New ‘Doubly Magic’ Research Reveals Role of Nuclear Shell
Researchers at the University of Tennessee, Knoxville, the Department of Energy’s Oak Ridge National Laboratory (ORNL), and six collaborating universities have performed an unprecedented nuclear reaction experiment that explores the unique properties of the “doubly magic” radioactive isotope of 132Sn, or tin-132.
The research, published in the journal Nature, is part of a broad scientific effort to understand nucleosynthesis, or the process by which the higher elements — those in the periodic table above iron — are created in the supernova explosions of stars. This research focused on the so-called r-process, responsible for the creation of about half of those heavy elements. This process involves interactions at very high energies of highly unstable and rare isotopes that do not naturally occur on earth but that can be created in the laboratory.
The research was performed at ORNL’s Holifield Radioactive Ion Beam Facility, a nuclear physics national user facility supported by the DOE Office of Science.
“Magic” nuclei define important way stations of relative stability as heavier elements are built up out of protons and neutrons, collectively known as nucleons.
Researchers relied on the “nuclear shell model” theory, which envisions the atomic nucleus as a series of shells, each representing a certain energy level and each containing a certain number of nucleons — protons or neutrons. As nucleons are added to the nucleus, they “fill” the successive shells: the first shell is filled with 2, the second with 8, and then on up to 20, 28, 50, 82 and 126, in succession. These numbers are “magic” because the nucleons in these shells are thought to be more strongly bonded — and hence relatively more stably configured — than the next nucleon that is added.
Tin-132 is a radioactive isotope of the familiar element tin with special properties–it is one of a small group of isotopes with a “magic” number of both protons and neutrons, making this nucleus “doubly magic.” It has 50 protons and 82 neutrons.
In this experiment, a neutron was transferred to a tin-132 nucleus to create tin-133, and the effects of adding this additional neutron were carefully measured.
“The experiment’s measurement is critical to benchmarking the nuclear shell model, to extrapolating theoretical nuclear models beyond the reach of current experimental facilities and to simulating the synthesis of nuclei heavier than iron in the cosmos,” said Kate Jones, assistant professor of physics and the paper’s lead author.
In the Nature paper, the team shows that tin-132 represents a good example of the shell model paradigm and that the properties of the states of tin-133 are to a large extent determined by the last, unpaired neutron.
“As such, tin-132 can now be used as a textbook example of a ‘doubly-magic’ nucleus and the principal benchmark for extrapolations to nuclei currently out of experimental reach that are crucial for production of heavy elements in stellar explosions,” Jones said. “Short-lived isotopes are important to astrophysical processes, and we want to understand how the heavy elements, such as those beyond iron, were produced.”
The Holifield facility enables nuclear scientists to produce beams of radioactive nuclei then separate a particular isotope for experimentation with the world’s most powerful electrostatic accelerator.
Nuclei can be classified in three ways: stable nuclei that never decay, known radioactive nuclei, and unknown, extremely short-lived nuclei. Doubly magic nuclei have properties that make them good launching pads to explore the structure of unknown nuclei with large neutron or proton numbers that do not naturally occur on Earth.
“A century ago, all nuclear physicists could study were the stable isotopes that make up all the atoms of elements of things around us,” Jones said.
However, nuclear processes — such as those that took place to create the matter around us — produce both stable elements and radioactive isotopes with nuclei that are either proton or neutron-rich.
Witold Nazarewicz, the Holifield Facility’s scientific director, explained how the experiments were performed at the ORNL facility.
“We produce some of the radioactive nuclei — the very short-lived nuclei like tin-132. We then use them as beams to further push the boundaries to get closer and closer to the territory of unknown nuclei,” Nazarewicz said.
Despite being comparatively strongly bound, tin-132 itself only lasts about 40 seconds. To produce the fleeting nuclei of tin-132, scientists shot protons at a uranium target, producing a primary beam of several kinds of radioactive nuclei from which tin-132 is carefully selected.
Jones and Nazarewicz said such pioneering research is rapidly changing basic theoretical models of nuclear structure because scientists’ understanding of the inner workings of the nucleus is altered the further into unstable, neutron-rich territory they are able to observe.
“It was only a decade ago we managed to produce beams of neutron-rich isotopes by proton-induced fission of uranium,” Nazarewicz said. “Now we see the nuclear structure shifting the more neutron-rich we get, and our understanding of the nuclear shells, which is basically textbook knowledge since the 1940s, will likely change.”
The research was supported by the DOE Office of Science, the National Science Foundation and the United Kingdom Science and Technology Funding Council.
ORNL is managed by UT-Battelle for the Department of Energy’s Office of Science.