Number 762, January 19, 2006
by Phil Schewe and Ben Stein
Fission Fragments Weighed
The fissioning of uranium results in a
variety of unstable neutron-rich nuclei. A team of scientists from
the University of Jyväskylä in Finland has for the first time made
high-precision mass measurements of a number of isotopes produced in
proton-induced fission reactions of uranium, including strontium,
zirconium, and molybdenum.
These so-called refractory elements are
hard to study as ionized beams because of their high boiling
points. Instead, the researchers reach a high level of precision by
coaxing the nuclei into a Penning trap, which employs a combination
of a strong magnetic field and a static quadrupole field to trap
ions.
In this kind of device, the particle's mass can be deduced
from the observed cyclotron motion -- that is, from the particle's
looping orbit in a strong magnetic field.
The reason for wanting
better isotope masses is that they provide information about nuclear
binding energies. The mass of the simplest compound nucleus, the
deuteron, for instance, is several million electron volts less than
the sum of the masses of its constituent proton and neutron. The
difference is the net binding energy.
In the case of the new
studies, the isotope masses are determined with a precision of
thousands of electron volts. By measuring the mass of several zirconium
isotopes of increasing neutron numbers, one can see subtle effects
in the complex structures of these nuclei. Astrophysicists, who
consider how larger nuclei are built inside stars or novas also will
be interested in knowing how nuclear mass increases with neutron
number.
Hager et al., Physical Review Letters,
upcoming article
Contact Ari Jokinen, ari.jokinen@phys.jyu.fi
Nuclear Molecule: Nature's Smallest Dumbbell
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
fragments.
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).
Scientists in Singapore have devised
magnetic micro-coils for moving beads along a microfluidic track.
Microfluidics -- the
transport of small objects or fluids around a microchip often
fabricated using the same lithographic techniques used for photonics
or electronics -- has many current and potential applications in
materials science and in bio/medical studies.
Researchers at three
Singapore organizations, the Institute of Microelectronics, the
Institute of Bioengineering and NanoTechnology, and the Nanyang
Technological University, have constructed several
different types of magnetized pillar structures which, when
energized, and move beads tens of microns around a microfluidic
chip.
Ramadan et al.,
Applied Physics Letters, 16 January 2006
Contact Qasem Ramadan, qasmr@ime.a-star.edu.sg
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