A Toroidal Mode in an Excited Nucleus
If a nucleus gets hit by a particle of the right energy, it can set the nucleus’s protons and neutrons sloshing past each other. The dominant vibrational modes are known as giant E1 resonances, but many other modes are possible, among them the toroidal dipole resonance (TDR). Predicted 50 years ago, the TDR involves protons and neutrons oscillating along nested loops that form a toroid inside the nucleus. It has yet to be unambiguously detected. But now Peter von Neumann-Cosel of the Technical University of Darmstadt in Germany and his collaborators have developed a theoretical description of the TDR whose predictions match the results of scattering experiments with nickel-58 nuclei [1].
In general, the interaction probability (cross section) of charged particles that electromagnetically scatter off nuclei has two components: a “longitudinal” one, which originates in the distribution of charge in the nucleus, and a “transverse” one, which originates in the current distribution. If an electron excites a TDR, the jiggling back and forth of protons causes the electron’s scattering cross section to have a transverse component. But such transverse components are not engendered exclusively by TDRs. To be sure of seeing a TDR, Neumann-Cosel and his collaborators realized that their theoretical description had to predict the scattering cross sections not only of electrons but of protons and gamma rays.
The researchers compared predictions made using their theory—a variant of density-functional theory—to electron, proton, and gamma-ray data from three separate experiments. So far, these experiments have examined only one nucleus, nickel-58, with the required resolution and range of scatterers, and the data were consistent with the theory. Measurements of more nuclei—notably, heavy ones—will be needed before declaring that the long-sought TDR has been found.
–Charles Day
Charles Day is a Senior Editor for Physics Magazine.
References
- P. von Neumann-Cosel et al., “Candidate toroidal electric dipole mode in the spherical nucleus 58Ni,” Phys. Rev. Lett. 133, 232502 (2024).