than a helium nucleus.
    • If you chose D: A proton alone is a hydrogen nucleus. An alpha particle is 2 protons and 2 neutrons together. Go to S4.6.1.

  2. Which kind of radiation is stopped by a sheet of paper?

    Tap your answer.

    1. Not this one. Beta goes through paper. A few millimetres of aluminium are needed to stop it. Try another.

    2. Not this one. Gamma goes through paper easily. It needs thick lead to be reduced a lot. Try another.

    3. Yes, that is right. Alpha ionises so strongly that it loses all its energy in a few centimetres of air, or in paper or skin.

    4. Not this one. Only alpha is stopped by paper. Beta and gamma go through it. Try another.

    See the answer

    Alpha

    Alpha ionises so strongly that it loses all its energy in a few centimetres of air, or in paper or skin.

    • If you chose A: Beta goes through paper. A few millimetres of aluminium are needed to stop it.
    • If you chose B: Gamma goes through paper easily. It needs thick lead to be reduced a lot.
    • If you chose D: Only alpha is stopped by paper. Beta and gamma go through it.

  3. Which kind of radiation is the most strongly ionising?

    Tap your answer.

    1. Yes, that is right. Alpha is heavy and has a charge of +2, so it knocks many electrons off atoms in a short path.

    2. Not this one. Beta ionises, but less than alpha. It is lighter and carries less charge. Try another.

    3. Not this one. Gamma goes the furthest, which is why it is the most penetrating. It ionises the least. Try another.

    4. Not this one. They do not. Alpha ionises the most and gamma the least. Try another.

    See the answer

    Alpha

    Alpha is heavy and has a charge of +2, so it knocks many electrons off atoms in a short path.

    • If you chose B: Beta ionises, but less than alpha. It is lighter and carries less charge.
    • If you chose C: Gamma goes the furthest, which is why it is the most penetrating. It ionises the least.
    • If you chose D: They do not. Alpha ionises the most and gamma the least.

  4. A source is tested with a counter and the background count has been taken away. What kind of radiation does it give out?

    A table with the columns Barrier and Counts/min. With the barrier none, the count is 412. With paper it is 409. With aluminium 3 mm it is 2.BarrierCounts/minnone412paper409aluminium 3 mm2
    Tap your answer.

    1. Not this one. Alpha would be stopped by the paper, and the count with paper would be near 0. It is 409. Try another.

    2. Not this one. Gamma would also go through the aluminium. Here the aluminium cuts the count almost to nothing. Try another.

    3. Not this one. Alpha would drop the count at the paper. It did not drop, so there is no alpha. Try another.

    4. Yes, that is right. Paper does not change the count much, so it is not alpha. Aluminium cuts it to almost nothing, so it is not gamma. That leaves beta.

    See the answer

    Beta, because paper hardly changes it and aluminium stops it.

    Paper does not change the count much, so it is not alpha. Aluminium cuts it to almost nothing, so it is not gamma. That leaves beta.

    • If you chose A: Alpha would be stopped by the paper, and the count with paper would be near 0. It is 409.
    • If you chose B: Gamma would also go through the aluminium. Here the aluminium cuts the count almost to nothing.
    • If you chose C: Alpha would drop the count at the paper. It did not drop, so there is no alpha.

  5. What happens inside the nucleus in beta minus decay?

    Tap your answer.

    1. Not this one. It is the other way. The atomic number goes up, so a proton is gained, from a neutron. Try another.

    2. Not this one. That describes alpha decay, where a helium nucleus leaves. Try another.

    3. Yes, that is right. The new proton raises the atomic number by 1. The electron leaves as the beta particle.

    4. Not this one. Shell electrons stay. A beta particle is made in the nucleus. Try another.

    See the answer

    A neutron changes into a proton and an electron.

    The new proton raises the atomic number by 1. The electron leaves as the beta particle.

    • If you chose A: It is the other way. The atomic number goes up, so a proton is gained, from a neutron.
    • If you chose B: That describes alpha decay, where a helium nucleus leaves.
    • If you chose D: Shell electrons stay. A beta particle is made in the nucleus.

  6. A nucleus of americium 241 gives out an alpha particle. What is the mass number of the new nucleus?

    Tap your answer.

    1. Not this one. You added 4. An alpha particle takes 4 away from the mass number. Try another.

    2. Yes, that is right. An alpha particle has mass number 4, so the mass number falls by 4: 241 − 4 = 237.

    3. Not this one. That would be the mass number after a beta or gamma decay. Alpha lowers it by 4. Try another.

    4. Not this one. You took away 2, which is the change in the atomic number. The mass number falls by 4. Try another.

    See the answer

    237

    An alpha particle has mass number 4, so the mass number falls by 4: 241 − 4 = 237.

    • If you chose A: You added 4. An alpha particle takes 4 away from the mass number.
    • If you chose C: That would be the mass number after a beta or gamma decay. Alpha lowers it by 4.
    • If you chose D: You took away 2, which is the change in the atomic number. The mass number falls by 4.

  7. From an earlier chapter

    A large sample of unstable nuclei is left alone. Which statement about when each nucleus decays is true?

    Tap your answer.

    1. Not this one. There is no order. Each nucleus decays at a random time. Go to S4.11.1. Try another.

    2. Not this one. They do not. The decays are spread over time. Go to S4.11.1. Try another.

    3. Yes, that is right. Decay is random. We cannot say when one nucleus will go, though a huge number behave in a steady way.

    4. Not this one. Position does not matter. Each nucleus has the same chance at any moment. Go to S4.11.1. Try another.

    See the answer

    Each one decays at a random time.

    Decay is random. We cannot say when one nucleus will go, though a huge number behave in a steady way.

    • If you chose A: There is no order. Each nucleus decays at a random time. Go to S4.11.1.
    • If you chose B: They do not. The decays are spread over time. Go to S4.11.1.
    • If you chose D: Position does not matter. Each nucleus has the same chance at any moment. Go to S4.11.1.

  8. Polonium 210 has atomic number 84 and decays by alpha emission. The picture shows the decay with the new nucleus hidden. What is the atomic number of the new nucleus?

    A nuclear equation with one term hidden. Polonium, symbol Po with 210 at the top and 84 at the bottom, gives a nucleus hidden behind a ?, plus an alpha particle, symbol He with 4 and 2.21084Po?42He
    See the answer

    82

    Alpha decay lowers the atomic number by 2: 84 − 2 = 82.

  9. A nucleus of iodine 131 has atomic number 53. It decays by beta emission. What is the atomic number of the new nucleus?

    See the answer

    54

    Beta decay raises the atomic number by 1: 53 + 1 = 54. The mass number stays at 131.

Keep it

Copy this into your exercise book.

Alpha: 2 protons, 2 neutrons, stopped by paper, most ionising. Beta: fast electron, stopped by aluminium. Gamma: wave, only reduced by thick lead. Alpha: mass −4, atomic −2. Beta: mass same, atomic +1. Gamma: no change.

Draw: A source with three rays: alpha stopped at paper, beta stopped at aluminium, gamma reaching lead.

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