Not even bismuth-209 lasts forever

Many heavy elements radioactively decay into lighter ones, although some do it faster than others. For decades, textbooks have listed bismuth-209 as the heaviest naturally occurring atom that never decays. A new experiment shows that the textbooks are wrong.

Using exquisitely sensitive, heat-detecting instruments known as bolometers, Pierre de Marcillac and his colleagues at the Institute for Space Astrophysics in Orsay, France, chanced upon signs that more than 100 bismuth-209 atoms had each spat out a helium nucleus–also known as an alpha particle–to become a lighter atom of thallium-205.

Theorists had predicted this particular decay more than 50 years ago. However, after a series of experiments conducted between 1949 and 1972 failed to turn up any evidence of the breakdown, those predictions faded into obscurity.

The rare disintegrations, reported in the April 24 Nature, were finally spotted as de Marcillac and his coworkers searched for something else–a hypothetical particle, the neutralino, that might be a component of the universe’s so-called dark matter (SN: 1/25/03, p. 51: Available to subscribers at In the Beginning: Dark matter builds galaxies, feeds quasars).

During a routine check for contamination in their bolometers, which happen to be built around crystals containing bismuth, the team noticed an unexpected alpha decay not listed in any reference tables.

Even so, those disintegrations occur so infrequently that an atom of bismuth-209 can still last just shy of forever. On the basis of the new decay data, the team calculates a half-life for bismuth-209 of some 19 billion billion years–roughly 1.4 billion times the current age of the universe.

Researchers didn’t have to wait anywhere near that long to detect the telltale alpha decays because the huge number of bismuth atoms in even a single bite-size bolometer crystal guarantees that some atoms will break down in a matter of days, if they break down at all, de Marcillac says.


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