Press release


Discovery of the natural radioactivity of bismuth

Paris, April 23, 2003


A team from the Institut d’Astrophysique Spatiale d’Orsay (IAS - CNRS, Université Paris XI) has just detected the decay of the bismuth isotope 209. This isotope is commonly regarded as the heaviest of the stable isotopes. Researchers were able to do this for the first time, thanks to a new type of detector known as a scintillating bolometer made of bismuth germanate. This type of measuring instrument is used to reveal the existence of enigmatic particles that could be one of the components of the Dark Matter in the universe. These results were published in the review, Nature, on April 24, 2003.

Since the 1940's, measurements of atomic mass and nuclear decay schemes have revealed that the bismuth isotope 209(1) suffered from a slight excess of weight. The only naturally abundant isotope of bismuth, isotope 209 should decay to the more stable thallium isotope 205, after rejecting a helium nucleus (made up of two protons and two neutrons), during "alpha" type decay. The energy recovered during this weight loss program is easily calculable and is measured at 3137 keV (kiloelectronvolt). Although experiments carried out over the past fifty years to observe this decay have all failed, they have nevertheless made it possible to deduce its rareness: its lifetime should be greater than 2 x 1018 years! This lifetime is so long that recent tables definitively characterized the bismuth isotope 209 as being stable.

A team from the IAS that recently announced the observation of its decay is involved in a program whose aim is the direct detection of Dark Matter in the universe, one of the biggest mysteries in cosmology, in the form of "supersymetrical" particles known as neutralinos. As part of a joint effort with a team from the University of Zaragoza in Spain, it initiated the ROSEBUD experiment in the Somport Tunnel, along the old railroad line between Pau and Canfranc in the Pyrenees Mountains in France. The detectors used to reach this objective are known as bolometers(2) .

Just like most of the groups involved in this type of detection, the IAS team is now in a research and development phase that should lead to the design of new detectors capable of discriminating between particles on the basis of their nature. Prototypes of scintillating bolometers in bismuth germanate ("BGO", with the chemical formula Bi4Ge3O123 and in calcium tungstate (CaWO4), weighing almost 50 grams, have been tested over the last two years at Canfranc and the IAS. In this technique, a pair of bolometers is enclosed in a light-reflecting cavity. The massive one is considered to be the target, so to speak, of future experiments and scintillates in response to ionizing events, whereas the other one, consisting of a thin germanium disk, absorbs and measures the photons emitted by the first. The target is cooled to a very low temperature of 20 millikelvins, or almost –273,13°.

During the night of March 14 to 15, 2002, during a calibration experiment in a 46-gram BGO bolometer, measurements made at the IAS revealed the existence of an "unknown line" linked to an alpha decay, measuring almost 3200 keV. In the morning, seven decays with these characteristics were observed on the monitor. After having verified that it was not the result of an electrical artifact, bibliographic research revealed that it was most probably the decay of the bismuth isotope 209. This hypothesis was validated beyond a doubt after having been subjected to many additional tests.

The reported lifetime of 209Bi is 1.9 x 1019 years - or approximately one billion times older than the present estimated age of the universe! The determined disintegration energy is precisely measured at 3137 keV. These two decay characteristics totally concur with theoretical forecasts, updated on the basis of more recent mass and energy tables.

This relatively easy detection in the laboratory of the decay of an isotope known to be stable until now is the unquestionable demonstration of the power of the technique used: scintillating bolometers cooled to very low temperatures will undoubtedly play a very important role in the future in the detection of rare events, highly unlikely nuclear reactions or in the research of infinite traces of radioactivity.

Experimental detection of a–particles from the radioactive decay of natural bismuth
Pierre de Marcillac, Noël Coron, Gérard Dambier, Jacques Leblanc & Jean-Pierre Moalic. Nature, April 24, 2003.

For more information:

(1) Two isotopes of the same element have a different number of neutrons (N). However, their number of protons (Z) and electrons is identical: they are chemically indistinguishable and have the same position (thus, the name isotope) on the periodic table of the elements of Mendeleïev. In addition to isotope 209 (Z=83; N=126), we know 32 other bismuth isotopes with lifetimes ranging from 50 microseconds to 3 million years that can be detected during nuclear reactions. These lifetimes are much too short in comparison to the age of the universe for these isotopes to have ever existed in measurable quantities.
(2) bolometer (etymology: from the Greek "bole": radiation, jet; "metron": measure)
The bolometer can be very schematically described as the assembly of a massive crystal, absorbing the radiation to be measured with a thermometer that is glued onto the crystal. The device is very sensitive when it is chilled to very low temperatures. Data generated by the thermometer make it possible to follow the energy history of all the events that occur in the crystal.

Researcher contacts:
Institut d’Astrophysique Spatiale
Pierre de Marcillac - Tel: +33 1 69 85 87 36

Noël Coron - Tel: +33 1 69 85 85 26

CNRS-INSU contact:
Philippe Chauvin. Tel: +33 1 44 96 43 36

Press contact:

Martine Hasler. Tel: +33 1 44 96 46 35