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
Stellar insights
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 (Fig. 3). 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.
A pioneer
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.”