Researchers have come one step closer to understanding unstable atomic nuclei. A team of researchers from RIKEN, the University of Tokyo and other institutions in Japan and Italy has provided evidence for a new nuclear magic number in the unstable, radioactive calcium isotope 54Ca. In a study published today in the journal Nature, they show that 54Ca is the first known nucleus with 34 neutrons (N) where N = 34 is a magic number.
The protons and neutrons inside the atomic nucleus exhibit shell structures in a manner similar to electrons in an atom. For naturally stable nuclei, these nuclear shells fill completely when the number of protons or the number of neutrons is equal to the 'magic' numbers 2, 8, 20, 28, 50, 82 or 126.
However, it has recently been shown that the traditional magic numbers, which were once thought to be robust and common for all nuclei, can in fact change in unstable, radioactive nuclei that have a large imbalance of protons and neutrons.
In the current study led by David Steppenbeck of the Center for Nuclear Study, the University of Tokyo, the team of researchers focused on 54Ca, which has 20 protons and 34 neutrons in its nucleus. They were able to study this nucleus thanks to the Radioactive Isotope Beam Factory (RIBF) at RIKEN, which produces the highest intensity radioactive beams available in the world.
In their experiment, a radioactive beam composed of scandium-55 and titanium-56 nuclei travelling at around 60% of the speed of light, was selected and purified by the BigRIPS fragment separator, part of the RIBF. The radioactive beam was focused on a reaction target made of beryllium. Inside this target, projectile fragmentation of the 55Sc and 56Ti nuclei occurred, creating numerous new radioactive nuclei, some in excited states. The researchers measured the energy of the γ rays emitted from excited states of the radioactive nuclei using an array of 186 detectors surrounding the reaction target.
The results of the experiment indicate that 54Ca's first excited state lies at a relatively high energy, which is characteristic of a large nuclear shell gap, thus indicating that N = 34 in 54Ca is a new magic number, as predicted theoretically by the University of Tokyo group in 2001. By conducting a more detailed comparison to nuclear theory the researchers were able to show that the N = 34 magic number is equally as significant as some other nuclear shell gaps.
"Our new measurement provides key data for the understanding of neutron-rich nuclei and will help pin down the treatment of nuclear forces in systems far from stability," explains David Steppenbeck.
"Enriching our knowledge of the structures of highly unstable nuclei and the nucleon-nucleon forces that drive nuclear shell evolution and the appearance or disappearance of the nuclear magic numbers in radioactive nuclei plays an important role in understanding astrophysical processes such as nucleosynthesis in stars," he adds.
For more information please contact:
D. Steppenbeck et al. "Evidence for a new nuclear 'magic number' from the level structure of 54Ca" Nature, 2013 DOI doi:10.1038/nature12522
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About the Nishina Center for Accelerator-Based Science
The Nishina Center for Accelerator-Based Science is a world-leading accelerator facility, home to the most powerful cyclotron in the world. Using its accelerators, researchers study the structure and properties of different types of unstable nuclei in an attempt to uncover the mystery of the origin of matter and for applications in the life and medical sciences. The center is unique in that it fosters close collaborations between theoretical and experimental scientists, not only in Japan at the Wako campus but also at RIKEN's overseas facilities in the USA and the UK.
About the University of Tokyo
The University of Tokyo was established in 1877 as the first national university in Japan. As a leading research university, the University of Tokyo offers courses in essentially all academic disciplines at both undergraduate and graduate levels and conducts research across the full spectrum of academic activity. The University aims to provide its students with a rich and varied academic environment that ensures opportunities for both intellectual development and the acquisition of professional knowledge and skills.
About the Center for Nuclear Study, the University of Tokyo
The Center for Nuclear Study (CNS) belongs to the Graduate School of Science, the University of Tokyo, which aims to support research and educational programs in nuclear physics. The primary mission of the CNS is the construction and operation of large research facilities, which are essential to nuclear physics. The CNS is therefore located inside RIKEN's Wako campus in order to pursue its major research objectives: heavy-ion science in a strong collaboration with RIKEN Nishina Center for Accelerator-Based Science, by using various beams produced by RIBF accelerators. The CNS has built several detectors and uses them in conjunction with RIBF accelerators. Such research projects are open to the international nuclear science community, being operated jointly by RNC and CNS.
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