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 elementyet discovered--with atomic number Z = 118. Despite the name, the island’s denizens would not be truly stable, just significantlylonger‑lived than their offshore neighbors. But until now the masses, andtherefore the binding energies, of transuranics have been determined onlyindirectly, by measuring the energies of αparticles released in long α‑decaychains down to nuclei of well‑measured mass. Such indirect determinations cansuffer from significant uncertainties and limitations. Now however, MichaelBlock and coworkers at the GSI Helmholtz Center for Heavy Ion Research inDarmstadt, Germany, have reported the first direct measurement of transuranicmasses. The team measured the masses of three short‑lived isotopes of nobelium(Z = 102) created by the fusion ofcalcium ions from GSI’s heavy‑ion accelerator with target lead nuclei. Theleast stable of the three has a half‑life of two seconds. Their masses weremeasured to within a few parts in 10⁸ in a precision Penning trap,an electromagnetic device that confines charged particles in a small cavity anddetermines their masses by measuring their cyclotron‑orbit frequencies in thetrap’s strong magnetic field. What makes the technique particularly challengingfor those transuranics whose creation requires fusion is primarily their painfullyslow production rates. (M.Block et al., Nature 463, 785, 2010 .) --Bertram Schwarzschild