Why Are Some Nuclei Stable?

Atomic nuclei contain protons and neutrons packed into an extremely small region.

Protons are positively charged, so they repel one another through the electrostatic force. At the same time, protons and neutrons experience the strong nuclear force, which can hold nucleons together when they are extremely close.

A nucleus is stable when the combination and arrangement of its protons and neutrons allow it to remain together without spontaneously changing.

An unstable nucleus eventually changes into a more stable arrangement by undergoing radioactive decay.

 

 

 

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The Forces Inside the Nucleus

Two interactions are especially important when thinking about nuclear stability.

Strong Nuclear Force

The strong nuclear force:

• Acts between nucleons.
• Is attractive over typical distances between neighbouring nucleons.
• Is extremely strong at nuclear distances.
• Acts only over a very short range.

It helps bind protons and neutrons together in the nucleus.

Electrostatic Repulsion

Every proton has a positive electric charge.

Therefore, every proton electrically repels every other proton in the nucleus.

This repulsion tends to push the nucleus apart.

Nuclear stability therefore depends partly on the balance between the attractive nuclear interaction and the electrical repulsion between protons.

 

 

 

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Why Are Neutrons Important?

Neutrons contribute to the attractive nuclear interactions between nucleons without adding additional electrical repulsion.

This makes neutrons particularly important in larger nuclei.

A nucleus with too few neutrons relative to its number of protons may be unstable.

However, simply adding more and more neutrons does not guarantee stability.

A nucleus can also become unstable if it contains too many neutrons.

This leads to an important concept:

Nuclear stability depends strongly on the balance between the numbers of neutrons and protons.

Neutron-to-Proton Ratio

The neutron-to-proton ratio compares the number of neutrons, N, with the number of protons, Z.

It can be written in ordinary text as:

Neutron-to-proton ratio = N / Z

For example, carbon-12 contains:

• 6 neutrons
• 6 protons

Therefore:

N / Z = 6 / 6 = 1

Carbon-12 is stable.

Stability in Light Nuclei

For many light stable nuclei, the number of neutrons is approximately equal to the number of protons.

For example:

Carbon-12

• 6 protons
• 6 neutrons
• N / Z = 1

Oxygen-16

• 8 protons
• 8 neutrons
• N / Z = 1

Calcium-40

• 20 protons
• 20 neutrons
• N / Z = 1

This does not mean that every nucleus with equal numbers of protons and neutrons is stable. It is a general pattern rather than an absolute rule.

Stability in Heavier Nuclei

As nuclei become larger, the situation changes.

A larger nucleus contains more protons, so there is more electrostatic repulsion.

Stable heavier nuclei therefore generally require more neutrons than protons.

For example, lead-208 contains:

• 82 protons
• 126 neutrons

Its neutron-to-proton ratio is approximately:

N / Z = 126 / 82 ≈ 1.54

Therefore, the stable neutron-to-proton ratio generally increases as nuclei become heavier.

 

 

 

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The Band of Stability

If neutron number is plotted against proton number for many nuclei, the stable nuclei occupy a region often called the band of stability or valley of stability.

For lighter nuclei, this region lies relatively close to:

N = Z

For heavier nuclei, the stable region shifts toward:

N > Z

This pattern shows that there is no single neutron-to-proton ratio that guarantees stability for every nucleus.

Instead, the most stable balance changes with nuclear size.

Too Many Neutrons

A nucleus with too many neutrons relative to its proton number may be unstable.

One way such a nucleus can move toward greater stability is through beta-minus decay.

During beta-minus decay, a neutron is transformed into a proton, with other particles also being emitted.

At the nuclear level:

neutron → proton + electron + antineutrino

The emitted electron is the beta-minus particle.

The important change in the nucleus is:

• Neutrons decrease by 1.
• Protons increase by 1.
• Nucleon number stays the same.

This can move a neutron-rich nucleus closer to the stable region.

Too Many Protons

A nucleus can also be unstable because it has too many protons relative to its number of neutrons.

Such nuclei may move toward greater stability through processes such as beta-plus decay or electron capture.

In beta-plus decay, a proton is transformed into a neutron, with other particles emitted.

The important nuclear change is:

• Protons decrease by 1.
• Neutrons increase by 1.
• Nucleon number remains unchanged.

This can move a proton-rich nucleus toward a more stable neutron-to-proton balance.

 

 

 

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Very Heavy Nuclei

For very large nuclei, having additional neutrons is eventually not enough to maintain stability.

The nucleus contains so many positively charged protons that electrical repulsion becomes increasingly significant.

Very heavy nuclei are therefore generally unstable.

They may undergo processes such as:

• Alpha decay
• Beta decay
• Spontaneous fission

For example, uranium-238 contains:

• 92 protons
• 146 neutrons

Despite having many more neutrons than protons, uranium-238 is still radioactive.

