Weighing transuranics

The binding energy of a nucleus—almost 1% of its mass—provides important information about its configuration of protons and neutrons. Theorists particularly want to know the binding energies of transuranic nuclear species approaching the so‑called island of stability predicted to lie not far beyond the most massive element yet discovered—with atomic number Z = 118. Despite the name, the island’s denizens would not be truly stable, just significantly longer‑lived than their offshore neighbors. But until now the masses, and therefore the binding energies, of transuranics have been determined only indirectly, by measuring the energies of α particles released in long α‑decay chains down to nuclei of well‑measured mass. Such indirect determinations can suffer from significant uncertainties and limitations. Now however, Michael Block and coworkers at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany, have reported the first direct measurement of transuranic masses. The team measured the masses of three short‑lived isotopes of nobelium (Z = 102) created by the fusion of calcium ions from GSI’s heavy‑ion accelerator with target lead nuclei. The least stable of the three has a half‑life of two seconds. Their masses were measured to within a few parts in 10⁸ in a precision Penning trap, an electromagnetic device that confines charged particles in a small cavity and determines their masses by measuring their cyclotron‑orbit frequencies in the trap’s strong magnetic field. What makes the technique particularly challenging for those transuranics whose creation requires fusion is primarily their painfully slow production rates. (M. Block et al., Nature 463, 785, 2010.) —Bertram Schwarzschild