By Paul Rincon BBC News Online science staff, at the BA festival

Each blast carries its own atomic signature

The samples have been collected through one of the longest-running continuous field experiments in the world.

Scientists matched fallout in them to specific nuclear tests conducted in the 1950s in Nevada, US, confirming the tests did in fact contaminate Europe.

Details are given at the BA Festival of Science being held in Exeter this week.

The study is a collaboration between the University of Southampton and Rothamsted Research, where the samples are stored.

Finding the fingerprint

It used mass spectrometry to measure levels of the radioactive elements plutonium, radium and caesium in the samples.

The researchers were able to tie plutonium in the Rothamsted samples to tests conducted by the US military in the Nevada desert in 1952 and 1953.

"Each weapons test has a characteristic signature of different isotopes; in this case it is two different plutonium isotopes, but it could be two different uranium isotopes," said Professor Keith Goulding of Rothamsted told the British Association's annual meeting.

The new mass spectrometers at Southampton enabled them to look at the ratios of the different isotopes in the Rothamsted samples and then compare those with the samples they have from the tests.

Nuclear fallout from tests at Bikini atoll in the Pacific and from the reactor meltdown at Chernobyl in the former Soviet Union can also be detected.

"By doing that, they were able to detect which tests produced which fallout," said Professor Goulding.

Unexpected consequences

Dr Ian Croudace, of Southampton, said the samples provided "the first evidence" that plutonium from the tests contaminated north-west Europe.

However, Professor Goulding stressed the amounts of fallout were small and therefore of little health concern.

The Rothamsted experiment was originally set up in 1843 to study the impact of different fertiliser regimes on crop yields and soil health.

Scientists are also using the experiments to study patterns in air pollution and climate change.

"A common feature of long-term field experiments is that they come to be used in ways their founders could never have predicted," said Professor Goulding.

"Events such as the industrial revolution, the introduction of unleaded petrol and acid rain can all be seen in the changing chemistry of the sample archive."