Mass Spectrometry Experiments Clinch the Magicity in Scandium

Noble gases, like Helium or Argon, are well known to feature special chemical and physical properties, such as a very low reactivity.  This is because, in atoms, electrons organize themselves in shell-like structures, and noble gases have the exact number of electrons to fill up all their shells. Any additional electron would be less bound to the atom and be more prone to participate in a chemical reaction. In the nucleus of an atom, protons and neutrons also organize themselves in similar structures. "Closed-shell" nuclei present special features that only emerge for specific "magic" numbers of protons or neutrons. They are more compact, harder to excite and, particularly, a tad bit lighter. The formation of such nuclear shells at specific magic numbers of protons or neutrons constitutes the backbone of our understanding of the nucleus. However, scientists are still trying to understand the rules behind when a shell is formed or not.

An emblematic situation occurs among nuclei with 32 and 34 neutrons. In the stable nuclei found on Earth, neither of those neutron numbers show any extraordinary property or signs of magicity. However, in some very rare and neutron-laden nuclei, that can only be forged at accelerator facilities such as the NSCL, these shells seem to make an appearance. Their presence is best spotted in calcium isotopes, where the effects of these emerging neutron shell closures gang up with those of a proton shell closure (the number of protons in calcium, 20, is a well-established magic number). Now, an experimental endeavor involving teams at NSCL and at the TRIUMF Laboratory in Canada spotted the signatures of a 32-neutron shell closure in the masses of scandium isotopes, with just one proton more than in the calcium case, but could not find evidence of a similar shell closure with 34 neutrons.

The experiments were carried out independently in each laboratory, using distinct methods of producing the scandium isotopes, and weighed them through different mass spectrometry techniques.  The two teams were able to provide new values for the masses of 6 neutron-rich isotopes of scandium: from 50Sc to 55Sc, each one with a precision on the order of one part in 100 million. Although the presence of a shell closure with 32 neutrons in scandium isotopes was already known, the high precision obtained enabled a meticulous characterization of these structural phenomena.  Combined, the results of the two experiments complete a detailed map on how the closed shell with 32 neutrons are formed and provide insights on how similar effects could appear with 34 neutrons. These results, jointly published in a recent Physical Review Letters, are of great value for the development of nuclear theories, since the emergence of new shell closures is highly sensitive to the underlying basic interactions between all the constituents of the nucleus.

Caption: the LEBIT Penning Trap Mass Spectrometer (disassembled in the picture) used for the mass measurements performed by the team at NSCL.