This shows why the neutron-to-proton ratio is important but is not the only factor determining nuclear stability.

What Is a Stable Isotope?

A stable isotope has a nucleus that does not spontaneously undergo radioactive decay.

Examples include:

• Hydrogen-1
• Carbon-12
• Carbon-13
• Oxygen-16
• Oxygen-18

These nuclei can persist without spontaneously changing into different nuclei.

What Is a Radioactive Isotope?

A radioactive isotope, or radioisotope, contains an unstable nucleus.

The nucleus spontaneously changes through radioactive decay.

Examples include:

• Hydrogen-3
• Carbon-14
• Cobalt-60
• Uranium-235
• Uranium-238

During radioactive decay, the nucleus emits particles and/or electromagnetic radiation and changes toward a more stable configuration.

 

 

 

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Example: Carbon Isotopes

Carbon provides a useful comparison.

Carbon-12

Carbon-12 contains:

• 6 protons
• 6 neutrons

It is stable.

Carbon-13

Carbon-13 contains:

• 6 protons
• 7 neutrons

It is also stable.

Carbon-14

Carbon-14 contains:

• 6 protons
• 8 neutrons

Carbon-14 is radioactive.

It undergoes beta-minus decay.

This demonstrates an important point:

Atoms can belong to the same element while having nuclei with different stabilities.

All three are carbon because they all contain 6 protons.

 

 

 

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Example: Hydrogen Isotopes

The three well-known isotopes of hydrogen also illustrate nuclear stability.

Hydrogen-1

• 1 proton
• 0 neutrons
• Stable

Hydrogen-2

• 1 proton
• 1 neutron
• Stable

Hydrogen-2 is also called deuterium.

Hydrogen-3

• 1 proton
• 2 neutrons
• Radioactive

Hydrogen-3 is also called tritium.

Tritium has an unstable neutron-to-proton balance and undergoes beta-minus decay.

Identifying an Unstable Isotope

Sometimes an isotope can be identified as unstable from known nuclear data.

For example:

¹⁴₆C

contains:

• 6 protons
• 14 − 6 = 8 neutrons

Carbon-14 is known to be radioactive.

Compare this with:

¹²₆C

which contains:

• 6 protons
• 6 neutrons

Carbon-12 is stable.

However, the neutron-to-proton ratio should not be treated as a simple formula that can always determine whether an unfamiliar isotope is stable.

Actual nuclear stability depends on several aspects of nuclear structure.

Why Do Unstable Nuclei Decay?

An unstable nucleus is in a configuration that can transform into a lower-energy, more stable configuration.

Radioactive decay allows this transformation to occur.

The nucleus may emit:

• Alpha particles
• Beta particles
• Gamma radiation
• Other particles in particular nuclear processes

The exact type of decay depends on the structure and energy of the nucleus.

Radioactive decay is therefore a nuclear process, not a chemical reaction.

Radioactive Decay Is Spontaneous

An important feature of radioactive decay is that it occurs spontaneously.

For a particular unstable nucleus, we cannot normally predict exactly when it will decay.

It might decay:

• Almost immediately
• Several seconds later
• Years later
• Millions or billions of years later

However, when very large numbers of radioactive nuclei are present, their overall behaviour is statistically predictable.

This leads to the concept of half-life, which is used to describe the rate of radioactive decay.

Stable Does Not Mean Unchanging in Every Situation

Calling a nucleus stable means that it does not spontaneously undergo radioactive decay under ordinary conditions.

It does not mean that the nucleus can never change.

Nuclei can undergo nuclear reactions if they interact with particles or other nuclei under suitable conditions.

The distinction is therefore between:

Spontaneous radioactive decay

and:

A nuclear reaction caused by an interaction

Stability Is More Complicated Than N / Z Alone

The neutron-to-proton ratio is an important guide, but real nuclear stability also depends on nuclear structure.

Important factors include:

• Number of protons
• Number of neutrons
• Nuclear energy
• Arrangement of nucleons
• Binding of nucleons within the nucleus

Some numbers of protons and neutrons produce particularly stable nuclear arrangements.

Therefore, two nuclei with apparently similar neutron-to-proton ratios can have different stabilities.

Nuclear Binding

A stable nucleus exists because energy would be required to separate its nucleons.

The nucleons are held together by nuclear interactions.

The concept of binding energy describes how strongly the nucleus is bound.

In general, nuclear stability is closely related to how energetically favourable the arrangement of nucleons is.

An unstable nucleus can sometimes transform into a configuration with greater stability by releasing energy through radioactive decay.

 

 

 

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Stable vs Radioactive Isotopes

Both stable and radioactive isotopes are still isotopes.

The difference concerns the stability of their nuclei.

