Researchers have long suspected that slow-moving, heavy particles such as alpha particles and neutrons should leave a characteristic pattern of damage. These kinds of radiation are known as “densely ionising” because they wreak havoc within a short tunnel; “sparsely ionising” X-rays or gamma rays spread their damage along a much longer path.

Because the damage from densely ionising radiation is so concentrated, it is far more likely to hit one chromosome several times, triggering deletions or reordering of its DNA.

Detecting intrachromosomal changes like these has been extremely difficult till now. Brenner’s team, together with a Russian group, took advantage of new dyes to “paint” bands on chromosomes. Image analysis software translates this into false-colour pictures, making it easy to spot any rearrangements.

The team used the technique to analyse chromosome 5 in thousands of blood cells from 31 people who had worked at a secret nuclear weapons facility near Ozyorsk in Russia. Though most of the workers were last exposed to densely ionising radiation from plutonium over 10 years ago, the team found a surprising amount of damage.

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