Background Radiation and Detection

Background radiation — the low-level ionising radiation present everywhere, all the time, from natural and artificial sources.

Where it comes from, roughly in order of contribution in the UK:

• Radon gas — by far the largest single source. A radioactive gas seeping from rocks, especially granite, and collecting in buildings.
• Rocks and buildings — naturally radioactive isotopes in granite, brick and concrete.
• Cosmic rays — high-energy radiation from space, partly absorbed by the atmosphere. Exposure is greater at altitude, so aircrew receive more.
• Food and drink — naturally occurring isotopes, notably potassium-40, in almost everything we eat. Bananas are a well-known example.
• Medical — X-rays, CT scans and radiotherapy. The largest artificial source.
• Nuclear industry and weapons fallout — a very small contribution.

Most background radiation is natural, not man-made, which surprises people.

Correcting for background. Any measurement of a source's activity includes background, so a careful experiment measures the background count rate first with no source present, then subtracts it from every reading:

corrected count rate = measured count rate − background count rate

Omitting this step is a standard place to lose marks in a practical question.

Detecting radiation:

Photographic film — radiation darkens photographic film, just as light does. Film badges worn by radiographers and nuclear workers are developed periodically to show the total dose received. They record a cumulative dose rather than giving a live reading.

Geiger–Müller tube — a tube of low-pressure gas with a high voltage across it. Incoming radiation ionises the gas, the freed charges are accelerated and cause an avalanche of further ionisation, and the resulting pulse of current is counted. Connected to a counter, it gives the count rate in counts per second.

Activity — the number of nuclei that decay each second, measured in becquerel (Bq). One becquerel is one decay per second. Note that the measured count rate is always less than the true activity, because a detector only intercepts the radiation heading its way.

Example: a Geiger counter reads 5 counts per second with no source in the room, and 53 with a source in place. The corrected count rate for the source is 53 − 5 = 48 counts per second.