Many naturally occurring isotopes are radioactive and thus we are constantly exposed to some ionising nuclear radiation.

Sources of ionising radiation include natural as well as man-made sources. Typically about 80% of our exposure is from naturally occurring sources.

Natural sources of radiation include:

• Rocks
• Radon gas in the air
• Food and drinks containing potassium (e.g. bananas, carrots and salt)

Artificial sources of radiation include:

• Building materials (e.g. concrete)
• Waste from nuclear power stations
• Medical X-rays

Example

When a GM detector is placed far away from any known source of radiation it shows a reading of 50 counts/min. (a) Explain why there is a reading on the GM counter. It is detecting background radiation. Show Answer   A source of radiation, P, with a half-life of 2 hours, is brought close to the GM detector. The counter initially reads 280 counts/min. (b) What is the initial activity (count rate) of source P alone? 280 – 50 = 230 counts/min Show Answer   (c) What reading will be shown on the counter after 4 hours have passed? 4 hours is two half-lives, thus the activity would be reduced to 25% of the original activity. i.e. activity of P alone would now be 230 ÷ 4 = 58 counts/min (2 s.f.) Including the background radiation, we would see a reading of: 58 + 50 = 108 counts/min on the counter. Show Answer

 

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