Some nuclei exhibit added stability that is attributed to the shell-like structure of the nucleus. Protons and neutrons occupy distinct energy levels, and large gaps between adjacent levels lead to extra binding and pronounced stability. The most extreme examples of such extra binding are the so-called doubly-magic nuclei, where the number of protons and neutrons are any combination of the magic numbers 2, 8, 20, 28, 50, 82, or 126. Nickel-56 is such a doubly-magic nucleus where both the proton and neutron number is 28. However, it is a radioactive nucleus (it decays by positron emission) and its distance from the valley of stable nuclei may challenge the shell stabilization of this nucleus. Several experimental tests have provided differing interpretations on the degree of magicity of Nickel-56. To help resolve the open questions regarding the behavior of Nickel-56, we have measured the magnetic dipole moment of Copper-57. In a simplified picture where the shell-stabilized structure of Nickel-56 is correct, Copper-57 can be represented as a single proton outside of the doubly-magic Nickel-56 core, as illustrated in the left side of the figure. A measurement of the magnetic properties of Copper-57 is one of the most sensitive tests to confirm the shell-like structure and the magicity of Nickel-56.
The magnetic dipole moment of Copper-57 was deduced by application of the Nuclear Magnetic Resonance technique, which is a similar technique to Magnetic Resonance Imaging used in the hospitals for functional diagnostics, but improves on sensitivity by a million*billion times using radiation-detection methods. In order for the sensitive test to be feasible, spatially aligned Copper-57 atoms were produced by colliding Nickel-58 ions from the coupled cyclotrons into a thick beryllium target. The A1900 fragment separator was used to isolate the desired Copper-57 ions from other reaction products before delivery to the Nuclear Magnetic Resonance apparatus.
The magnetic dipole moment of Copper-57 was found to be 2.0 nuclear magnetons, nearly 2 times smaller than the prediction of 3.79 nuclear magnetons that assumes an inert Nickel-56 core coupled to a single proton as presented in the left side of the figure. The magnetic dipole moment of Copper-57 is better explained by assuming that Nickel-56 is not a doubly-magic nucleus, as pictured on the right side of the figure. We have concluded that a significant breaking of the nuclear shell-like structure has been observed at Nickel-56.
Reference K. Minamisono et al., Phys. Rev. Lett., accepted for publication
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