Background Radiation

Background radiation is the low-level radiation that exists everywhere in the environment. It is always present and must be accounted for in any measurement of radioactivity.

Natural sources (about 85% of total)

• Radon gas from rocks and soil (the largest single source). Radon seeps into buildings and is inhaled.
• Rocks and soil containing naturally radioactive minerals (e.g. granite contains uranium).
• Cosmic rays from space (high-energy particles from the Sun and other stars). More intense at higher altitudes.
• Food and drink containing naturally radioactive isotopes (e.g. potassium-40 in bananas, carbon-14 in all organic matter).
• The human body itself contains radioactive isotopes (mainly potassium-40).

Artificial sources (about 15% of total)

• Medical procedures (X-rays, CT scans, radiotherapy). This is the largest artificial contribution.
• Nuclear weapons testing (fallout from past tests).
• Nuclear power stations (small controlled releases).
• Industrial sources (gauges, sterilisation).

Accounting for background radiation

When measuring the activity of a radioactive source in the lab:

1. Measure the background count rate first (with no source present).
2. Subtract the background count rate from all subsequent readings.

$\text{corrected count rate} = \text{measured count rate} - \text{background count rate}$

Common errors and how to correct them

• Thinking background radiation comes mainly from nuclear power. It is mostly natural (radon gas is the biggest source).
• Forgetting to subtract background radiation when calculating the activity of a source.

How to apply this in an exam

List the main sources (natural and artificial). For experimental questions, explain why you measure the background count first and subtract it from all readings.