Exploring the mysteries of atom nuclei at RIKEN’s Radioactive Isotope Beam Factory

Tohru Motobayashi
Team Leader, SAMURAI Team
Coordinator, Research Instrument Group
RIBF Research Division
RIKEN Nishina Center for Accelerator-Based Science
Atomic nuclei come in various shapes, from the commonly imagined spheres to shapes more akin to rugby balls or mandarins. It is generally understood that the number of protons and neutrons determines whether a nucleus will be spherical or non-spherical. With the successful creation of short-lived unstable nuclei, however, scientists have found that certain nuclei predicted to be spherical by the conventional theory are often quite non-spherical. “Theoretically, there are about 10,000 different atomic nuclei, but less than one-third of these have been studied. There may be many interesting and strongly deformed atomic nuclei. Creating and investigating nuclei that have never been seen before will surely contribute to our understanding of the atomic nucleus,” says Tohru Motobayashi from the RIKEN Nishina Center for Accelerator-Based Science in Wako, Japan. Motobayashi is using the center’s Radioactive Isotope Beam Factory (RIBF) to elucidate the shapes and properties of atomic nuclei, obtaining remarkable insights into how elements are created. The RIBF, a next-generation heavy-ion accelerator research facility, is capable of creating an intense beam of any of about 4,000 unstable nuclei, making it one of the world’s most useful tools for nuclear studies.

The RIBF, creating a thousand new radioisotopes

Motobayashi’s work is predominantly with the RIBF, which went into full-scale operation in 2007. The RIBF includes linear accelerators, ring cyclotrons, radioisotope beam generation and separation facilities and other experimental instruments (Fig. 1). The superconducting ring cyclotron (SRC) at the heart of the RIBF is 18 meters in diameter and weighs 8,300 tons. “What we are studying in this large-scale facility is atomic nuclei with diameters of just several femtometers. With the RIBF, we are creating a lot of unstable nuclei and studying their shapes and properties, which allows us to elucidate how elements are synthesized,” he says.

All matter consists of atoms, which in turn consist of an atomic nucleus and electrons, and the atomic nucleus itself is comprised of protons and neutrons. The element represented by an atom is determined by the number of protons, and there are about 90 naturally occurring elements, from hydrogen with one proton to uranium with 92. Each element, however, can have multiple ‘isotopes’—atoms with the same number of protons but different numbers of neutrons. Including the isotopes, about 300 stable atoms are found in nature. Including unstable isotopes, however, there are about 10,000 in total. Most of the unstable isotopes are not found in nature because they quickly decay into one of the stable forms. “The difference between the numbers of protons and neutrons in stable nuclei is not very large. We are studying unstable nuclei, particularly those with many more neutrons than protons. The RIBF allows us to create and study unstable nuclei that differ considerably from stable nuclei.”

In the RIBF, ions of various elements are accelerated in stages up to 70% of the speed of light and bombarded against target nuclei. The collisions strip off protons and neutrons to produce various unstable nuclei, which are then separated into a beam line and subject to analysis. With the help of numerous instruments installed on the beam lines, scientists can determine the shape, mass, lifetime and other properties of the nuclei. Although such experiments using radioisotope beams have been conducted since the 1980s, previous facilities have only been able to create about 3,000 different atomic nuclei. The RIBF, which can accelerate a wider range of starting elements from hydrogen to uranium, is expected to make it possible to create 1,000 new types of unstable nuclei that have never been observed before.

Disappearance of the magic number in neutron-rich nuclei

The interest in neutron-rich nuclei originates from a discovery in 1995 by a RIKEN team including Motobayashi that the nucleus of magnesium-32 (32Mg) was significantly deformed from the expected spherical shape. Magnesium has 12 protons and normally 12 neutrons. The 32Mg isotope with mass number of 32 (total number of protons and neutrons) has an extra eight neutrons, making it particularly neutron rich, and the discovery of deformation in such an isotope attracted a great deal of public attention. “Atomic nuclei were generally regarded as uniform mixtures of protons and neutrons. However, as scientists began to create neutron-rich nuclei in experiments, they discovered that certain unstable atomic nuclei can have characteristics remarkably different from those for stable nuclei. For example, in ‘neutron halo’ nuclei, the neutrons are localized around the rim of the nucleus. Even so, the deformation of 32Mg was still quite unexpected,” says Motobayashi.

