Nuclear Physics: The Heaviest Atom

Near the beginning of time, the universe almost certainly contained many elements heavier than uranium, the heaviest element that exists naturally on earth. Gradually these "transuranium" elements disappeared, decomposing by radioactive decay into lighter and more stable elements. During the past few decades, however, at least eleven transuranium elements and their isotopes have reappeared, thanks to the ingenuity of man. In their latest atomic synthesis, nuclear physicists have produced the heaviest atom known to man, a new isotope of the element mendelevium, which itself was first artificially created in 1965.

As is often the case in science, researchers at the University of California's Lawrence Radiation Laboratory were attempting to synthesize an entirely different isotope when mendelevium 258 was created. A team led by Nuclear Chemist E. Kenneth Hulet was using the laboratory's heavy ion linear accelerator to bombard a tiny amount of einsteinium (a transuranium element discovered in 1952) with alpha particles which consist of two protons and two neutrons. "We expected the alpha particles to join with the heavier isotope of einsteinium," says Hulet, "and then decay by a process called 'electron capture' to fermium 258."

Delightful Half Life. Because the most recently discovered transuranium elements decompose quickly, the scientists hurriedly analyzed the einsteinium target after the bombardment. To their surprise, they discovered a minute amount—fewer than 30,000 atoms—of a mysterious and heavy isotope, which they later identified as mendelevium 258. Even stranger, the isotope—unlike many of its transuranium counterparts —appeared to be in no rush to disappear. The California scientists eventually determined that its half life (the time in which half the atoms of an element decay) was nearly two months. This compared, for example, with only eight seconds for lawrencium 257, until now the heaviest of the known atoms.

No one yet understands why mendelevium 258 is so long-lived. "It's possible," speculates Hulet, "that because of the structure of the nucleus, certain kinds of decay are hindered." Whatever the reason, scientists are delighted. The long half life will enable them eventually to accumulate more substantial amounts of the new isotope and to study its properties at leisure. Even more important, mendelevium stays around long enough to make a good target for high-velocity particle accelerators. And it is by the bombardment of uranium and transuranium elements that even heavier elements and their isotopes have been created.

Hulet hopes to continue creating and identifying ever-heavier atoms. "We want to investigate the very limits of matter," he says. Much more than mere scientific curiosity could be involved. It was in an attempt to create transuranium elements that scientists first bombarded a rare isotope, uranium 235, with slow neutrons. Investigating the strange reaction that resulted, they discovered nuclear fission.