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Page from an NSCL logbook used in the experiment
that successfully created three super-heavy isotopes of magnesium
and aluminum. The partially visible scrawled phrase "Let the
celebrations begin!" reflects the excitement about the discovery.
Dave Morrissey, one of the paper's coauthors, in
the NSCL data acquisition area.
Photos: �
NSCL
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Particles that comprise atomic nuclei, protons
and neutrons, are held together by the nuclear force. One of the four
fundamental forces that collectively describe the interactions of all
matter in the cosmos, the nuclear force, has been the subject of
scientific inquiry since the 1930s.
Despite much progress in nuclear physics during the subsequent decades,
understanding of how the nuclear force and other effects play out
inside nuclei is far from complete. For example, even today scientists
aren�t sure exactly what combinations of protons and neutrons can make
up most atomic nuclei.
One way experimental nuclear physicists explore this issue is by using
accelerator facilities to create reactions that, in effect, kluge
together piles of protons. An element is defined by its number of
protons. For example, hydrogen has one proton; helium, two protons;
oxygen eight protons, uranium, 92 protons. Whenever physicists
establish a new proton limit, they invariably garner attention for
conjuring new elements. In October 2006, a team of Russian and
American scientists generated worldwide headlines for creating an
element with 118 protons, the most protons ever recorded in a single
nucleus.
Another way to probe nuclear stability is to see how many neutrons can
be loaded onto nuclei of more quotidian elements, which is the focus
of much of the work at NSCL. Elements can exist as different isotopes,
which contain the same number of protons but different numbers of
neutrons. As an example, the most abundant stable isotope of carbon
has six protons and six neutrons. However, trace amounts of carbon-13
and carbon-14 � with seven and eight neutrons respectively � also can
be found on Earth.
The neutron-limit, referred to as the neutron-dripline, is a basic
property of matter. Yet remarkably, despite more than a half-century
of inquiry, scientists know the dripline location only for the eight
lightest elements, hydrogen to oxygen. So one very basic question �
what�s the heaviest isotope of a given element that can exist" �
remains unanswered for all but eight of the hundred or so elements on
the Periodic Table.
In an experiment that ran earlier this year at NSCL, researchers
successfully created and detected three new super-heavy isotopes of
magnesium and aluminum: magnesium-40, with 12 protons and 28 neutrons;
aluminum-42, 13 protons and 29 neutrons; and aluminum-43, 13 protons
and 30 neutrons. If the everyday version of aluminum were a 160-pound
adult, aluminum-43 would be a muscular, 255-pound heavyweight.
"Evidence of particle stability for magnesium-40 obtained at NSCL is a
major step in the field of rare isotope physics," said Hiro Sakurai,
chief scientist at RIKEN in Japan, who was not involved in the
research. The RIKEN research institute in Saitama, Japan, is home to
the world's most powerful accelerator facility for creating
radioisotope beams.
The fleeting appearance of these three nuclear newcomers is
significant for several scientific and technical reasons.
First, when is comes to magnesium, the results indicate that the
dripline extends at least as far as, and possibly beyond,
magnesium-40. The isotope wasn�t detected in several dripline-focused
experiments conducted around the world since 1997 and the research
community had begun to suspect that it was beyond the bounds of
stability. Though it�s difficult to compare across disciplines,
physicists� success in detecting three magnesium-40 isotopes in the
course of an 11-day experiment is roughly similar to the achievement
of biologists who finally snap an image of an elusive and
thought-to-be-extinct animal after years of traipsing through the
jungle.
"The discovery of the hitherto unknown heaviest magnesium and aluminum
isotopes at NSCL is a milestone in rare isotope research and is a
great accomplishment for the worldwide scientific community exploring
unstable nuclei close to the so-called neutron dripline," said Horst
Stocker, director of Gesellschaft fur Schwerionenforschung, GSI, who
was not involved in the research. Darmstadt, Germany-based GSI is one
of the world's top accelerator facilities for producing heavy-ion
beams for research.
Second, aside from being a similarly interesting outlier, aluminum-42
carries added importance since it is a near-dripline nucleus with an
odd number of neutrons. Isotopes of lighter elements that toe the edge
of existence generally have even numbers of neutrons due to the fact
that neutrons naturally pair up inside nuclei. With an even number of
neutrons, the nuclei in effect have a tidy, complete set of such pairs
that collectively form a sort of energetic scaffolding that increases
stability.
According to one of the leading theoretical models, aluminum-42
shouldn�t exist. That it does suggests that the dripline may in fact
tilt in the direction of more novel, neutron-rich isotopes, an
implication that will help to extend nuclear theory and point the way
to future experiments.
The NSCL result "alters the landscape of known nuclei, it alters our
understanding of the forces that bind nuclei into stable objects, and
it has important implications for future attempts with next-generation
facilities to map the evolution of nuclear structure and existence
into the most weakly bound nuclei," said Rick Casten, D. Allan Bromley
Professor of Physics at Yale University, also not involved in the
research.
The experimental technique itself also is noteworthy. Creating and
measuring rare isotopes is always needle-in-a-haystack work that
requires researchers to hunt for a few desired nuclei from a swarm of
fast-moving and mostly known and therefore less interesting particles.
But in this experiment, NSCL researchers achieved a hundred- to
thousand-fold boost in their ability to filter out what can be thought
of as junk. They did so by essentially jury-rigging the facility to
filter the beam twice. The result was an ability to detect and measure
isotopes so rare that they represent less than one in every million
billion particles that passed by the detectors.
The dual filtering process, more properly known as two-stage
separation, is a fixture in most new and planned facilities for rare
isotope beam research, including the proposed upgrade of NSCL. This
experiment marks one of the first uses of two-stage separation in the
world and the first time the technique has been tried at NSCL, which
typically filters and purifies particles only once in its A1900
separator.
NSCL detectors returned just one blip of data consistent with the
existence of aluminum-43. This generally isn�t enough to count as a
discovery, according to the conventions of nuclear science. However,
more than 20 instances of its immediate neighbor, aluminum-42, were
observed. Because of this relative abundance and the fact that, due to
pairing, the 30 neutrons in aluminum-43 should prove more stable than
the 29 neutrons in aluminum-42, the solitary signature of aluminum-43
etched in the data logs carries more than usual amount of credibility.
"Experiments such as these are paving the way into the new era of
nuclear structure studies that technological developments are opening
to investigation for the first time ever," said Yale's Casten.
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