Oak Ridge Associated Universities 2006 Annual Report

Scientists Explore Extreme Regions
of Nuclear Stability
To explore the origins of our universe, scientists
study the very short-lived, exotic nuclei that
are involved in astrophysical processes, such as
supernovae, or the explosions of stars.
Working in the extreme regions of nuclear
stability (see Chart of Nuclei, right), during the
past year ORAU’s UNIRIB consortium and the
UNIRIB/Rutgers University Center of Excellence
for Radioactive Ion Beam Studies for Stewardship
Science have attempted to measure exotic nuclei by
completing experiments on neutron-deficient tin-
100 and initiating experiments on the very neutronrich
tin-132. Exploring nuclei in these two unstable
regions is sometimes referred to as searching for
the holy grail of nuclear physics.
Tin-100 and -132 are considered to be “doubly
magic,” or extremely stable. A nucleus has two
numbers: a number of protons and a number of
neutrons. Certain numbers (2, 8, 20, 28, 50, 82,
126) of protons or neutrons bring extra stability
to nuclei because they produce closed, or complete,
shells; they are known as “magic” numbers. The
tin-100 isotope is doubly magic as it has 50
protons and 50 neutrons; tin-132 is also doubly
magic with 50 protons and 82 neutrons. Very few
nuclei are doubly magic, and even fewer can be
measured.
“It’s an example of real science being performed at
ORAU, and it’s exciting science. ORAU’s UNIRIB
is doing what it set out to do, which is partnering
with universities to explore the forefront of nuclear
physics,” said UNIRIB Director Ken Carter.
The first experiment, on the neutron-deficient side,
was conducted by using a process known as double
alpha decay, where exotic nuclei of xenon-109
decays to tellurium-105 and ultimately to tin-101.
The results were published in the scientific journal
Physical Review Letters in August 2006, and the
experience gained will be applied to future attempts
to search for the alpha-decay chain of xenon-108
to telerium-104 to tin-100.
The second experiment, conducted in September
2006, used a transfer reaction by taking a beam of
very neutron-rich tin-132, produced at ORNL’s
HRIBF, and transferring one neutron to it.
Image Information:
Kate Jones, principal investigator on the tin-132 research
at UNIRIB, and Sean Liddick, post-doctoral researcher
on the tin-100 research, hold a silicon detector, similar to
ones used in their experiments.
Researchers measured resulting protons from the
reaction on a beam of tin-132 in order to study
the effect of the doubly magic properties on the
neighboring nucleus of tin-133.
These experiments were made possible due to
an investment in technologies and techniques
(developed by UNIRIB and its partners over the
past three years), such as the Oak Ridge Rutgers
University Barrel Array (ORRUBA) detectors
and the new advanced signal analysis techniques
with UNIRIB’s digital electronics. The HRIBF
at ORNL is also currently the only place in the
world that can produce radioactive ion beams at the
energies required to perform transfer reactions that
are very powerful tools for studying nuclei.
Chart of Nuclei
Proton-rich Nuclei
100
Sn
Line of Stability
132
Sn
Neutron-rich Nuclei
Scientists at UNIRIB have been exploring nuclei in two extreme
regions of nuclear stability, shown here in this slice of the
nuclidic chart, tin-100 and tin-132 (the chemical symbol for tin is
Sn). Exploring nuclei in these two unstable regions is sometimes
referred to as searching for the holy grail of nuclear physics.
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