Theme E — Nuclear and Quantum Physics

E.3.2 Background Radiation — IB Physics HL Revision Notes & Topical Questions

5 min read 9 parts Lesson 19 of 45 1 quiz

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Lesson summary

E.3.2 Background Radiation — IB PHYSICS SL notes: Identify the sources of background radiation and correct a measured count rate for it.Background radiation — the ionising radiation always present in the environment, whether or not a deliberate source is nearbyA Geiger–Müller tub…

Why it matters
Rocks and soil
Cosmic rays
Food and drink
Nuclear medicine
Nuclear accidents

Part 1 of 7What background radiation is

Background radiation

Background radiation the ionising radiation always present in the environment, whether or not a deliberate source is nearby
A Geiger–Müller tube on a bench with all sources removed still records counts.

Why it matters

Why it matters it must be accounted for, otherwise a measured activity is too high

Sources

Sources natural and artificial

Variation

Variation the level varies with location, altitude and building materials
Granite bedrock or poorly ventilated basements give much higher count rates than a coastal town with sandy soil.

Part 2 of 7Natural sources

Radon gas

Radon gas it comes from uranium in rocks, soil and building materials such as stone and brick
Uranium decays into radon, which is an alpha emitter and is dangerous if inhaled in large quantities.
It is colourless, tasteless and odourless, so it is detected with a Geiger counter.
It is often the largest single contribution, and in the UK by far the largest.
It seeps into buildings and its level varies from place to place.

Rocks and soil

Rocks and soil granite and some building materials contain uranium, thorium and potassium-40, giving gamma radiation

Cosmic rays

Cosmic rays high-energy particles from the Sun, supernovae and other cosmic events, mainly protons and alpha particles
Collisions with air molecules produce secondary radiation, including gamma rays.
The dose rises with altitude, so air travellers receive more.

Carbon-14

Carbon-14 all organic matter contains a tiny amount, and living things keep the amount almost constant by replacing their carbon

Food and drink

Food and drink naturally occurring radioactive elements such as potassium-40 in bananas are present in tiny, harmless amounts

Part 3 of 7Artificial sources

Nuclear medicine

X-rays, CT scans, radioactive tracers and radiation therapy
Medical procedures are the largest part of the artificial share.

Fallout

Fallout radioactive material thrown into the air after nuclear weapons explosions
It is presently very low but would rise in areas of testing.

Nuclear accidents

Nuclear accidents events such as Chernobyl give a large dose and can devastate areas for centuries, but are now extremely rare

Nuclear waste

Nuclear waste it contributes little to background but is dangerous to those handling it

Industry and power

Industry and power routine releases make a small contribution

Overall

Overall for most people the artificial share of the dose is a minority, with waste and fallout under 1% in the UK

Part 4 of 7Correcting for background

Corrected count rate

Corrected count rate measured count rate minus background count rate
Both must be expressed per unit time before subtracting.

Measuring the background

Measuring the background record counts with the source removed
For example, 24 counts in 1 minute is 24 counts per minute, or 0.4 counts per second.

Measuring the source

Measuring the source record the count rate with the source in place, then subtract the background

Improving accuracy

Improving accuracy repeat readings and take averages, and count over a long time

Practical habits

Keep other sources well away from the detector.
Measure background at the start and end of the experiment.
Use the same position and counting time for each reading.

Distance method

Distance method when a source is moved far enough away, the count rate becomes constant and equals the background

Part 5 of 7Uncertainty in counts

Random uncertainty

Random uncertainty the fractional uncertainty of N counts is about 1/√N
100 counts give about 10% and 10 000 counts give about 1%.

Subtraction

Subtraction when two counts are subtracted, the uncertainties add
A weak source near background gives poor precision and needs a long counting time.

Detection limit

Detection limit if the source rate is smaller than the random fluctuations in the background, the source cannot be distinguished

Part 6 of 7Why it matters in experiments

Decay curves

Decay curves the same background value is subtracted from every reading, and the corrected values are plotted

Without correction

Without correction the curve levels off at a non-zero count rate instead of falling towards zero
The half-life found from the graph is then wrong.

Safety

Safety regulators compare the extra dose from a hospital procedure or power plant with natural variation in background

Part 7 of 7More from the specification

1Radon gas decay in environmental atoms

Radon gas decay in environmental atoms spontaneous alpha emissions from atmospheric radon atoms provide a means of natural environmental background radiation.

2Sources of environmental background radiation

Background radiation originates from cosmic rays, terrestrial radioactive rocks (radon gas), and artificial medical or industrial sources.

3Nuclear binding energy and mass defect

The binding energy of a nucleus corresponds to the mass defect between constituent nucleons and the intact nucleus via E = delta m * c².

4Corrected Geiger-Muller count rates

Accurate measurement of radioactive source activity requires subtracting ambient background count rate from raw GM detector readings.

5Nuclear binding energy

Nuclear binding energy energy required to completely separate an atomic nucleus into individual constituent protons and neutrons.

6Strong nuclear force

Strong nuclear force attractive short-range force acting equally between nucleons to overcome electrostatic repulsion in stable nuclei.

7Radioactive isotopes decay

Radioactive isotopes decay unstable isotopes in an environmental sample decay spontaneously over half-lives spanning days to millions of years.

8Decay equation

An equation in nuclide notation with balanced nucleon and proton numbers.

9Neutron

Neutral particle in the nucleus, relative mass 1.

10Isotope

Atom of the same element with a different number of neutrons.

11Isotopes

Nuclei with the same proton number but different neutron numbers.

12Electron

Negatively charged particle in an atom.

13Binding energy

The minimum energy required to separate the nucleons of a nucleus to infinity.

14Penetrating power

How far radiation travels through a material.

15Nuclear notation

Symbol showing mass number and atomic number.

16Beta-plus decay

Proton changes to a neutron with emission of a positron and a neutrino.

Key terms

Term	Meaning
Background radiation	Ionising radiation always present in the environment.
Radon	Radioactive gas from uranium decay, a major natural source.
Cosmic rays	High-energy particles from space producing secondary radiation.
Corrected count rate	Measured count rate minus background count rate.
Count rate	Number of detected counts per unit time.
Geiger–Müller tube	Detector that produces a pulse for each ionising particle entering it.

Avoid these mistakes

Forgetting to subtract background, or subtracting counts taken over different times — convert to counts per unit time first.

Assuming background is constant everywhere — it changes with location, altitude and building materials.

Thinking background is purely man-made — most of it is natural.

Using a very short background measurement, which gives a large fractional uncertainty.

Saying background comes from the source itself — it is present when the source is removed.

Last updated: 18 October 2026