What Is Background Radiation?

Background radiation is the ionising radiation that is continuously present in our surroundings, even when no obvious radioactive source is nearby.

Everyone is exposed to some background radiation throughout their lives.

It comes from a mixture of:

• Natural sources, which occur naturally in the environment.
• Human-made sources, which result from human activities involving radiation.

Background radiation levels are not identical everywhere. They vary with location, altitude, geology, buildings, medical exposure, and other factors.

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Natural and Human-Made Sources

Most background radiation experienced by the general population comes from natural sources.

Important natural sources include:

• Radon gas
• Rocks and soil
• Cosmic radiation
• Radioactive isotopes inside our bodies
• Radioactive materials in food and water

Human-made sources include:

• Medical X-rays
• CT scans
• Nuclear medicine
• Some industrial and research activities
• Residual radioactive material from past nuclear weapons testing and nuclear accidents

The relative contribution of each source depends strongly on where a person lives and their individual circumstances.

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Natural Sources of Background Radiation

Radon Gas

Radon is often one of the largest natural contributors to radiation exposure.

Radon is a radioactive gas produced naturally in decay chains involving uranium present in rocks and soil.

Because radon is a gas, it can:

• Move through cracks and spaces in the ground.
• Enter buildings through foundations.
• Accumulate in poorly ventilated spaces.
• Be inhaled into the lungs.

The amount of radon varies greatly from one location to another because different types of rock contain different concentrations of uranium.

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Why Radon Levels Vary

Two neighbouring regions can have very different radon levels.

Important factors include:

• Local geology
• Uranium content of rocks
• Soil type
• Cracks and openings beneath buildings
• Building design
• Ventilation

Buildings can sometimes allow radon to accumulate to concentrations considerably greater than those found outdoors.

This is one reason indoor background radiation can vary between buildings in the same general area.

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Rocks and Soil

Many rocks naturally contain small quantities of radioactive isotopes.

Examples include isotopes of:

• Uranium
• Thorium
• Potassium

These radioactive materials have existed naturally in Earth's crust for billions of years.

Their decay produces radiation that contributes to the background radiation around us.

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Building Materials

Some building materials are made from naturally occurring rock.

Materials such as:

• Granite
• Stone
• Concrete
• Brick

can contain small quantities of naturally radioactive isotopes.

The amount varies according to the materials and where they originated.

This does not mean that ordinary stone or concrete buildings are automatically dangerous. It simply means that natural radioactivity in construction materials can make a small contribution to background radiation.

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Cosmic Radiation

Earth is continuously exposed to high-energy particles arriving from space.

These are known as cosmic rays.

When cosmic rays enter Earth's atmosphere, they collide with atoms and molecules and can produce additional particles and radiation.

The atmosphere therefore acts as a partial shield.

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Effect of Altitude

Cosmic radiation generally increases with altitude.

At sea level, there is a large amount of atmosphere above us.

At high altitude, there is less atmosphere available to absorb the incoming radiation.

Therefore:

Higher altitude → generally greater cosmic radiation exposure

People living at high elevations therefore tend to receive more cosmic radiation than people living near sea level.

Airline passengers and flight crews also experience increased cosmic radiation while flying at high altitude.

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Radioactive Isotopes in Food and the Body

Small amounts of radioactive material occur naturally in food, water, and living organisms.

Examples include:

• Potassium-40
• Carbon-14

Potassium is an essential element required by the body.

Natural potassium contains a very small proportion of radioactive potassium-40.

As a result, the human body naturally contains a small amount of radioactive material.

This is normal and has been true throughout human history.

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Carbon-14

Carbon-14 is produced naturally in Earth's atmosphere through interactions involving cosmic radiation.

Carbon dioxide containing carbon-14 can enter plants through photosynthesis.

It then moves through food chains.

As a result, living organisms naturally contain small quantities of carbon-14.

After an organism dies, it stops exchanging carbon with the environment. The carbon-14 already present gradually decays.

This is the basis of radiocarbon dating.

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Human-Made Sources of Background Radiation

Medical Radiation

For many people, the largest source of human-made radiation exposure comes from medicine.

Medical uses of ionising radiation include:

• X-rays
• CT scans
• Nuclear medicine
• Some cancer treatments

Medical radiation is deliberately used because it can provide important diagnostic or therapeutic benefits.

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X-Rays

X-rays can pass through the body and are absorbed differently by different tissues.

This makes them useful for producing images of structures such as bones.

A medical X-ray therefore exposes a patient to a controlled amount of ionising radiation.

The amount depends on:

• The body part being examined
• The imaging equipment
• The number of images required
• The imaging procedure

Medical professionals aim to obtain the required diagnostic information while keeping radiation exposure appropriately controlled.