Predicting General Trends

The following patterns are useful when considering nuclear stability:

Light stable nuclei

Usually have similar numbers of protons and neutrons.

Heavier stable nuclei

Usually contain more neutrons than protons.

Very neutron-rich nuclei

Are often radioactive.

Very proton-rich nuclei

Are often radioactive.

Very heavy nuclei

Are generally radioactive.

These are useful trends, but they should not be treated as perfect rules for every individual isotope.

Common Mistakes

Assuming Stable Nuclei Always Have Equal Numbers of Protons and Neutrons

This is approximately true for many light stable nuclei.

Heavy stable nuclei normally contain more neutrons than protons.

Assuming More Neutrons Always Increase Stability

Additional neutrons can help stabilize larger nuclei, but too many neutrons can also make a nucleus unstable.

Assuming the N / Z Ratio Alone Determines Stability

The ratio is an important indicator, but nuclear stability also depends on nuclear structure and binding.

Confusing Radioactive Isotopes with Different Elements

Carbon-12 and carbon-14 are both carbon because they both contain 6 protons.

Carbon-14 being radioactive does not make it a different element.

Thinking Radioactive Decay Is a Chemical Reaction

Radioactive decay changes the nucleus.

Chemical reactions primarily involve electrons.

Thinking Scientists Can Predict When One Particular Nucleus Will Decay

Individual radioactive decay events are unpredictable.

Scientists can predict the statistical behaviour of large numbers of nuclei.

Assuming Stable Means the Nucleus Can Never Change

Stable nuclei do not spontaneously decay, but they can participate in nuclear reactions under suitable conditions.

Check Your Understanding

1. What is meant by nuclear stability?

2. Which force helps bind nucleons together?

3. Why do protons repel one another?

4. Why are neutrons important for the stability of larger nuclei?

5. What is meant by the neutron-to-proton ratio?

6. Calculate N / Z for carbon-12.

7. Why do heavier stable nuclei generally contain more neutrons than protons?

8. What is the band of stability?

9. Why can a nucleus with too many neutrons be unstable?

10. Carbon-14 contains 6 protons and 8 neutrons. How does its composition differ from carbon-12?

11. Explain the difference between a stable isotope and a radioactive isotope.

12. Why are very heavy nuclei generally unstable?

13. Explain why uranium-238 can be radioactive even though it contains considerably more neutrons than protons.

14. Why is radioactive decay described as spontaneous?

15. Explain why the neutron-to-proton ratio is useful but cannot by itself perfectly predict nuclear stability.

Key Terms

• Nuclear stability – the ability of a nucleus to remain without spontaneously undergoing radioactive decay.
• Stable isotope – an isotope whose nucleus does not spontaneously decay.
• Radioactive isotope – an isotope with an unstable nucleus that undergoes radioactive decay.
• Neutron-to-proton ratio – the ratio of the number of neutrons to the number of protons in a nucleus.
• Strong nuclear force – the interaction responsible for binding nucleons within nuclei.
• Electrostatic repulsion – the repulsive interaction between positively charged protons.
• Band of stability – the region containing stable nuclei when neutron number is plotted against proton number.
• Radioactive decay – the spontaneous transformation of an unstable atomic nucleus.
• Radioisotope – another term for a radioactive isotope.
• Beta-minus decay – a radioactive process that converts a neutron into a proton while emitting a beta-minus particle and an antineutrino.
• Binding energy – the energy associated with binding nucleons together in a nucleus.
• Half-life – the time required for the expected number of undecayed nuclei in a large sample to decrease to half its original value.

Key Takeaways

• Nuclear stability depends on the balance of interactions and the arrangement of protons and neutrons in the nucleus.
• The strong nuclear interaction helps bind nucleons together, while positively charged protons electrically repel one another.
• The neutron-to-proton ratio is an important indicator of nuclear stability.
• Many light stable nuclei have approximately equal numbers of neutrons and protons.
• Heavier stable nuclei generally require more neutrons than protons.
• Too many or too few neutrons relative to the proton number can result in an unstable nucleus.
• Very heavy nuclei are generally radioactive.
• Stable isotopes do not spontaneously undergo radioactive decay.
• Radioactive isotopes contain unstable nuclei that spontaneously decay.
• Radioactive decay allows an unstable nucleus to transform toward a more stable, lower-energy configuration.
• Carbon-12 and carbon-13 are stable, while carbon-14 is radioactive.
• The neutron-to-proton ratio is useful for understanding stability, but it is not the only factor involved.
• Nuclear stability is ultimately connected to nuclear structure, interactions between nucleons, and nuclear binding energy.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

A Relative Comparison

A general comparison can be made without assuming exact percentages:

These descriptions are general. Actual contributions can differ considerably.

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.

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.

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.

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.

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

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

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.

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.

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.

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.

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.