Advancing the frontiers of nuclear physics
10 August 2007 (Volume 2 Issue 8)
RIKEN Nishina Center for Accelerator-Based Science – Part 1
RIBF – Radioactive-Isotope Beam Factory
At RIKEN’s Wako campus in the rolling hills of Saitama outside of Tokyo, researchers at the RIKEN Nishina Center for Accelerator-Based Science (the RIKEN Nishina Center) are busy extending the frontiers of nuclear physics. Using cutting-edge equipment, including the world’s most powerful cyclotron, heavy ions are accelerated to close to the speed of light and smashed apart to create unstable isotopes. Scientists believe that these unstable isotopes play an essential role in the synthesis of the elements that form our universe.
The RIKEN Nishina Center, inaugurated April 1, 2006, includes the Radioactive Isotope Beam Factory (RIBF), consisting of five particle accelerators, including a linear accelerator, four ring cyclotrons, an RI beam separator and experimental apparatus.
The pride and joy of the facility is its superconducting ring cyclotron (SRC). Commissioned in 2006, it is located 20 meters underground, both for radiation shielding and to position its 8,300 metric tons solidly on a gravel bed. Its extremely powerful superconducting electromagnets make it possible to accelerate ions to create intense beams of unstable isotopes.
The ions start their journey through the facility in the linear accelerator, and are then sped up through three ring cyclotrons before entering the SRC and emerging as a beam of ions traveling at up to 70% of the speed of light. This beam then strikes a production target and the resulting fragments are converted into an RI beam containing thousands of different types of isotopes. This beam, to be useful for research, has to be purified to leave only the particles of interest. This is the job of BigRIPS, the first of a new generation of RI beam separators. Others are now being built in Germany and planned in the US.
“The role of BigRIPS is to collect the beam and separate the different types of isotopes spatially,” says Toshiyuki Kubo, director of the BigRIPS facility. “It does this by analyzing the beam with 14 extremely powerful superconducting quadropole magnets. By carefully tuning the magnetic fields, we can separate the different kinds of ions, which are affected differently by the strong magnetic fields and focus the ions of interest on the secondary target, where their properties are studied.”
According to Akira Goto, director of the Accelerator Development Group, RIBF provides “intensity that is the highest in the field, making this the best facility in the world for the production and study of RI beams.”
In March 2007, in the first experiment ever performed at RIBF, a neutron-rich isotope of palladium, Pd125, was identified.
An important part of the center’s research activities focuses on the study of so-called super heavy elements. In July 2004, the new heaviest element, with the atomic number 113, was created for the first time using the high-intensity ion beam from the heavy-ion linac, which is the first stage of the RIBF accelerator complex. We hope that the new element will be named ‘rikenium’ or ‘japonium’.
RIBF’s heavy ion beams are also being used in biological research, for example to induce genetic mutations in plants. RIKEN researchers recently developed a highly salt-resistant strain of rice, as well as new varieties of flowers.
Free and open collaboration—a crucial requirement for research
The field of nuclear physics thrives on free and open collaboration. But until now, external researchers had to hold positions within RIKEN, and that may have limited their free use of equipment in RIKEN.
Moves are now afoot to open the RIKEN Nishina Center’s cyclotron facilities to external researchers. “This is an important and highly advanced facility, and should be considered a common instrument for all of humanity,” says Toshimi Suda, the director of the RIKEN Nishina Center’s User Liaison and Support Group.
The key to the new system is an independent body, the Program Advisory Committee (PAC), consisting of 17 top researchers from around the world, most representing large accelerator facilities. Its chairman is the director of GSI (Gesellschaft für Schwerionenforschung), one of major accelerator facilities in Germany.
The PAC is strictly independent of the RIKEN Nishina Center to ensure that proposals are evaluated on the basis of their scientific merit and feasibility only.
Interest has been intense. Since declaring the facility open six months ago, the PAC has received 32 proposals for experiments corresponding to 550 days of beam time.
Heiko Scheit—Exploring the edges of existence
Bizarre isotopes of ordinary, stable elements that contain large numbers of extra protons or neutrons, known as ‘exotic nuclei’, are a current focus of nuclear physicists. Created in the high-energy environment of the RIKEN Nishina Center’s cyclotrons, these extremely unstable and short-lived isotopes—especially in their death throes as they decay and throw off gamma radiation—offer insights into the most fundamental properties of nature.
Heiko Scheit is a staff researcher at the RIKEN Nishina Center, performing experiments on exotic nuclei using the center’s new SRC.
“These particles are very difficult to produce and difficult to study, since the more exotic they are, the more unstable and short-lived they become,” he explains. So far about 3,000 have been observed and studied, and about 7,000 more are predicted by theory but have never been observed in the laboratory.
“Of particular interest in the study of the structure of exotic nuclei, are the so-called ‘magic numbers’. Experiments investigating nuclides near stability have shown that some of them are especially stable and difficult to excite: these nuclides have a particular number of protons and neutrons, magic numbers, 2, 8, 20, 28, 50, 82 and 126.”
These magic numbers form the cornerstone of the most successful nuclear model, the nuclear shell model. It is not clear, however, whether the common magic numbers (the ones listed above) also hold for exotic nuclei. In addition, various theories predict the properties of the most stable nuclei very well, but in exotic isotopes, once the number of protons and neutrons starts to diverge, yawning gaps appear between predicted and observed properties.
When Scheit and his team conduct experiments in October this year, they hope to find interesting new properties of these nuclei. “We also want to perform a systematic study of them, so that the results can be compared to theory over a wide range of proton and neutron numbers,” Scheit adds. “That’s where the challenge is in nuclear theory.”
Specifically, he is interested in the so-called ‘island of inversion’, a region of the nuclear chart where exotic and rare isotopes have been identified but whose properties have not been established.
This kind of research advances nuclear theory, by testing the extent to which current models accurately describe the most exotic nuclei. It is also of interest, for example, to astrophysicists investigating the origin of the elements. “Exotic nuclei play a key role in this process,” explains Scheit, “and reliable extrapolations have to be employed to predict properties of the most exotic nuclides that cannot yet be synthesized in the laboratory.”
Very high energies and very intense beams are needed for this work. “The lab here is currently one of the best in the world, and will be for at least the next 5–10 years,” Scheit notes. “With this facility I think we can extend the region of known nuclei much further.”
You also need teamwork, which is available in abundance in the international environment of the RIKEN labs, according to Scheit. Despite the mix of cultures and languages, he has never had problems communicating or discussing issues with his Japanese or foreign colleagues. “Things are done a bit different here, but the differences are minor.”
He came to RIKEN last November with his wife and one-year-old son from Germany, after a short stay in the US. They are enjoying life and work in Japan and plan to stay a long time.
