Any experiments with radioactivity should allow for the radiation from natural sources and other 'man-made' sources. This is known as background radiation and may come from several origins:
(a) cosmic radiation - including radiation from solar flares
(b) radioactive rocks (radon gas) - background radiation is greater over rocks such as granite
(c) radioactive contamination of the apparatus
(d) fallout from nuclear tests, mainly in the 1950s and 1960s
(e) radioactive potassium and carbon in our bodies
The total sum of all these sources is likely to be small - some 20 to 50 counts per minute.
The background radiation received by a person in a year is roughly equal to the dose received in 5 minutes by a 1 Curie source of cobalt 60 placed one metre away.
For example, a piece of granite from the edge of Dartmoor in the UK gave a count rate of 34 counts per minute when measured at the author's school. This should be compared wiith the normal background count in the school of 19 to 20 counts per minute, although this did rise in the UK after the Chernobyl reactor disaster.
For eighteen months after the reactor destruction at Chernobyl there were restrictions on the sale of lambs form North Wales and the Finnish government put a safety limit of 1200 Bq per kg on reindeer meat.
