When matter accumulates on the surface of a neutron star, the ultra-dense remnant of a dead star, an explosion occurs, akin to a giant thermonuclear bomb. In this inferno, nuclei of elements not found on Earth are born within seconds. To understand which ones, their masses must be known with great precision. Physicists at RIKEN in Japan used an instrument that, like a very precise scale, catches and weighs individual nuclei as they fly through a magnetic field.
They measured, for the first time with high precision, the masses of three rare nuclei: rhodium-91, palladium-92, and cadmium-96. The uncertainty was only about 10 kiloelectronvolts, as if you weighed a person and were off by no more than the weight of a single eyelash. Plugging the new numbers into computer models of the explosions revealed that the flow of nuclear reactions shifts: more nuclei with mass numbers around 90 are produced, while heavier ones are born less often. This changes our picture of which elements are ejected into space during such explosions.
These measurements are like refining a map for a traveler. Now, models of stellar explosions can more accurately predict how much of each element is produced. This, in turn, helps us understand where many elements in the universe come from, including those in our own bodies.
The mass spectrograph at RIKEN is so sensitive that it can distinguish two nuclei that differ in mass by less than one billionth of a gram.
In science fiction, explosions on neutron stars are sometimes described as sources of rare elements for advanced civilizations — now we know exactly which nuclei are born there.