Alpha detector. An artist's conception of a chamber containing a decaying bismuth germanate crystal.

Bismuth Not So Stable After All

After being caught suffering a breakdown, the heaviest stable element on the periodic table has just lost its title. Clever new experiments with bismuth-209 have shown that the heavy metal actually does decay, a theoretical prediction that had defied verification by experimenters until now. The technique could help scientists search for dark matter, a fundamental--and fundamentally unknown--component of the universe.

Physicists have long thought that bismuth-209 was stable. But as more precise measurements of mass became possible, theorists suggested that the heavy element should decay to become the element thallium-205, an alpha particle, and a packet of extra energy. Since the 1950s, researchers have tried to detect this decay, but with no luck.

Success came serendipitously to researchers at the Institut d'Astrophysique Spatiale in Orsay, France, who were trying to develop a device that could detect elusive particles of dark matter. The researchers, led by astrophysicist Pierre de Marcillac, cooled a crystal of bismuth germanate to near absolute zero, then looked for tiny amounts of heat and light given off. These emissions had exactly the same energy as the predicted alpha particle, the team reports online 24 April in Nature. When an alpha particle escapes a bismuth nucleus, it perturbs the crystal, releasing its energy in a tiny burst of heat and an even smaller flash of light.

"It's a very tough experiment," says Eric Norman of the Lawrence Berkeley National Laboratory in California, who attempted it himself in the late 1990s. The key to the team's success is the extraordinary cold conditions they created, he says. At ultralow temperatures, bismuth germanate is more sensitive to the small amount of energy released by exiting alpha particles and creates more noticeable changes in the crystal's temperature.

With bismuth dethroned, lead, the previous element in the periodic charts, now claims the title of heaviest stable element. But although bismuth is no longer the paragon of stability, it's no flake either. The researchers found that the element has what may be the longest half-life of any isotope: about 20,000,000,000,000,000,000 years.

It's "an exciting measurement," says Henry Stroke, a physicist at New York University in New York City, but the technique is even more exciting. Norman says miniscule temperature changes in supercooled bismuth germanate--similar to those detected in the new study--might also betray the presence of passing particles of dark matter, the so-far invisible stuff that makes up about a quarter of the universe.

Related sites
Institut d'Astrophysique Spatiale
Information about bismuth (now partly outdated)
Information about alpha particles