Chapter 4 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

S4.11.4 Chapter 4

Background radiation

A radiation counter reads some counts every minute, often 15 to 30, with no source anywhere near it. Where do they come from?

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.

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

By the end you can

  • Name natural sources of background radiation and ones made by people
  • Explain why background radiation differs from place to place

Radiation is not rare. You are being hit by it as you read this. It comes from the ground, the air, space and even your own body. We call this always present radiation background radiation. Life on Earth has always lived with it.

To describe how much, we need a unit. The sievert (Sv) measures how much harm radiation may do to the body. It takes account of the kind of radiation. The total you receive is your dose.

A sievert is a very large dose, so we use small units. A millisievert (mSv) is one thousandth of a sievert. A microsievert (µSv) is one thousandth of a millisievert. So 1 mSv = 1 000 µSv.

Do not mix up two ideas. A detector gives a count rate, in counts each minute. A dose, in sieverts, is about the effect on a body. They are linked, but they are not the same number.

Most background radiation is natural. These are the sources, with their share of the average natural dose of about 2.4 mSv a year, over the whole world. The figures are rounded world averages from the United Nations committee that studies the effects of radiation.

A pie chart of the natural background dose. The sectors are labelled radon 180°, ground 75°, space 60° and food 45°. The sectors are told apart by white, hatching, dots and cross hatching.radon180°ground75°space60°food45°
About half of the natural dose is radon. Each sector is its angle out of 360°. Rounded world averages.

Radon is a radioactive gas. Uranium is in tiny amounts in many rocks and soils. As it decays, it makes radium and then radon. The gas seeps up out of the ground. Radon 222 gives out alpha particles and has a half-life of about 3.8 days.

Four boxes joined by arrows going down. The first says uranium in rock. The second says radon gas rises. The third says it collects in a closed room. The fourth says it is breathed in.uranium in rockradon gas risesit collects in a closed roomit is breathed in
Out of doors, radon spreads out in the air. In a closed room it can build up.

Radon matters because it is a gas that we breathe in. Alpha particles cannot get through skin, but inside the lungs they strike living cells. In most homes the level is low. Over many years a high level raises the chance of lung cancer, and good ventilation lowers it.

Rocks and soil also give gamma rays, from uranium, thorium and potassium 40. Buildings made of stone, brick or concrete give some too. Food and drink hold potassium 40 and carbon 14, and these are in our bodies, with thousands of decays each second.

Cosmic rays come from space, from the Sun and from far beyond. They are mostly fast charged particles. Air above us takes much of their energy. So the dose rises with height, on a mountain top or in an aeroplane, where there is less air above.

Some background radiation is made by people. By far the largest part of that is medical: X-rays and scans. X-rays are not made by decay, but they ionise like the radiation in this book. The nuclear industry and fallout from past weapon tests add very little on average.

A bar chart with the axis labelled dose (mSv a year). The scale runs from 0 to 3 in steps of 1. The bar for natural reaches 2.4 and the bar for medical reaches 0.6.0123naturalmedicaldose (mSv a year)
About 2.4 mSv a year is natural and about 0.6 mSv is medical, as world averages. Medical doses vary a lot between people.

Worked example

The chart shows an average natural dose of 2.4 mSv a year and a medical dose of 0.6 mSv a year. Find the total dose and the percentage that is medical.

  1. total = 2.4 + 0.6 = 3.0 mSv.
  2. fraction that is medical = 0.6 ÷ 3.0 = 0.2.
  3. As a percentage, 0.2 × 100 = 20%.

Answer: 3 mSv, 20%

Background radiation is not the same everywhere. A house on granite, which holds more uranium, may have more radon than a house on clay. A few places on Earth have rocks or sands with so much thorium or uranium. There the background is many times the average.

Height changes it too, because of cosmic rays. Buildings change it: a sealed, draught-free room holds more radon than an airy one. Food changes it a little. So a person's dose depends on where they live, how high, and what the house is built of.

A table of rounded doses with the columns Exposure and Dose. A banana is 0.1 µSv. A day of background is 7 µSv. A chest X-ray is 20 µSv. A long flight is 40 µSv.ExposureDosea banana0.1 µSva day of background7 µSva chest X-ray20 µSva long flight40 µSv
Typical values, rounded. A long flight here means one across an ocean. They vary with the place, the machine and the flight.

This gives a sense of size. A chest X-ray gives about three days of natural background. Doctors only use scans when they help, and the benefit of finding an illness is far greater than the small dose. Large doses are another matter. They are the subject of the next chapter.

Try it

Add up a year of doses

You need: paper, a pencil.

