Background Radiation

Background radiation is the low-level ionising radiation that is present everywhere in our environment, all the time, regardless of whether a radioactive source is nearby. It is not a modern invention or the result of nuclear power — it has existed throughout Earth's history and even reaches us from outer space. Every second, your body is struck by hundreds of radioactive particles and photons from natural and artificial sources, yet your cells routinely repair this damage. Understanding background radiation is essential both for assessing real radiation risks and for conducting accurate laboratory measurements. The sources of background radiation fall into two broad categories: natural and artificial. Natural sources dominate, accounting for roughly 85% of the average person's annual dose in most countries. The main natural contributors are radon gas (a radioactive noble gas that seeps from rocks and accumulates in buildings), terrestrial radiation from radioactive isotopes in rocks and soil such as uranium-238, thorium-232 and potassium-40, cosmic rays arriving from space, and radiation from food and drink we consume. Artificial sources include medical procedures such as X-rays and CT scans, occupational exposure, and a small residual contribution from nuclear weapons testing and nuclear power. In any physics experiment involving a Geiger-Müller (GM) tube or other radiation detector, the detector will register counts even when no radioactive source is present. This background count rate must be measured separately and subtracted from all experimental readings to obtain the corrected count rate — the count rate genuinely attributable to the source under investigation. Failing to apply this correction systematically overestimates the activity of a source, introducing a constant positive error into every measurement.

Key takeaways

Radon Gas
Terrestrial Radiation from Rocks and Soil
Cosmic Rays
Food, Drink and Medical Sources
Measuring and Correcting for Background

Radon Gas

Radon-222 is a naturally occurring radioactive noble gas produced by the radioactive decay chain of uranium-238 in rocks and soil, particularly granite. Because it is a gas, it can diffuse out of the ground and accumulate inside buildings with poor ventilation. Radon decays by emitting alpha particles, making it the single largest contributor to natural background radiation — about 50% of the UK population's average annual dose. Areas built on granite, such as parts of Cornwall and Aberdeen in the UK, experience significantly higher radon levels. Ventilating buildings and sealing floors reduces radon accumulation.

Terrestrial Radiation from Rocks and Soil

Rocks contain trace quantities of naturally occurring radioactive isotopes. The most significant are uranium-238 (half-life 4.5 billion years), thorium-232 (half-life 14 billion years), and potassium-40 (half-life 1.25 billion years). These isotopes have existed since the Earth formed and decay slowly, emitting alpha, beta, and gamma radiation. Building materials derived from rock — bricks, concrete, granite worktops — carry this radioactivity into our homes. Potassium-40 is also present in soil and dissolves into groundwater, contributing to the dose we receive from food and drink.

Cosmic Rays

The Sun and other stars continuously bombard Earth with high-energy particles — mainly protons and alpha particles — called primary cosmic rays. When these particles slam into nuclei in the upper atmosphere, they produce secondary showers of muons, neutrons, pions, and gamma rays. At ground level, muons are the dominant component. The atmosphere provides significant shielding: people living at high altitudes (such as in Denver, Colorado, or La Paz, Bolivia) or those who frequently fly in aircraft receive a meaningfully higher cosmic ray dose than people at sea level. Astronauts receive far higher doses still.

Food, Drink and Medical Sources

Radioactive isotopes enter the food chain naturally. Carbon-14 (produced in the atmosphere by cosmic ray interactions with nitrogen) is present in all living organisms. Potassium-40 is found in bananas, potatoes, nuts, and dairy products. Polonium-210, a decay product of uranium, concentrates in seafood. These internal sources contribute a few percent of our annual dose. Medical sources — primarily diagnostic X-rays and CT scans — are the largest artificial contributor, accounting for roughly 14% of the UK average dose. Nuclear fallout and nuclear power together contribute less than 1%.

Measuring and Correcting for Background

To measure background radiation in a school laboratory, place a GM tube away from any known sources and record the total counts over a long time period (typically 5–10 minutes) to improve statistical reliability. Divide by the time to get the background count rate in counts per second (cps) or counts per minute (cpm). This value is then subtracted from every raw count rate measured with a source present. Because radioactive decay is a random process, count rates fluctuate statistically — taking a longer measurement time reduces the percentage uncertainty due to this randomness, following the principle that larger samples give more reliable averages.

The Key Equation

C_{\text{corrected}} = C_{\text{source}} - C_{\text{background}}

C_{\text{corrected}} — Corrected count rate (counts per second (cps)): The count rate due solely to the radioactive source, after removing the environmental contribution.
C_{\text{source}} — Measured count rate (counts per second (cps)): The raw count rate recorded by the detector with the source present, including background contributions.
C_{\text{background}} — Background count rate (counts per second (cps)): The count rate measured with no source present, representing natural and artificial background radiation in the laboratory.

Worked Example

Problem. A student places a GM tube near a radioactive source and records 840 counts in 60 seconds. She then removes the source and records 180 counts in 5 minutes with no source present. Calculate the corrected count rate due to the source alone.

C_{\text{corrected}} = 14.0 - 0.60 = 13.4 \text{ cps}

Answer. The corrected count rate is 13.4 counts per second.

Figure

Diagram

Origins · 1 minMarie Curie and how this was discovered

Marie Curie

1867–1934 · Polish-French

Marie Curie coined the term 'radioactivity' and discovered that it was an atomic property of certain elements, not a chemical reaction. She discovered polonium and radium, and developed techniques to isolate radioactive isotopes. Her meticulous measurements of ionisation currents produced by radioactive materials laid the groundwork for understanding that background ionisation is always present — an observation crucial to the development of radiation detection methods. Marie Curie remains the only person to have won Nobel Prizes in two different sciences — Physics (1903) and Chemistry (1911). Her original laboratory notebooks are still so radioactive that researchers must sign a waiver and wear protective equipment before handling them.

Types of Nuclear Radiation — Background radiation consists of alpha, beta, and gamma emissions; understanding the properties and penetrating power of each type is needed to appreciate which sources contribute to background at ground level.
Half-Life and Radioactive Decay — The corrected count rate is used in half-life experiments; understanding that decay is random and that background must be subtracted is essential for accurately plotting decay curves.
Activity and Count Rate — Activity is the rate of decay of a source measured in becquerels; count rate is the detector's reading. The relationship between these quantities relies on correctly accounting for background.
Radiation Dose and Risk — Evaluating the health risk from a radiation source requires comparing it to background levels, which provides a natural reference point for assessing what constitutes a significant additional exposure.