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April 12, 2013

Silicon’s double magic

The observation of a deformed atomic nucleus for a symmetric isotope of silicon suggests new forces at work

small image of nuclear shapes of silicon isotopes

Figure 1: Nuclear shapes of silicon isotopes. The 42Si nucleus should have a spherical shape similar to that for 36Si, but instead has a pancake shape.

© 2013 Satoshi Takeuchi, RIKEN Nishina Center for Accelerator-Based Science

Silicon is mainly known for its use in the electronics industry, but its study may also reveal new details about the most fundamental forces of nature. Observations by Satoshi Takeuchi and fellow scientists from the RIKEN Nishina Center for Accelerator-Based Science have now shown that the silicon isotope 42Si has a deformed atomic nucleus rather than the expected spherical structure, suggesting the presence of new types of forces in atomic cores1.

The atomic nucleus contains protons and neutrons, where the number of neutrons can vary to give rise to various isotopes of a given element. The nucleus of the 42Si isotope has 14 protons and 28 neutrons—‘magic numbers’ of each resulting in perfectly filled nuclear energy levels or ‘shells’. 

The nuclei of isotopes with magic numbers of protons or neutrons are usually perfectly spherical. Yet this is not always the case. “Forces may exist in the nucleus that break the shell stability caused by the magic number,” says Takeuchi. The deformation caused by this breakdown of shell stability has been seen before for isotopes with a large proton–neutron imbalance. However, the observation of deformation in 42Si, with magic numbers in both protons and neutrons, is particularly significant and is expected to help scientists to understand the cause of these deformations.

The experiments on 42Si were only possible because of the facilities available to the RIKEN research team—the Radioactive Isotope Beam Factory (RIBF) for the production of a beam of 42Si isotopes, and the DALI2 gamma-ray detector for efficient study of nuclear states. “We could not perform such experiments previously because they would have taken 100 or 1,000 times longer, and no other group in the world would conduct such a study,” explains Takeuchi.

The results of the experiments suggest that 42Si has a pancake-shaped nucleus (Fig. 1). This deformation differs from that for other isotopes, hinting at the involvement of a different deformation mechanism. Takuchi’s team is already planning further experiments to investigate what such a mechanism might be. “We are going to study isotopes such as nuclei around regions of 78Ni and 132Sn, which have magic numbers similar to 42Si,” says Takeuchi. “To look for isotopes with unexpected stability2, such as 24O, could also be of interest.” Clarification of nuclear deformation is expected to expand our understanding of fundamental physical processes such as the evolution of stars and the formation of chemical elements in the Universe. 

References

  1. Takeuchi, S., Matsushita, M., Aoi, N., Doornenbal, P., Li, K., Motobayashi, T., Scheit, H., Steppenbeck, D., Wang, H., Baba, H. et al. Well developed deformation in 42Si. Physical Review Letters 109, 182501 (2012). doi: 10.1103/PhysRevLett.109.182501 (Link)
  2. Tshoo, K., Satou, Y., Bhang, H., Choi, S., Nakamura, T., Kondo, Y., Deguchi, S., Kawada, Y., Kobayashi, N., Nakayama Y. et al. N = 16 spherical shell closure in 24O. Physical Review Letters 109, 022501 (2012). doi: 10.1103/PhysRevLett.109.022501 (Link)
  3. Heavy oxygen holds it together. RIKEN Research December 2012  (Link)