Dimensions
  • Issue 2  |
  • January 2010

Even Superheavies Need a Little Protection

Superheavies

Superheavy elements synthesized in cold- and
hot-fusion reactions

How heavy can the periodic table go? Scientists have long had the capability to create new, “superheavy” elements by fusing lighter nuclei. The existence of a system formed in a heavy-ion fusion reaction is often brief, however, because the nucleus heats up in the process and must find a way to shed energy and cool back down to a more stable state. Often this is accomplished through fission, where the nucleus simply breaks apart. While much progress has been made in recent years, it’s still a challenge to find the optimal combination of beam and target—as well as the kinematic conditions—to maintain the structural integrity of the superheavies and lead to the formation, at reasonable rates, of new elements. This is due in part to the relationship between fission barriers and excitation energy—the energy difference between the ground and excited state. The conventional wisdom has been that fission barriers, which stabilize the nucleus, tend to disappear at higher excitation energies.

Superheavies

Calculated inverse barrier damping parameter for
even-even superheavy nuclei

The fastest decrease of fission barriers with excitation
energy is predicted for deformed nuclei around N =164
and spherical nuclei around N =184 that are strongly
stabilized by shell effects. On the other hand, for the
transitional nuclei around N=176, synthesized in hot fusion
reactions, the barrier damping is relatively weak.

Using microscopic density functional theory, nuclear theorists from the University of Tennessee/Oak Ridge National Laboratory carried out calculations [1,2] showing that fission barriers of excited superheavy nuclei vary rapidly with particle number, pointing to the importance of shell effects even at large excitation energies. The results are consistent with recent experiments where superheavy elements were created by bombarding an actinide target with 48-calcium; yet even at high excitation energies, sizable fission barriers remained. Not only does this reveal clues about the conditions for creating new elements, it also provides a wider context for understanding other types of fission, such as that used in reactors to provide energy.

[1]   “Fission barriers of compound superheavy nuclei,”J.C.Pei, W.Nazarewicz, J.A. Sheikh,  and A.K. Kerman, Phys. Rev. Lett. 102, 192501 (2009).

[2]   “Systematic study of fission barriers of excited superheavy nuclei,” J.A. Sheikh, W. Nazarewicz, and J.C. Pei, Phys. Rev. C 80, 011302(R) (2009).

This work was supported in part by the National Nuclear Security Administration under the Stewardship Science Academic Alliances program through Grant DE-FG03-03NA00083 and by the US Department of Energy, Nuclear Physics, under Contract Nos.  DE-FG02-96ER40963 (University of Tennessee), DE-AC05-00OR22725 with UT-Battelle, LLC (Oak Ridge National Laboratory), and DE-FC02-07ER41457 (UNEDF SciDAC Collaboration).