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CT Scans

A CT scan uses X-rays from many different directions to produce detailed images of structures inside the body.

CT scans can provide much more detailed information than many conventional X-rays.

Because multiple X-ray measurements are involved, a CT examination can result in a larger radiation dose than a simple X-ray examination, although the dose varies substantially with the type of scan.

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Nuclear Medicine

Some medical procedures introduce small amounts of radioactive substances into the body.

These substances are called radioactive tracers.

Detectors outside the body can detect radiation emitted by the tracer and provide information about the function of organs and tissues.

Radioactive materials can also be used in some medical treatments.

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Industry and Research

Radioactive materials and radiation are also used in industry and scientific research.

Applications include:

• Measuring the thickness of materials
• Detecting leaks
• Inspecting welds
• Sterilising equipment
• Scientific experiments
• Tracing the movement of substances

These activities can contribute to radiation exposure, particularly for workers who use radiation sources, although such work is normally subject to radiation-safety controls.

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Nuclear Power

Nuclear power stations contain large quantities of radioactive materials.

During normal operation, radiation and radioactive materials are carefully contained and monitored.

For most members of the public, routine exposure associated with normally operating nuclear power facilities is generally a small component compared with major natural sources and, for many individuals, medical exposure.

However, nuclear facilities require strict monitoring because accidental releases can increase environmental radiation levels.

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Nuclear Weapons Testing

Atmospheric nuclear weapons tests carried out mainly during the twentieth century released radioactive materials into the environment.

Some radioactive material was transported around the world through the atmosphere before eventually being deposited on land and water.

This material is often called radioactive fallout.

Today, residual fallout generally represents a relatively small component of background radiation in most locations, although its historical and environmental importance remains significant.

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Nuclear Accidents

Major nuclear accidents can release radioactive materials into the environment.

The effects can vary greatly depending on:

• The type and quantity of radioactive material released
• Weather conditions
• Distance from the source
• Local geography
• Cleanup and containment measures
• The half-lives of the released isotopes

Areas affected by significant releases may experience elevated radiation levels for extended periods.

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Comparing Sources of Background Radiation

Contributions Are Not the Same Everywhere

It is tempting to create one fixed percentage chart showing where background radiation comes from.

However, such a chart would only apply to a particular population, place, and time period.

For example, one person's exposure might be dominated by radon, while another person's exposure might be strongly influenced by medical imaging.

Therefore, it is more useful to understand the major factors controlling exposure.

For many populations:

• Natural sources make a substantial contribution.
• Radon can be a major natural source.
• Cosmic radiation contributes everywhere but increases with altitude.
• Terrestrial radiation depends on local geology.
• Medical exposure can be an important human-made contribution.
• Contributions from nuclear power during normal operation are generally much smaller than major natural and medical sources.

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A Relative Comparison

A general comparison can be made without assuming exact percentages:

These descriptions are general. Actual contributions can differ considerably.

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Why Background Radiation Varies

Geology

Local geology can have a major effect on background radiation.

Some rocks contain greater concentrations of naturally radioactive elements than others.

Areas containing uranium-rich rocks may therefore have:

• Greater terrestrial radiation
• Greater radon production

This can produce significant regional differences.

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Altitude

Altitude affects exposure to cosmic radiation.

At higher elevations:

• The atmosphere is thinner.
• Less shielding is present above you.
• More cosmic radiation reaches the surface.

Therefore, mountain communities generally experience more cosmic radiation than coastal communities at sea level.

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Building Design

Buildings can influence background radiation, particularly through radon.

Factors include:

• Foundation design
• Cracks in floors
• Ventilation
• Basement construction
• Building materials

A poorly ventilated building in a high-radon area may have considerably greater indoor radon concentrations than a well-ventilated building nearby.

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Lifestyle and Occupation

Background exposure can also vary between individuals.

For example:

• Frequent flyers receive more cosmic radiation.
• Airline crews spend substantial amounts of time at high altitude.
• Some workers use radioactive materials or radiation-producing equipment.
• Some patients undergo more medical imaging than others.

Therefore, two people living in the same city can still have different annual radiation exposures.

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Background Radiation and Radiation Detectors

Suppose you switch on a radiation detector in a laboratory with no radioactive source nearby.

The detector may still record radiation.

For example:

Background count rate = 18 counts per minute

Now suppose a radioactive source is placed nearby and the detector reads:

Measured count rate = 83 counts per minute

The approximate count rate due to the source is:

Source count rate = measured count rate − background count rate

Therefore:

Source count rate = 83 − 18

Source count rate = 65 counts per minute

This correction is important when carrying out radiation experiments.

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Why Background Measurements Fluctuate

Because radioactive decay is random, background count rate is not perfectly constant.

