Argonne scientists take giant step forward in understanding
exotic nuclei
ARGONNE, Ill. (Aug. 29, 2007) – Developing good predictive powers of how all
nuclei work is critical to advance our understanding of the universe. The vast
nuclear landscape, which is thought to consist of about 6,000 isotopes is not
well charted and half the nuclei remain unknown.
Only about 300 isotopes are stable and exist in the world around us. A lot
has been learned about these stable nuclei, but researchers at the U.S. Department
of Energy's Argonne National Laboratory are developing techniques to uncover
the secrets of some of the most unstable and exotic counterparts.
While the exotic isotopes are not normally present on earth, some of them
play critical roles in the working of the stars, especially in the element-producing
process of nucleosynthesis.
Darek Seweryniak and his colleagues from Argonne's Physics Division in collaboration
with scientists from the University
of Maryland, University of
Notre Dame and University
of Edinburgh, have focused their attention on a new isotope
of tin, tin-100 (100Sn), one of the very rare "doubly-magic" nuclei. The
doubly-magic assignment refers to the special cases where both protons and
neutrons fill their quantum shells so that super-stability is achieved.
The properties of stable doubly-magic nuclei, such as oxygen-16 (16O), calcium-40
(40Ca) and lead-208 (208Pb) are well known, but the challenge is to determine
the extent to which current models can be extrapolated to describe nuclei situated
far from the line of stability, in this case, near the "proton dripline," where
nuclear existence ends. Establishing the properties of this nucleus and its
immediate neighbors builds a solid platform from which the properties of all
proton-rich intermediate mass nuclei can be developed. To date, only a handful
of 100 Sn nuclei have ever been synthesized, in France and Germany, but almost
nothing is known about its shell structure.
Using the Argonne Tandem Linac Accelerator System (ATLAS) facility in a 10-day
experiment, the researchers started by synthesizing tin-101 (101Sn) and measuring
its lowest excited state. 101Sn consists of a 100Sn core with the extra valence
neutron orbiting in discreet quantum states in the field of the core.
Gamma
rays emitted during the transition between these states were detected and
analyzed in Gammasphere, the world's most powerful gamma-ray detector, and
101Sn nuclei were identified with the Fragment Mass Analyzer. The properties
of the gamma rays reveal the sequence of states and their spacing, and can
be used to directly challenge the reliability of contemporary nuclear models.
In the short term, the researchers expect to find further quantum states in
101Sn and learn more about the core this way. However, the experiment provides
a key technical step toward the study of 100Sn itself in the future.
The detailed results of these findings were published in the journal Physical
Review Letters in July. [Phys.Rev.Lett.
99, 022504 (2007)]
Funding for this work was provided by the Department of Energy's Office
of Nuclear Physics, part of the Office of Science.
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of Science.
For more information, please
contact Steve McGregor (630/252-5580 or media@anl.gov)
at Argonne.
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