Much like an atom’s electrons, an atom’s nucleus can exist in ground and excited states. It can also emit photons—typically at gamma-ray wavelengths—as the nucleus transitions between those states. The most easily observed of these transitions, which are classified by their “multipolarity,” are the low-order ones (dipolar and quadrupolar), which can be modeled as the emission of oscillating dipoles or quadrupoles. As the order increases, the transitions become less probable—and their names more intricate. Previously, the fifth-order transition (known as triacontadipole) was the highest-order transition observed. Now Alan John (AJ) Mitchell of the Australian National University and colleagues have provided conclusive evidence for the sixth-order transition (hexacontatetrapole) [1].
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