Evidence For New Nuclear Magic Number

Researchers from RIKEN and the University of Tokyo have provided evidence for a new nuclear magic number in the unstable and radioactive calcium isotope 54Ca.

AsianScientist (Oct. 25, 2013) – Researchers from RIKEN and the University of Tokyo have come one step closer to understanding unstable atomic nuclei.

The study, published in the journal Nature, provides evidence for a new nuclear magic number in the unstable and radioactive calcium isotope 54Ca. Together with Japanese and Italian colleagues, they show that the nucleus of 54Ca has 34 neutrons (N), where N=34 is a magic number.

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.

Led by David Steppenbeck from the Center for Nuclear Study at 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 with help from the Radioactive Isotope Beam Factory (RIBF) at RIKEN.

In their experiment, a radioactive beam, composed of scandium-55 and titanium-56 nuclei traveling at around 60 percent of the speed of light, was selected and purified by the BigRIPS fragment separator. 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,” said 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 said.

The article can be found at: Steppenbeck D et al. (2013) Evidence For A New Nuclear ‘Magic Number’ From The Level Structure Of 54Ca.

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Source: RIKEN.
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