Just as electrons orbit around the nucleus of an atom, neutrons and protons move around in orbits within the nucleus itself. There are multiple nuclear orbits, and each can hold up to a certain number of neutrons or protons. An atomic nucleus is spherical and particularly stable when one of its orbits is fully filled with neutrons or protons, and the number of neutrons or protons in a filled orbit is known as the ‘magic number’: 2, 8, 20, 28, 50, 82 and 126. “The atomic nucleus of 32Mg was found to be deformed even though it has a magic neutron number of 20. This discovery was referred to as ‘the disappearance of magic numbers’ and was much discussed among scientists. The experiment with 32Mg was our first neutron-rich nucleus experiment. Our findings could be attributed to beginner’s luck, but it motivated me to delve into the study of neutron-rich nuclei, which I found very interesting,” says Motobayashi.

Later, the atomic nuclei of neon-30 (30Ne, 20 neutrons) and magnesium 34 (34Mg, 22 neutrons), both with ten extra neutrons, were also found to be deformed, prompting scientists to start paying attention to atomic nuclei with mass numbers of around 30 and neutron numbers around 20, which appeared to be considerably deformed (Fig. 2). “Understanding why these nuclei are deformed requires investigation of other unstable nuclei in the high-deformation region. However, these nuclei are so unstable that they are extremely difficult to generate. Conventional accelerators failed to create adequate amounts of such unstable nuclei, forcing scientists to turn to the RIBF for this research.”

The rugby ball-shaped atomic nucleus of 32Ne

As soon as the RIBF went into operation in 2007, an experiment was conducted to create new unstable nuclei by accelerating and splitting the atomic nucleus of uranium. This experiment resulted in the successful creation of palladium-125 (125Pd) and -126 (126Pd) as the first fruits of the RIBF. Scientists also started experiments to investigate the properties of a range of unstable nuclei. The first target was the nucleus of neon-32 (32Ne), which consists of ten protons and 22 neutrons. To produce 32Ne, the scientists accelerated ions of calcium-48 (48Ca) and bombarded them into a beryllium (Be) target to strip protons and neutrons from the 48Ca nucleus, resulting in various unstable nuclei. The 32Ne nuclei were then separated from the population of nuclei and collated into a beam, which was then used to investigate the shape of the atomic nucleus.

“We can estimate the shape of an atomic nucleus by rotating it,” explains Motobayashi. “When we bombard a carbon target with the 32Ne beam, nuclear interactions can occur that excite and rotate the 32Ne nuclei. The excited nuclei emit gamma-rays as they rotate back and return to their original unexcited state. The larger the deformation of the nuclei, the slower the rotational speed and the lower the energy of the emitted gamma-rays. In this way, we can study the shape of the nuclei by measuring the energy of the gamma-rays using a gamma-ray detector. We found that the energy of gamma-rays was significantly lower than expected for 32Ne, proving that the nuclei are considerably deformed from spherical. A report on this finding was published in July 2009.” The deformation of 32Ne was the largest ever observed for neon isotopes. It was also found that the larger the neutron number in this region, the greater the deformation.

The gamma-ray detector used to determine the shape of 32Ne, called the detector array for low-intensity radiation or DALI-2, was developed under the leadership of Motobayashi. Determining the energy of gamma-rays accurately is not easy because the gamma-ray emission of fast-moving particles, such as the unstable nuclei generated by the RIBF, which move at about 60% of the speed of light, increases in energy if the particles are moving toward the detector, and decreases in energy when they are moving away from the detector. This is known as the Doppler effect, and to deal with it accurately, DALI-2 has about 160 detectors installed around the reaction point. “It seems like the detectors are arranged at random, and that’s exactly what we intended,” says Motobayashi with a smile. “If you do the calculations, you can arrange the detectors in strict pentagonal or hexagonal arrays. But once set, it is not possible to rearrange the detectors for a new experiment. With the random arrangement, we can easily change the location of each detector, making it a very flexible configuration. We can hold discussions on how to improve the measuring methods, and we can learn even from mistakes. This approaches help to yield new discoveries.”

Using the RIBF, the 32Ne experiment took only about eight hours, much faster than expected. “The RIBF successfully provided a thousand times the number of nuclei as anticipated. That’s a good misjudgment, because we were prepared to spend at least half a year on the experiment,” says Motobayashi.

One possible explanation for the severe deformation of 32Ne is an effect known as ‘orbital inversion’, by which neutrons reverse their usual ordering from lower- to higher-energy orbits. This results in neutrons populating higher-energy orbits even when lower-energy orbitals are not fully filled. The region in the nuclear chart where this phenomenon is observed is also known as ‘the island of inversion’. “The energy of gamma-rays observed through the 32Ne experiment cannot be explained without the concept of orbital inversion or the assumption of two nuclear orbits that are extremely close to each other. We expect to be able to understand the causes of the strong deformation of atomic nuclei by taking advantage of the RIBF to investigate unstable nuclei in the island of inversion and adjacent regions.”

