Two new elements have emerged from the rubble of nuclear collisions, a team of Russian and American scientists report. The superheavy elements provide further evidence that an "island" of relatively stable new atomic nuclei lies just beyond experimenters' grasp.
Nuclear physicists and chemists have long known that certain atomic nuclei hold together better than others. The protons and neutrons within a nucleus nestle into series of quantum-mechanical shells, and certain "magic numbers" of protons and neutrons fill the shells and confer additional stability. For example, 82 is a magic number for protons and 126 is a magic number for neutrons; lead, whose nuclei contain 82 protons and typically 125 neutrons, is the heaviest naturally occurring stable element. All heavier elements undergo radioactive decay.
However, nuclear scientists have long suspected that heavier elements exist and theorized an "island of stability" with nuclei containing nearly 114 protons and 184 neutrons. In recent years nuclear physicist Yuri Oganessian of the Joint Institute for Nuclear Research in Dubna, Russia, nuclear chemists Mark Stoyer and Kenton Moody of Lawrence Livermore National Laboratory in California, and their colleagues have succeeded in producing elements 114 and 116 by slamming calcium nuclei into heavier nuclei. Although hardly stable, the new elements decay more slowly than their lighter cousins, suggesting that they occupy the shallows around the island of stability.
Now the team has produced element 115 by blasting calcium-48 nuclei, which have 20 protons and 28 neutrons, into atoms of americium-243, which have 95 protons and 148 neutrons. Last summer, the team pumped nearly 9 billion-billion calcium-48 nuclei into foils containing americium. Out of the collision popped four nuclei of element 115. They decayed inside their detector through a sequence of so-called alpha decays, in which the nucleus spits out a knot of two protons and two neutrons. So along the way, the nuclei produce element 113, which also had not been observed before, the team reports in the 2 February issue of Physical Review C.
The nuclei last only a few hundredths of a second and have no obvious practical use. Their real value comes in helping researchers develop a more complete understanding of the strong force that holds the nucleus together, says Richard Casten, a nuclear physicist at Yale University in New Haven, Connecticut. The new nuclei are also about 11 neutrons short of the magic 184. To reach the island of stability, researchers will need an accelerator that produces beams of rare radioactive nuclei, such as the proposed Rare Isotope Accelerator, says Konrad Gelbke of Michigan State University in East Lansing.