  1. Write: background for a year is about 2 400 µSv.
  2. Add 2 chest X-rays. Each is 20 µSv. Write the sum.
  3. Add 1 flight across an ocean, which is 40 µSv.
  4. Add the three lines to get the total for the year.
  5. Work out what share of the total the extra doses are.

What you will see: The extra doses are small next to the natural background, a few percent. But they are real, and they add to it. Doses add up, whatever the source.

If you cannot try it, this is what happens.
A table with the columns Source and Dose. Background for a year is 2 400 µSv. 2 chest X-rays are 40 µSv. 1 flight is 40 µSv. The total is 2 480 µSv.SourceDosebackground2 400 µSv2 chest X-rays40 µSv1 flight40 µSvtotal2 480 µSv

A wrong idea many people have

Radiation is only made by people, in nuclear plants, bombs and hospitals.

Why it feels right: We hear about radiation when something goes wrong in an industry, or when we are given a scan.

The test that breaks it: Take a radiation counter far from any plant or hospital. It still counts. The counts come from rocks, air and space. Your own body has potassium 40 in it.

What is true: Radiation was here long before people. Most of the average dose is natural. Human sources add a share, and medical scans are the largest of them.

Key words

background radiation
the radiation that is always present, mostly from natural sources.
dose
the radiation a body receives, measured by the harm it may do.
sievert
the unit of dose. Small doses are in mSv or µSv.
radon
a radioactive gas that comes from uranium in rock.
cosmic rays
fast charged particles from space, stronger at height.

Back to your guess

The answer is B: Natural sources all around us, such as rocks, air, space and food.

Radiation is always around us. Most of it is natural. It comes from rocks and soil, from a gas called radon, from space, and from our food and bodies.

Check yourself

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

  1. Which of these is a natural source of background radiation?

    Tap your answer.

    1. Yes, that is right. Radon comes from uranium that is naturally in rocks and soil. It has always been there.

    2. Not this one. X-rays used in medicine are made by people with machines. They are a human source. Try another.

    3. Not this one. That was made by people, in nuclear explosions. It is a human source, and a very small one today. Try another.

    4. Not this one. Many smoke alarms hold a tiny source made by people. It adds almost nothing to the average dose. Try another.

    See the answer

    Radon gas from rocks

    Radon comes from uranium that is naturally in rocks and soil. It has always been there.

    • If you chose B: X-rays used in medicine are made by people with machines. They are a human source.
    • If you chose C: That was made by people, in nuclear explosions. It is a human source, and a very small one today.
    • If you chose D: Many smoke alarms hold a tiny source made by people. It adds almost nothing to the average dose.

  2. Which of these is a source of background radiation made by people?

    Tap your answer.

    1. Not this one. Cosmic rays come from space. They are natural. Try another.

    2. Not this one. Radon comes from natural uranium in rocks. It is natural. Try another.

    3. Not this one. Potassium 40 is a natural isotope that all living things take in. Go to S4.6.3. Try another.

    4. Yes, that is right. Scans and X-rays use radiation made by people. Medical use is the largest human source of dose, on average.

    See the answer

    Medical X-rays and scans

    Scans and X-rays use radiation made by people. Medical use is the largest human source of dose, on average.

    • If you chose A: Cosmic rays come from space. They are natural.
    • If you chose B: Radon comes from natural uranium in rocks. It is natural.
    • If you chose C: Potassium 40 is a natural isotope that all living things take in. Go to S4.6.3.

  3. Why can one house have more radon than another house?

    Tap your answer.

    1. Not this one. Radon comes from the ground. A house does not make it, though it can trap it. Try another.

    2. Not this one. Radon is a product of the decay of uranium. Electricity has nothing to do with it. Try another.

    3. Yes, that is right. Uranium decays to radium and then to radon gas. Rock with more uranium gives more radon. A closed house can also trap it.

    4. Not this one. Radon comes from the ground, not from space. Cosmic rays come from space. Try another.

    See the answer

    The rock under it may hold more uranium, which decays to radon.

    Uranium decays to radium and then to radon gas. Rock with more uranium gives more radon. A closed house can also trap it.

    • If you chose A: Radon comes from the ground. A house does not make it, though it can trap it.
    • If you chose B: Radon is a product of the decay of uranium. Electricity has nothing to do with it.
    • If you chose D: Radon comes from the ground, not from space. Cosmic rays come from space.

  4. Why do people in aeroplanes receive a higher dose of cosmic rays?

    Tap your answer.

    1. Not this one. Temperature does not matter. It is the amount of air above that counts. Try another.

    2. Yes, that is right. The air above acts as a shield. The higher you go, the less air is above you, and the more cosmic radiation reaches you.

