Excitation effects on the nuclear-fission process in the heaviest elements

doi:10.1016/0375-9474(85)90082-X

Nuclear Physics A

Volume 436, Issue 3, 8 April 1985, Pages 499-505

https://doi.org/10.1016/0375-9474(85)90082-X Get rights and content

Abstract

Calculations have been carried out of the fission barriers of excited nuclei for elements with Z = 102−108. The effect of nuclear heating is taken into account according to conventional quantum-statistical formulae. A realistic Woods-Saxon potential is employed to calculate single particle states. The- degree of deviation of the fissioning nucleus shape from axial symmetry and changes in the depth of the intermediate minimum are discussed.

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preprint JINR D7-83-644, Dubna

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We present the first attempt of systematically investigating the effects of shell correction energy for a dynamical process, which includes fusion, fusion–fission and quasi-fission processes. In the superheavy mass region, for the fusion process, shell correction energy plays a very important role and enhances the fusion probability when the colliding partner has a strong shell structure. By analyzing the trajectory in three-dimensional coordinate space with the Langevin equation, we reveal the mechanism of the enhancement of the fusion probability caused by ‘cold fusion valleys’. The temperature dependence of shell correction energy is considered.
Nuclear data sheets for A = 248,252,256,260,264*
1999, Nuclear Data Sheets
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It is well known that the heaviest elements of the periodic table have been synthesized in the cold fusion of magic nuclei of Pb with Z ⩽ 26 ions. Because of dynamic limitations for fusion under strong Coulomb interaction of nuclei, the cross-sections of cold fusion reactions diminish exponentially with growing compound nucleus atomic number. For element Z = 110 produced in the reaction ²⁰⁸Pb(⁶²Ni,n)²⁷¹110, the expected cross-section is 10⁻³⁶ cm². In still more asymmetric reactions, when isotopes of actinoid elements irradiated with relatively light ions (Z ⩽ 12) are used as the target material, the compound nuclei possess an excitation energy of approx. 50 MeV. At this energy the nuclear shell effects are strongly suppressed and, as a result, in the case of hot compound nuclei of transactinoid elements the fission barrier is practically absent. The transition of these nuclei into the ground state depends strongly on the dynamic properties of the system with respect to the fission degree of freedom. Experimental studies were going on in two directions: (1) determination of the fission time by measuring the prefission neutrons (of CfFm nuclei) in a wide interval of excitation energies; (2) direct synthesis of known nuclides with Z = 102–105 in reactions with ions of ²²Ne, ²⁶Mg, ²⁷Al and ³¹P when final nuclei are produced in the ground state after the evaporation of five or six neutrons from the excited compound nuclei (E_x = 50–60 MeV). The dependence of the reaction cross-section (HI, 5–6n) on the atomic number of the compound nucleus in different target—ion combinations points to the possibility of synthesizing new elements in hot fusion reactions. The advantage of these reactions arises from the use of neutron-rich nuclei like ²⁴⁸Cm and ²⁴⁹Cf which allows us to synthesize nuclei close to the deformed shell N = 162, for which a considerable growth of stability against spontaneous fission is predicted. Experimental set-ups and methods of detecting rare events of formation and decay of transactinide nuclei are described.
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The qualitative differences between the experimental mass distributions observed in the collisions ³²S(192 MeV) + ²³⁸U and ⁴⁰Ar(220 MeV) + ²³²Th leading to the same compound system Z_tot = 108 are explained within a simple diffusion model. It is demonstrated that there generally exist two different modes of the mass relaxation process. In the “fast fusion” channel the mass-asymmetry increases rapidly, whereas in the “quasifission” process the system drives slowly towards symmetry. The relative contributions of both channels to the cross section depend sensitively on the position of the injection point in the driving potential.
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^∗: On leave of absence from The Institute of Physics, the M. Curie-Sklodowska University, Lublin, Poland.

^∗∗: On leave of absence from The Institute of Physics, Technical University, Warsaw, Poland.

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Excitation effects on the nuclear-fission process in the heaviest elements

Abstract

Nucl. Phys.

Nucl. Phys.

Nucl. Phys.

Nucl. Phys.

Phys. Lett.

Nucl. Phys.

Nucl. Phys.

Nucl. Phys.

Ann. of Phys.

Nucl. Phys.

preprint JINR D7-83-644, Dubna