Elements with more than 100 protons are made exclusively by combining the nuclei of lighter elements. The heaviest elements are typically created by smashing calcium-48 atoms accelerated to 10% the speed of light into a target of one of the actinides. The nuclei collide with high enough energies that they can overcome the electromagnetic repulsion between them and fuse.
But oganesson, with 118 protons, seems to be the limit for fusion involving 48Ca. To make an element with 119 or 120 protons, researchers would have to combine Ca and its 20 protons with einsteinium or fermium, respectively, elements that are difficult to work with and cannot yet be produced in large quantities. Now Jacklyn Gates of Lawrence Berkeley National Laboratory and her colleagues have demonstrated that the use of titanium, which has 22 protons, provides a potential route toward adding new elements to the periodic table. After adjusting the experimental design used for Ca, the team created the superheavy, albeit already known, element livermorium.
A primary reason 48Ca has proven to be such a useful tool for element makers is because it has so-called magic numbers of neutrons and protons. Theoretical work had indicated that although the titanium-50 nucleus isn’t as strongly bound as 48Ca, it would be the next best progenitor of superheavy elements. As the group found out, the looser nucleus wasn’t prohibitive; the real challenge was in the engineering. The beam setup needed to be redesigned because Ti has a melting point twice as high as Ca’s, but the surrounding magnets still need to be kept cool.
Rather than jumping right into trying to create a new element, the team started with a known one. The researchers produced one atom of Lv, element 116, approximately every 10 days, versus a production rate of one atom every few days in previous 48Ca experiments.
Creating elements with 119 or 120 protons is especially enticing because those nuclei are thought to be near the island of stability, where specific combinations of protons and neutrons may last a full second without decaying; the half-life of Og’s most stable isotope is about 1 ms. (See the article by Yuri Ts. Oganessian and Krzysztof P. Rykaczewski, Physics Today, August 2015, page 32.) Gates and colleagues are working to overcome additional engineering hurdles before they attempt to blast Ti at californium and try for element 120. (J. M. Gates et al., Phys. Rev. Lett. 133, 172502, 2024.)
