Element 115 Has Been Discovered

Element 115 has been discovered at the Joint Institute for Nuclear Research (JINR) in Dubna, Russia. JINR physicists and their longtime collaborators from Lawrence Livermore National Lab in the US produced 4 atoms of the new superheavy element by striking a target of americium-243 atoms with a beam of calcium-48 ions. The beam energy used, 248 MeV, was chosen to produce just the right energy conditions for making the amalgamated nucleus but not causing it to break up, at least not right away.

The long lifetime observed for element 115 suggests that physicists might be getting closer to the "island of stability," the presumed region on the chart of possible nuclear isotopes for which certain combinations of protons and neutrons (collectively known as nucleons, the regular constituents of all nuclei) are much more stable than some of the other heavy nuclei made artificially at accelerators.

In general, nature doesn't produce elements heavier than uranium (element 92) and scientists must resort to colliding smaller nuclei to build up heavyweight elements. In previous experiments conducted by the same team at Dubna, evidence has been recorded for elements 114 and 116.

One sequential decay event corresponding to element 118 was also seen. (Claims for a separate discovery of element 118 by a group at the Lawrence Berkeley National Lab in the US were later withdrawn.)

In the new experiment, using the same approach, a beam of calcium-48 atoms (atomic number, or Z, equal to 20) was plowed into a target of americium-243 atoms (Z=95). By bringing together element 95 with element 20, four atoms of element 115 were created.

The nuclei of these precious atoms apparently lived for 90 msec. They expired in the following way: by decaying first to element 113 by the emission of an alpha particle (a nuclear morsel consisting of two protons and two neutrons); thence to element 111 by alpha emission again; and then by three more alpha decay steps to element 105 ("Dubnium") which, after the delay of a whole day (almost an eternity in nuclear physics) from the time of the original interaction, finally fissioned.

Besides being a very difficult physics experiment to carry out, this work represents a great feat of nuclear chemistry, since it entailed sifting 4 atoms out of trillions of candidates. In other words, the gas-filled separator, employing chemistry, proved to be just as important as the accelerator.

In the past decade or so even-Z superheavy nuclei---112, 114, 116, 118---were sought at Dubna chiefly because of the facility's intense beams of Ca-48 and the ready availability of even-Z actinide targets.

By the way, this experiment also marks the discovery of a second element, 113, which had not been seen before either. (Oganessian et al., Physical Review C, February 2004; contact Yuri Oganessian at JINR, oganessian@flnr.jinr.ru, 011-7-09621-62151; Ken Moody at Livermore, 925-423-4585, moody3@llnl.gov, or Mark Stoyer, mastoyer@llnl.gov, cell phone 301-661-1169; background article by Oganessian in Scientific American, Jan 2000.)