Background radiation and radiation safety

Key ideas

Background radiation is the low level of ionising radiation present in the environment at all times, from sources that are not the one being investigated. It is always present and must be accounted for when measuring a radioactive sample.

Sources of background radiation

• Natural sources (the large majority):

  • Cosmic rays — high-energy radiation arriving from space.
  • Rocks and soil — naturally occurring radioactive isotopes in the ground (e.g. in granite).
  • Radon gas — a radioactive gas that seeps from the ground, especially in some geological areas.
  • Radioactive isotopes naturally present in food and the human body (e.g. potassium-40).

• Artificial (human-made) sources (a small minority):

  • Medical uses, such as X-rays and nuclear medicine.
  • Nuclear industry and, historically, weapons testing fallout.

• When measuring the count rate of a radioactive source, the background count rate must be measured separately (with the source removed) and subtracted from the total reading to find the count rate due to the source alone.

Why radiation is hazardous

• Ionising radiation can knock electrons out of atoms and molecules in living cells, including DNA.
• This ionisation can damage or kill cells, or cause mutations that may lead to cancer.
• The degree of harm depends on the type of radiation, the dose received, and whether the source is outside or inside the body:
  • Alpha is the most dangerous inside the body (e.g. inhaled or swallowed), because its strong ionising ability then acts directly on internal tissue at close range.
  • Gamma is the most dangerous from outside the body, because it is the only type able to penetrate deep enough to reach internal organs from an external source.

Reducing radiation exposure

Exposure to radiation from an external source can be reduced using three principles:

• Shielding — place an absorbing material (appropriate to the radiation type, e.g. lead for gamma) between the source and the person.
• Distance — stay as far as practical from the source; radiation spreads out and intensity falls rapidly with distance.
• Time — minimise the time spent near the source, since total exposure builds up the longer you are near it.

Practice question

A student measures a count rate of $220$ counts per minute near a radioactive source. With the source removed, the detector still reads $20$ counts per minute. What is the count rate due to the source alone, and where does the remaining reading come from?

Worked solution: $220 - 20 = 200$ counts per minute due to the source. The $20$ counts per minute with the source removed is background radiation, from natural sources such as cosmic rays and radioactive material in rocks and soil.