Discovery of 45 rare neutron-rich isotopes provides clues to the stellar formation of heavy elements
Many of Earth’s resources, from copper to more precious metals, are often taken for granted, but the only place to make more of these elements lies at the interior of stars. Despite this cosmological reality, however, scientists are yet to fully understand the stellar process that produces many of the heavier elements in the periodic table. Now, with the discovery of 45 new, rare isotopes by a team of scientists lead by Toshiyuki Kubo at RIKEN’s Nishina Center in Wako, hopes are high to establish an understanding of the nuclear process that produces roughly half the elements heavier than iron. The discoveries, published in the Journal of the Physical Society of Japan1, are some of the first results from the Radioactive Isotope Beam Factory (RIBF), a next-generation heavy-ion accelerator designed to explore the structure of exotic, neutron-rich isotopes.
All of the known elements and their isotopes are collected in the ‘Table of Nuclides’, a continuously updated chart that is organized according to how many protons and neutrons each isotope contains.
The RIBF was designed to explore the outer limits of this chart, near the so-called neutron ‘drip-line’, where nuclei can be produced only by collisions in particle accelerators. These nuclei contain so many neutrons they survive for only fractions of a second before decaying to more stable forms.
Neutron-rich isotope research is important for understanding how stars produce elements of the periodic table. Fusion, where two high-energy nuclei merge, occurs in stars and can form elements up to iron. However, scientists believe that roughly half of the elements heavier than iron are produced by the so-called ‘r-process’, where r stands for rapid. During the r-process, a nucleus is bombarded and bloated with neutrons so rapidly that it has no time to stabilize by beta decay; instead, it decays through a series of unstable intermediate nuclei. According to theoretical models, many of the rare isotopes discovered using the RIBF act as the intermediate nuclei in the r-process.
“If we understand the structure of the nuclei of these new neutron-rich isotopes, we can better understand the path and pace of the r-process and how the process is constrained by temperature and density,” says Mike Famiano, a member of Kubo’s team.
The rapid-fire flux of neutrons required for the r-process likely only occurs at the interior of exploding stars called supernova. As such, the RIBF research is providing a unique glimpse into a rare and distant stellar process.
Break and measure
The RIBF produces rare isotopes by accelerating ionized uranium-238—an element heavy enough to break into other large nuclei—to close to the speed of light and then smashing these ions into a target of beryllium or lead. The collision causes the uranium nucleus to undergo fission and split into smaller nuclear ‘fragments’ that are collected and analyzed in the fractions of a second before they decay.
It was in such fragments that Kubo and colleagues discovered the 45 new isotopes, which span the periodic table from manganese to barium. To produce fragments over this wide range, Kubo’s team designed a means of identifying the nuclear fragments quickly and accurately, and the RIBF accelerator group designed a cyclotron capable of accelerating uranium.
The RIBF cyclotron uses powerful superconducting magnets to cycle the uranium ions through an accelerating voltage multiple times, until the ions reach speeds 70% of the speed of light.
The ‘brains’ of the RIBF is the in-flight separator, dubbed ‘BigRIPS’, which analyzes the fragments of the fissile uranium. Superconducting magnets in the separator force the fast-moving nuclei to fan out with different curvatures, allowing the team to determine the atomic number and the ratio of charge to mass of each nucleus—some of which were produced only once in the collision.
Kubo and his team’s results not only provide insights into the stellar production of heavy elements, but also enable them to test the limits of theoretical models for more stable nuclei. Kubo says they will next focus on the new isotopes palladium-128 and nickel-79 because they are similar to the nuclei with a so-called ‘magic’ number of neutrons or protons—2, 8, 20, 28, 50 and 82—which are extraordinarily stable. Palladium-128 has 82 neutrons, while nickel-79 has one more than the magic number of 50 neutrons. Near the neutron drip-line, however, nuclei may have different magic numbers, a possibility that the new isotopes will allow nuclear physicists to test.
As the first next-generation accelerator for studying rare isotopes, the RIBF is in prime position to keep opening new doors in nuclear physics research. Similar facilities are under construction in Germany and in the US and Kubo points out that the teams working at three new-generation facilities are already collaborating with each other. Given the funding necessary to plan, design and construct such large facilities—on the order of 500 million US dollars (50 billion yen)—the results from RIKEN’s RIBF will continue to provide motivational fuel for these efforts.
“The discovery of new, rare isotopes is the first validation of the extended capability of these new-generation facilities,” explains Kubo. The aim now is to increase the intensity of the uranium beam at RIBF by 1,000 times higher than present. “We expect to discover many new isotopes and expand the frontier of nuclear physics to a large extent.”
About the Researcher
Toshiyuki KuboToshiyuki Kubo was born in Tochigi, Japan, in 1956. He received his BS degree in physics from The University of Tokyo in 1978, and his PhD degree from the Tokyo Institute of Technology in 1985. He joined RIKEN as an assistant research scientist in 1980, and was promoted to research scientist in 1985 and to senior research scientist in 1992. He spent time at the National Superconducting Cyclotron Laboratory of Michigan State University in the USA as a visiting physicist from 1992 to 1994. In 2001, he became leader of the BigRIPS team, and was promoted to group director of the research instruments group at the RIKEN Nishina Center in 2007. He is in charge of the design, construction, development and operation of major research instruments, as well as related infrastructure and equipment, at the RIKEN Nishina Center. His current research focuses on the production of rare isotope beams, in-flight separator issues, and the structure and reactions of exotic nuclei.
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