TTU physicists help uncover ‘magic’ nature of tin

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This portion of the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory houses the world’s largest electrostatic accelerator. This insulating column is 100 feet high by 33 feet in diameter.
A team of nuclear physicists with Tennessee Tech University connections is the first to explore and confirm the “magic” nature of a short-lived isotope of tin. Their results are published in tomorrow’s issue of the journal Nature, a highly respected international science weekly.

Scientists have been working in recent years with elements known to have a “magic” number of protons and neutrons. Many isotopes decay so rapidly that their nature can’t be measured readily. By experimenting with closely related isotopes, researchers seek to learn more about how atoms work.

In short, they want to understand nuclear explosions without actually doing them.

The scientists were able to study the “magic” qualities of Tin-132 (an isotope) by measuring characteristics on a neighboring isotope, Tin-133. Isotopes are different types of atoms of the same chemical element, each having a different number of neutrons. In analogy with electrons in atoms, certain numbers of neutrons or protons correspond to closed shells in nuclei, which are exceptionally stable.  Tin-132 has 50 protons and 82 neutrons, both known to be a “magic numbers” for stable nuclei, but Tin-132 is radioactive, and it was not known whether similar “magic” qualities would persist for such an exotic isotope.

“This experiment had been planned by a number of facilities around the world. This particular experiment has been listed as a benchmark,” said Raymond L. Kozub, TTU physics professor who is also a researcher at the Oak Ridge National Laboratory. “We just had the opportunity to do it before everybody else.”

Kozub says curiosity-driven research such as this has additional, national security motivations.

“This research is partially supported by the Center of Excellence for Stewardship Science, a collaboration involving several universities (led by Rutgers University) and national laboratories, which is funded by the National Nuclear Security Administration of the Department of Energy. In addition to providing data on unstable nuclear species of importance to nuclear structure and nuclear astrophysics, one of the aims of the center is to serve national nuclear security interests such as non-proliferation, nuclear forensics, and stockpile stewardship, which is critical in today’s era without nuclear testing,” he said. “Information such as that obtained in this experiment can help to understand the reactions that occurred in previous nuclear explosions without further testing of such devices.”

Kozub is among a team of scientists who conducted the highly complex experiment and subsequent analysis. The team was lead by Kate Jones, assistant professor of physics and astronomy from the University of Tennessee-Knoxville. Physics professor John F. Shriner Jr. of TTU is also a co-author, as are former TTU undergraduates D.W. Bardayan (now with ORNL’s physics division) and B.H. Moazen (now at Louisiana State University) as well as former TTU faculty member C. D. Nesaraja (now also with ORNL’s physics division).

The experiment was conducted at the Holifield Radioactive Ion Beam Facility at Oak Ridge, currently the only facility in the world with the necessary combination of equipment and expertise. Among Kozub’s many roles in the work was to design and develop experimental tools and techniques, set up the equipment and unique detectors, run the experiment, and analyze and interpret the data including theoretical calculations.

According to a press release from Nature, the experiment’s results confirm the successful shell model of nuclear structure and will help to predict the properties of more exotic nuclei such as those involved in the synthesis of the heaviest elements.

Atomic nuclei have a shell structure analogous to that of electrons in atoms, which gives special properties to nuclei with closed shells of protons and/or neutrons. Tin-132, with 50 protons and 82 neutrons, is doubly magic, but with a half-life of only 40 seconds its properties have been difficult to examine.

Kozub and his co-writers report in the Nature article on the technically challenging experiment in which they added single neutrons to Tin-132 to create Tin-133. By measuring the spectrum of quantum states available to the added neutron, the authors were able to show that the characteristics of Tin-133 nucleus are determined almost completely by this single neutron, confirming the closed-shell character of Tin-132. This finding extends the validity of the shell model to neutron-rich nuclei, and provides a benchmark for predicting the properties of nuclei even farther from stability, including those involved in neutron capture reactions in supernovae.

Understanding the single-particle states of the double-shell closure of the isotope is essential to predicting the properties of thousands of currently unmeasured nuclei, such as those involved in the explosion of supernovae. Such explosions are responsible for creation of more than half of the world’s elements.

“After all, we’re all made of stardust,” Kozub said.

Kozub said theoretical calculations based on the work described in the just-published paper in Nature will likely be performed by a number of physicists around the world. He and his collaborators have also conducted experiments on Tin-130 with similar results that will be published at some point in the future. He has obtained U.S. Department of Energy funding to conduct future experiments on Tin-126 and Tin-128, he said.

Nature is a weekly international journal publishing the finest peer-reviewed research in all fields of science and technology on the basis of its originality, importance, interdisciplinary interest, timeliness, accessibility, elegance and surprising conclusions. It was published for the first time in 1869.

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