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A team working at ISOLDE has now achieved this feat by producing thorium-229 nuclei in the isomeric state in a novel way and investigating the nuclei using a technique called vacuum-ultraviolet spectroscopy. The wavelength of the observed light corresponds to an isomer's energy of 8.338 electronvolts (eV) with an uncertainty of 0.024 eV - a value that is seven times more precise than the previous most precise measurements. Significant to the team's success was the production of isomeric thorium-229 nuclei via the so-called beta decay of actinium-229 isotopes, which were made at ISOLDE and incorporated in calcium fluoride or magnesium fluoride crystals. "ISOLDE is currently one of only two facilities in the world that can produce actinium-229 isotopes," says the main author of the paper, Sandro Kraemer. "By incorporating these isotopes in calcium fluoride or magnesium fluoride crystals, we produced many more isomeric thorium-229 nuclei and increased our chances of observing their rad