Bulletin of the American Physical Society
APS April Meeting 2010
Volume 55, Number 1
Saturday–Tuesday, February 13–16, 2010; Washington, DC
Session S2: Fusion, Fission and Super Heavy Element Production |
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Sponsoring Units: DNP Chair: Daniel Shapira, Oak Ridge National Laboratory Room: Thurgood Marshall East |
Monday, February 15, 2010 3:30PM - 4:06PM |
S2.00001: Status of the search for the heaviest elements Invited Speaker: The heavy element group at Lawrence Livermore National Laboratory (LLNL) has had a long tradition of nuclear and radiochemistry dating back to the 1950's. Some of the most exciting work has taken place in the last decade in collaboration with the Flerov Laboratory of Nuclear Reactions in Dubna, Russia, with the discovery of five new elements - 113, 114, 115, 116, and 118. By pushing the boundaries of the periodic table, we can start to answer some of the most fundamental questions of nuclear science, such as the locations of the next ``magic numbers'' of protons and neutrons, and the possibility of an ``Island of Stability'' where nuclides would have lifetimes much longer than those currently observed in the heaviest elements. We have already seen evidence of extra-stability in the heaviest nuclides, which leads to half-lives that are long enough for us to perform chemistry on these isotopes one atom at a time. In this presentation, recent results and future directions of heavy element science will be presented. [Preview Abstract] |
Monday, February 15, 2010 4:06PM - 4:42PM |
S2.00002: Fission Barriers of Compound Superheavy Nuclei Invited Speaker: The dependence of fission barriers on the excitation energy of the compound nucleus impacts the survival probability of superheavy nuclei synthesized in heavy-ion fusion reactions. In this work [1,2], we investigate the isentropic fission barriers by means of the self-consistent nuclear density functional theory. The relationship between isothermal and isentropic descriptions is demonstrated. Calculations have been carried out for $^{264}$Fm, $^{272}$Ds, $^{278}$Cp, $^{292}$114, and $^{312}$124. For nuclei around $^{278}$Cp produced in ``cold fusion" reactions, we predict a more rapid decrease of fission barriers with excitation energy as compared to the nuclei around $^{292}$114 synthesized in ``hot fusion'' experiments. This is explained in terms of the difference between the ground-state and saddle-point temperatures. \\[4pt] [1] J.C. Pei, W. Nazarewicz, J.A. Sheikh and A.K. Kerman, Phys. Rev. Lett. {\bf 102}, 192501 (2009).\\[0pt] [2] J.A. Sheikh, W. Nazarewicz, and J.C. Pei, Phys. Rev. C {\bf 80}, 011302(R) (2009). [Preview Abstract] |
Monday, February 15, 2010 4:42PM - 5:18PM |
S2.00003: Hot fusion or cold fusion, best route to the SHEs? Invited Speaker: Elements 102-113 have been synthesized using cold fusion reactions (Pb or Bi target nuclei, massive projectiles., E*=13 MeV, high survival probabilities,significant fusion hindrance). The production cross sections decrease with increasing Z$_{CN}$ with a cross section of 27 fb being measured for element 113. Synthesis of elements 102-108 by hot fusion reactions (actinide target nuclei, intermediate mass projectiles, E*=30-50 MeV, low survival probability, small fusion hindrance) shows decreasing production cross sections for Z=102 to Z=108 and then the cross sections level out at a few pb out to Z=118. Upper limit cross sections for the production of Z=120 nuclei in hot fusion reactions are $\sim $ 0.1 pb. How should one go forward to make nuclei with Z $>$ 120 or with large neutron numbers, N $\sim $ 184? The cross section for the production of an evaporation residue, $\sigma _{EVR}$, is $\sigma _{EVR} =\sigma _{CN} W_{sur} $ where $\sigma _{CN}$ is the complete fusion cross section and W$_{sur}$ is the survival probability of the completely fused system. The complete fusion cross section can be written as $\sigma _{CN} =\sum\limits_{J=0}^{J_{\max } } {\sigma _{capture} (E_{c.m.} ,J)P_{CN} (} E_{c.m.} ,J)$ where $\sigma _{capture} (E_{c.m.} ,J)$ is the capture cross section and P$_{CN}$ is the probability that the projectile-target system will evolve inside the fission saddle point to form a completely fused system rather than reseparating (quasifission). I have used this formalism to make estimates of the best reactions to make new heavy nuclei using stable and radioactive beams. I conclude that stable beams offer the best opportunities to make new chemical elements and that radioactive beams offer new opportunities to make nuclei to study the atomic physics and chemistry of the heaviest elements. The radioactive beam reactions involve the light neutron-rich projectiles interacting in hot fusion reactions. If time permits I will also discuss recent experiments to make heavy nuclei using multi-nucleon transfer reactions. [Preview Abstract] |
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