For example, repeated one-minute measurements might give:

• 18 counts
• 22 counts
• 17 counts
• 21 counts
• 19 counts

This does not necessarily mean the background radiation source has changed.

Random variation is expected.

Scientists can improve their estimate by:

• Measuring for a longer period
• Repeating measurements
• Calculating an average

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Worked Example: Average Background Count

Suppose five measurements give:

16, 19, 18, 21, 16 counts per minute

Total:

16 + 19 + 18 + 21 + 16 = 90

Average:

90 / 5 = 18

Therefore:

Average background count rate = 18 counts per minute

If a source produces a measured count rate of 118 counts per minute:

Corrected source count rate = 118 − 18

Therefore:

Corrected source count rate = 100 counts per minute

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Background Radiation Is Not the Same Everywhere

There is no single universal background radiation level.

A detector reading depends on:

• Location
• Altitude
• Local geology
• Radon concentration
• Building materials
• Nearby radiation sources
• Detector type
• Measurement time

For experimental work, students should therefore measure the local background count rather than assuming a universal value.

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Common Mistakes

Thinking Background Radiation Is Entirely Human-Made

Much background radiation comes from natural sources that existed long before modern technology.

Thinking Background Radiation Is Entirely Natural

Medical procedures and other human activities can also contribute to radiation exposure.

Assuming Every Location Has the Same Background Level

Background radiation varies with geology, altitude, buildings and other environmental factors.

Assuming Nuclear Power Is Always the Largest Source

For the general public during normal operation, nuclear power is generally a much smaller source than major natural sources and, for many people, medical exposure.

Forgetting About Radon

Radon can be one of the largest contributors to natural radiation exposure.

Thinking Background Count Should Stay Exactly Constant

Radioactive processes are random, so detector readings naturally fluctuate.

Forgetting to Subtract Background Count

When investigating a radioactive source:

Source count rate = measured count rate − background count rate

Confusing Activity with Count Rate

Activity is the actual number of nuclear decays per second.

Count rate is the number of events detected by a particular detector per unit time.

A detector normally records only some of the radiation emitted by a source.

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Check Your Understanding

1. Define background radiation.

2. Give four natural sources of background radiation.

3. Give three human-made sources of radiation exposure.

4. Explain how radon gas is produced.

5. Why can radon levels vary between different locations?

6. Why does cosmic radiation increase with altitude?

7. Explain why living organisms naturally contain small quantities of radioactive material.

8. Why can medical imaging make an important contribution to an individual's radiation exposure?

9. Explain why there is no single percentage breakdown of background radiation that applies everywhere.

10. How can local geology affect background radiation?

11. Explain how building design can affect radon exposure.

12. A detector measures 76 counts per minute near a source. The background count is 14 counts per minute. Calculate the corrected count rate.

13. Why might repeated measurements of background radiation give slightly different results?

14. Why should background radiation be measured during a radiation experiment?

15. Explain why two people living in the same city could receive different amounts of radiation exposure.

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Key Terms

• Background radiation – ionising radiation continuously present in the environment.
• Natural background radiation – radiation originating from naturally occurring sources.
• Human-made radiation – radiation exposure resulting from human activities.
• Radon – a naturally occurring radioactive gas produced in radioactive decay chains in rocks and soil.
• Cosmic radiation – radiation associated with high-energy particles arriving from space.
• Terrestrial radiation – radiation originating from naturally radioactive materials in Earth's crust.
• Radioactive fallout – radioactive material deposited in the environment following atmospheric nuclear releases.
• Background count rate – the detector count rate measured without the investigated source present.
• Count rate – the number of radiation events recorded by a detector per unit time.
• Activity – the number of nuclear decays occurring per second.
• Becquerel (Bq) – the SI unit of radioactive activity, equal to one decay per second.
• Ionising radiation – radiation capable of removing electrons from atoms or molecules.

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Key Takeaways

• Background radiation is the ionising radiation continuously present in our surroundings.
• Background radiation comes from both natural and human-made sources.
• Important natural sources include radon, rocks and soil, cosmic radiation, and radioactive isotopes in living organisms.
• Radon can be a major source of natural radiation exposure.
• Cosmic radiation generally increases with altitude.
• Local geology influences terrestrial radiation and radon levels.
• Medical imaging is an important human-made source of radiation exposure for many people.
• Radiation from normally operating nuclear facilities is generally a relatively small contribution to public exposure compared with major natural sources.
• The relative contributions from different sources vary with location and individual circumstances.
• Background radiation levels can vary because of geology, altitude, building design, ventilation, occupation and lifestyle.
• Detector measurements of background radiation fluctuate because radioactive processes are random.
• Background count should be measured and subtracted when determining the count rate produced by a radioactive source.