Background Radiation

Background radiation is the low-level ionising radiation that is always present in the environment. It comes from natural and artificial sources and is detected even when no radioactive source is deliberately placed near a detector.

Natural sources

Natural sources account for roughly 85% of total background radiation exposure:

1. Radon gas (the largest single contributor): a radioactive gas released from certain rocks, especially granite. It seeps into buildings from the ground.
2. Cosmic rays: high-energy particles from space. Exposure increases with altitude.
3. Rocks and soil: some contain naturally occurring radioactive isotopes such as uranium-238 and potassium-40.
4. Food and drink: trace amounts of radioactive carbon-14 and potassium-40 are present in all living organisms.

Artificial sources

Artificial sources account for roughly 15% of background radiation:

1. Medical procedures: X-rays and radiotherapy.
2. Nuclear power and weapons testing: very small contribution for most people.
3. Industrial uses: thickness monitoring and sterilisation equipment.

Why background radiation matters in experiments

When measuring the activity of a radioactive source, the detector also picks up background radiation. To find the true count rate from the source alone, you must:

1. Measure the background count rate (with no source present) for several minutes and calculate the average count per minute.
2. Subtract this background count rate from every reading taken with the source present.

This corrected value is the count rate due to the source only.

Variation

Background radiation levels vary by location. Areas with granite bedrock (high radon) or high altitude (more cosmic rays) have higher background levels. The average annual dose in the UK is about 2.7 millisieverts.