    3. Not this one. The Sun is not much closer. And radiation does not make people radioactive. Try another.

    4. Not this one. Cosmic rays come from space. An aeroplane only flies where there is less air to stop them. Try another.

    See the answer

    There is less air above them to take the energy of cosmic rays.

    The air above acts as a shield. The higher you go, the less air is above you, and the more cosmic radiation reaches you.

    • If you chose A: Temperature does not matter. It is the amount of air above that counts.
    • If you chose C: The Sun is not much closer. And radiation does not make people radioactive.
    • If you chose D: Cosmic rays come from space. An aeroplane only flies where there is less air to stop them.

  5. A radiation counter with no source anywhere near it still reads some counts every minute. What are these counts?

    Tap your answer.

    1. Yes, that is right. Radiation from rocks, air, space and food is always present. A counter detects it, so you must subtract it from a reading.

    2. Not this one. A working counter does this. The counts come from natural radiation around it. Try another.

    3. Not this one. A source is not needed. Rocks, air and space give counts everywhere. Try another.

    4. Not this one. A counter does not store radiation. It only counts what reaches it. Try another.

    See the answer

    Background radiation

    Radiation from rocks, air, space and food is always present. A counter detects it, so you must subtract it from a reading.

    • If you chose B: A working counter does this. The counts come from natural radiation around it.
    • If you chose C: A source is not needed. Rocks, air and space give counts everywhere.
    • If you chose D: A counter does not store radiation. It only counts what reaches it.

  6. What is the biggest single natural source of background radiation in many places?

    Tap your answer.

    1. Not this one. Cosmic rays are important at height, but at ground level they are a smaller share. Try another.

    2. Not this one. Food gives a small share, about an eighth of the natural dose. Try another.

    3. Yes, that is right. On average, radon gives about half of the natural dose. It is the largest share in many places.

    4. Not this one. Medical scans are made by people, so they are not a natural source. Try another.

    See the answer

    Radon

    On average, radon gives about half of the natural dose. It is the largest share in many places.

    • If you chose A: Cosmic rays are important at height, but at ground level they are a smaller share.
    • If you chose B: Food gives a small share, about an eighth of the natural dose.
    • If you chose D: Medical scans are made by people, so they are not a natural source.

  7. From an earlier chapter

    A sample has a corrected count rate of 600 counts/min and a half-life of 10 min. What is its corrected count rate after 20 min?

    Tap your answer.

    1. Not this one. That is after one half-life, 10 min. 20 min is two half-lives. Go to S4.11.3. Try another.

    2. Not this one. You divided by 3. Each half-life divides by 2. Go to S4.11.3. Try another.

    3. Not this one. That is after 3 half-lives. 20 min is only two. Go to S4.11.3. Try another.

    4. Yes, that is right. 20 ÷ 10 = 2 half-lives. 600 → 300 → 150. The corrected count rate is 150 counts/min.

    See the answer

    150 counts/min

    20 ÷ 10 = 2 half-lives. 600 → 300 → 150. The corrected count rate is 150 counts/min.

    • If you chose A: That is after one half-life, 10 min. 20 min is two half-lives. Go to S4.11.3.
    • If you chose B: You divided by 3. Each half-life divides by 2. Go to S4.11.3.
    • If you chose C: That is after 3 half-lives. 20 min is only two. Go to S4.11.3.

  8. The bar chart shows the yearly dose of one person from four sources. What is the total yearly dose in mSv?

    A bar chart of one person's yearly dose, with the axis labelled dose (mSv a year). The scale runs from 0 to 2 in steps of 0.5. The bar for radon reaches 1.5, space 0.5, food 0.5 and scans 1.5.00.511.52radonspacefoodscansdose (mSv a year)
    See the answer

    4 mSv

    Add the four bars: 1.5 + 0.5 + 0.5 + 1.5 = 4 mSv.

  9. A pie chart of the natural dose in one town has a sector of 216° for radon. What percentage of the dose is radon?

    See the answer

    60%

    The whole circle is 360°. 216 ÷ 360 = 0.6, which is 60%.

  10. A person has 3 chest X-rays of 20 µSv each and 2 flights of 40 µSv each in a year. What total dose do these add, in µSv?

    See the answer

    140 µSv

    3 × 20 = 60 µSv from the X-rays. 2 × 40 = 80 µSv from the flights. 60 + 80 = 140 µSv.

Keep it

Copy this into your exercise book.

Background radiation is always there. Natural: radon (about half), rocks, cosmic rays, food. Made by people: mostly medical. It varies with rock, height and buildings. Dose is in sieverts: 1 mSv = 1 000 µSv.

Draw: A pie chart of natural background with radon about half, then ground, space and food.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (788 KB). It opens on any phone.