Although bismuth-209 is commonly thought to be the heaviest stable isotope that exists in nature, theory suggests that it should be metastable and decay via alpha-particle emission to thallium-205. This decay is not easy to measure because the alpha particles generated have very little energy, which means that the isotope decays at a very low rate.
The equipment used by the Orsay team consists of two “heat and light” detectors that are enclosed in a reflecting cavity and cooled to 20mk. The first detector- containing bismuth-209, germanium and oxygen – undergoes a slight temperature rise when it absorbs an alpha particle. This temperature change is measured in the form of a voltage pulse whose amplitude is directly proportional to the energy released. The second detector, made from a thin disk of germanium, registers the light flashes from alpha-particle events.
The team performed two measurements, one with 31 grams of bismuth in the detector and the other with 62 grams. The scientists registered 128 alpha-particle events over 5 days and found an unexpected line in the spectrum at 3.14 MeV - now attributed to bismuth-209 decay. The half-life was calculated to be (1.9 +/- 0.2 ) x 1019 years, which is in good agreement with the theoretical prediction of 4.6 x 1019 years.
The technique could be also be used to accurately detect beta and gamma decays. “The experiment is a by-product of our search for dark matter,” team member Pierre de Marcillac told PhysicWeb. “Other kinds of decays such as protons from proton-rich nuclei could be studied by the same method but this will have to be proved!”