Physicists have for the first time observed visible light--a soft green glow--coming from the nucleus of an atom. The glow illuminates the atom from inside by exciting the surrounding electron shells. Physicists hope to exploit the phenomenon, reported in the current Physical Review Letters, to probe the interplay between an atom's nucleus and its electrons.
Atoms routinely emit photons of visible light, but they come from electrons jumping between energy levels around the nucleus. The nucleus has its own energy levels, too, which depend on its shape, spin, or vibration. But the forces in the nucleus are so strong that the gaps between energy levels are measured in thousands or millions of electron volts (eV)--thousands of times greater than the gaps between electron levels. As a result, an excited nucleus relaxing into the ground state usually emits invisible, high-energy gamma radiation.
Thorium-229 is an exception. George Irwin, a physicist at Idaho State University in Pocatello, and Kinney Kim, a physicist at North Carolina Central University in Durham, studied thorium's football-shaped nucleus. A combination of its asymmetric shape and the spin of the protons and neutrons in its nucleus creates an excited state a mere 3.5 eV higher than the ground state--a gap so small than when the nucleus relaxes, it emits an ultraviolet photon. "It just so happens that [the states] are close together," says Charles Reich, a physicist who has retired from the Idaho National Engineering and Environmental Laboratory (INEEL) in Idaho Falls. "It's a statistical fluke."
But that fluke has dramatic consequences, Irwin and Kim discovered when they excited thorium nuclei with gamma rays. Ordinarily, the photon emitted by an excited nucleus is so energetic that it kicks an electron right out of the atom. "But in [thorium's] case, the energy is so low that it causes the excitation of outer electrons," says Irwin. When the ultraviolet photon emitted by the nucleus is absorbed by an electron, the electron is promoted to a higher energy level and the excess energy emerges as green light.
"Like most experiments, it needs to be verified, but it's a very interesting phenomenon," says Richard Helmer, a physicist at INEEL. He adds that it may open up "an interesting new energy region to study the interactions between the nucleus and the atom." Reich agrees: "Studying those interactions should provide a lot of information."