Weighing the heaviest elements

Despite the steady influx of elements added to the periodic table in recent years, scientists’ understanding of those newly synthesized atoms is limited. For example, researchers have only been able to estimate, via analyses of decay products, the mass numbers of short-lived superheavy elements with proton number Z > 103. Now, using a novel mass separator, Jacklyn Gates of Lawrence Berkeley National Laboratory and her colleagues have determined the combined proton and neutron counts of isotopes of nihonium and moscovium, the first direct mass measurement of superheavy elements.

Using the 88-inch cyclotron at Berkeley Lab, the researchers fired a beam of calcium-48 ions (Z = 20) onto targets containing americium-243 (Z = 95). Roughly 1 of the 10¹⁸ particles produced per day by the collisions was a fused nucleus of Mc (Z = 115). Magnetic fields in a helium-filled cavity separated out all but the desired ions, which were then slowed down, confined within a 1 mm³ trap, and reaccelerated. The experiment’s final, pivotal component, called FIONA (For the Identification of Nuclide A; pictured above), used perpendicular electric and magnetic fields to send the ions into a looped trajectory before they reached an array of silicon detectors. The ions’ exact trajectory, and thus where they struck the detectors, depended on their mass-to-charge ratio. By analyzing where the ions implanted and measuring subsequent alpha decays, Gates and her colleagues were able to determine that the two decay chains they observed in 30 days of beam time had begun with ²⁸⁸Mc and ²⁸⁴Nh.

The results not only confirm the estimated mass numbers of the Mc and Nh isotopes in previous experiments, but they also kick-start new efforts to precisely measure superheavy elements. By fitting FIONA with photon detectors and other sensors, Gates and her colleagues hope to probe such properties as the shape of nuclei. Other facilities, such as the Joint Institute for Nuclear Research in Russia, are pursuing similar studies (see the article by Yuri Oganessian and Krzysztof Rykaczewski, Physics Today, August 2015, page 32 ). The new research could provide clues as to how many more spots need to be filled on the periodic table before reaching the theorized island of stability, on which certain superheavy nuclei should be long-lived. (J. M. Gates et al., Phys. Rev. Lett., in press .)