An oxygen molecule is
a small dumbbell less than a nanometer across: two oxygen atoms with
two electrons flying between acting as the bonding agent. Now, an
international consortium has succeeded in making a dumbbell far
smaller: a beryllium-10 nucleus consisting of two alpha particles
(nuclear fragments containing two protons and two neutrons) with two
neutrons flying between acting as a sort of nuclear bonding agency.
This nuclear dumbbell is only a few fermis (10-15 m) across (see
figure at Physics News Graphics).
These tiny oblong
nuclei are made by colliding a beam of helium-6 nuclei into a gas of
helium-4 atoms. (The helium-6 nuclei, which are themselves a novelty,
were made by shooting protons at lithium.)
The berillium-10 nuclei created
in this way don't live very long. With a lifetime of about 10-21
seconds, they fly apart, usually back into helium-4 and helium-6
Martin Freer (M.Freer@bham.ac.uk) says that the
beryllium results support the idea that nuclei sometimes behave like
atomic systems in that they can be thought of as a core of particles
with extra "valence" particles (electrons/neutrons) exchanged
between cores. Several
exotic shapes are thought to be possible among the light nuclei.
Carbon-12, for instance, can exist as a triangular arrangement of
three alpha particles and oxygen-16 as a tetrahedron of alphas. But
these nuclei are tightly bound, so their exotic geometry cannot be
discerned. But berillium-10's prolate shape can be seen clearly through
the rotational behavior of the decaying system.
Freer is part of a team from the Universities of Birmingham and
Surrey (U.K.), Université Catholique de Louvain and University of
Leuven (Belgium), Université de Caen (France), and the Rudjer
Boskovic Institute (Croatia).
Freer et al., Physical Review Letters, upcoming article