Motobayashi points out that a future challenge is how to distinguish the roles of protons and neutrons. The energy of gamma-rays merely provides information related to the shape of the atomic nucleus, which consists of both protons and neutrons. It is possible to determine the behavior of protons in an atomic nucleus by measuring the lifetime of the excited state, by bombarding it with electrons, or by using the ‘Coulomb excitation’ method, which was used in the discovery of 32Mg nuclear deformation. However, there remains no effective method to clarify the distribution of neutrons. Motobayashi is now working on a method in which unstable nuclei are bombarded into a solid or liquid hydrogen target. “The hydrogen nucleus consists of a single proton. As protons can have a strong effect on neutrons in atomic nuclei, we should be able to clarify the behavior of neutrons by bombarding hydrogen with the unstable nuclei. We have started experiments using liquid hydrogen targets, and we are now producing results including the finding that 32Mg is ‘softly’ deformed,” says Motobayashi. The atomic nucleus of 32Mg is now known to look like a rugby ball, but its shape is not fixed, and is in fact quite flexible. Studies are now being conducted to clarify the details.

Exploring the ‘island of inversion’ as one of the mysteries of elemental synthesis

“I want to find out whether there are other islands of inversion in the nuclear chart. I am particularly interested in the regions adjacent to magic numbers 28 and 50. This study is also related to element synthesis, another subject of research in our team.”

Among the naturally occurring elements, hydrogen, helium and lithium were created immediately following the Big Bang, while other elements up to the atomic number of iron were created through nuclear reactions in stars. About half of the heavier elements, from iron to uranium, are considered to have been created in a period of just one second by supernovae at the end of the life of massive stars. A leading hypothesis that explains elemental synthesis is the theory of rapid neutron-capture, known as the ‘r-process’, by which atomic nuclei rapidly absorbed neutrons at a high temperature and pressure driving the continuous creation of unstable nuclei, which then decayed into stable nuclei. However, no solid evidence for this theory has been found as yet (Fig. 2). “We plan to create the world’s first unstable nuclei that are considered to have been created through the r-process, and attempt to ascertain the facts by reproducing the elemental synthesis process. At present, the RIBF is the only available facility that can serve this purpose. In this study, the role of magic numbers is very important. I am interested in how the island of inversion affects the nucleosynthesis process.” The team has already started conducting experiments to create some of the unstable nuclei that may been created through the r-process and to investigate their lifetimes.

The RIBF evolves

Along with experiments, RIKEN is also promoting the design and development of new instrumentation (see Fig. 1). A typical example is the ‘Rare-RI’ ring, which is capable of measuring the mass of single short-lived unstable nuclei that may have been produced by the r-process. “Measuring the mass of each nucleus is based on RIKEN’s unique idea that no unstable nuclei should be wasted because they are precious.”

Motobayashi is leading the development of another instrument called SAMURAI, a superconductiing analyzer for multiple particles from radioisotope beams, which is due to be completed in 2011. “This instrument will enable accurate, simultaneous and wide-ranging measurements of multiple particles created by the reaction of unstable nuclei in terms of type, energy and movement tracks in all directions. SAMURAI will promote our understanding of the atomic nucleus and help elucidate the origins of elements. It will also contribute to the establishment of a new nuclear theory that could explain the behavior of both stable and unstable nuclei.”

The RIBF is attracting the attention of nuclear researchers around the world, and the number of inquiries from researchers who want to use the RIBF for experiments is increasing rapidly. The team is planning to accept experimental ideas using SAMURAI on an international basis. Of course, Motobayashi also has his own ideas for using the instrument. “You won’t want to miss it,” he says with a smile.

About the Researcher

Tohru Motobayashi

Tohru Motobayashi was born in Tokyo, Japan, in 1949. He graduated from the Faculty Sciences at The University of Tokyo in 1972, and obtained his PhD in 1977 from the same university. After one year of postdoctoral training at RIKEN, he moved to Osaka University as a research associate, where he started his career in experimental nuclear physics. In 1984, he returned to Tokyo as lecturer at Rikkyo University, where he was promoted to associate professor in 1986 and professor in 1994. In 2002, he was appointed as a chief scientist at RIKEN, where he led the Heavy Ion Nuclear Physics Laboratory until March 2010. He is currently RIBF Synergetic Use Coordinator and leader of the SAMURAI Team. His research is on the structure and reaction of nuclei far from the stability line and nuclear phenomena related to nuclear synthesis in the universe. He leads the research field by introducing new experiments using the radioisotope beams at RIKEN’s accelerator facilities.

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