A team from the Institut dAstrophysique
Spatiale dOrsay (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
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
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 aparticles 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:
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.
Institut dAstrophysique Spatiale
Pierre de Marcillac - Tel: +33 1 69 85 87 36
Noël Coron - Tel: +33 1 69 85 85 26
Philippe Chauvin. Tel: +33 1 44 96 43 36
Martine Hasler. Tel: +33 1 44 96 46 35