than 114 have been seen but none have been independently confirmed.)

The nuclear shell game

Nuclear stability is thought to be based in part on shell structurea model in which protons and neutrons are arranged in increasing energy levels in the atomic nucleus. A nucleus whose outermost shell of either protons or neutrons is filled is said to be "magic" and therefore stable. The possibility of finding "magic" or "doubly magic" isotopes of superheavy elements (with both proton and neutron outer shells completely filled) led to predictions of a region of enhanced stability in the 1960s.

The challenge is to create such isotopes by bombarding target nuclei rich in protons and neutrons with a beam of projectiles having the right number of protons, and also rich in neutrons, to yield aby alpha emission before spontaneous fission interrupted the process.

"This was a very deliberate strategy," says Ellison, "because we hoped to track the isotopes that resulted from subsequent alpha decays farther down into the main body of the chart of nuclides, where the relationships among isotope number, shell structure, and stability are better understood. Through this connection, and by observing the energy of the alpha decays, we could hope to learn something about the accuracy of predictions of the shell structure of the heaviest elements."

The sum of protons and neutrons of 48Ca and 242Pu is 114 protons and 176 neutrons. To make the desired "neutron poor" 285114 nucleus, one having only 171 neutrons, first required a beam of 48Ca projectiles whose energy was carefully adjusted to excite the resulting