PRC87(2013)057601-ROYER-Superheavynuclei.pdf (original) (raw)

Low energy incomplete fusion and its relevance to the synthesis of super heavy elements

EPJ Web of Conferences, 2015

To study the presence of incomplete fusion at energies around the Coulomb-barrier and to understand its dependence on various entrance-channel parameters, the incomplete fusion fractions have been deduced (i) from excitation function measurements for 18 O, 13,12 C+ 159 Tb, and (ii) from forward recoil range measurements for 12 C+ 159 Tb systems, at low energies (<7MeV/A). The data have been analyzed within the framework of compound nucleus decay, which suggests the production of xn/pxn-channels via complete fusion, as these are found to be well reproduced by PACE4 predictions, while, a significant enhancement in the excitation functions of α-emitting channels has been observed over the theoretical ones, which has been attributed due to the incomplete fusion processes. Further, the incomplete fusion events observed in case of forward recoil ranges have been explained on the basis of the breakup fusion model, where these events may be attributed to the fusion of 8 Be and/or 4 He from 12 C projectile to the target nucleus. For better insight into the underlying dynamics, the deduced fractions of incomplete fusion have been compared with other nearby systems as a function of various entrance channel parameters. The incomplete fusion has been found to be sensitive to the projectile's energy and alpha-Q-value of the projectile.

Entrance Channel Effect on the Formation of Heavy and Superheavy Nuclei

Journal of the Physical Society of Japan, 2003

We study the effect of the entrance channel and the shell structure of reacting massive nuclei on the fusion mechanism and the formation of evaporation residues of heavy and superheavy nuclei. In the framework of the combined dinuclear system concept and advanced statistical model, we analyze the 40 Ar + 176 Hf, 86 Kr + 130 Xe and 124 Sn + 92 Zr reactions leading to 216 Th à ; the 32 S + 182 W, 48 Ti + 166 Er, and 60 Ni + 154 Sm reactions leading to 214 Th à ; the 40 Ar + 181 Ta reaction leading to 221 Pa à ; the 48 Ca + 248 Cm reaction leading to the 296 116 compound nucleus. In our calculations of the excitation functions for capture, fusion and evaporation residues we use the relevant variables such as mass-asymmetry of nuclei in the entrance channel, relative distance between nuclear centers, shell effect and shape of colliding nuclei and such characteristics of the reaction mechanism as potential energy surface, driving potential, the dependence of capture, fusion cross sections and survival probability of compound nucleus on the orbital angular momentum. As a result we obtain a beam energy range for the capture of the nuclei before the system fuses and the À n =À f ratio at each step along the de-excitation cascade of the compound nucleus. Calculations allow us to reach useful conclusions about the mechanism of the fusion-fission process, that is in competition with the quasifission process, and the production of the evaporation residues.

Systematics of Heavy-Ion Fusion Reactions at Extreme Sub-Barrier Energies

Progress of Theoretical Physics Supplement, 2004

A new measurement of fusion reactions down to 10 nb has been performed for the system 64 Ni + 64 Ni. Coupled-channels analyses have been carried out for the systems 60 Ni + 89 Y and 64 Ni + 64 Ni. They demonstrate that coupled-channels calculations are unable to reproduce the unexpected steep falloff of the cross sections at extreme sub-barrier energies. Heavy-ion fusion excitation functions are also analyzed in terms of the S factor, as this offers a pragmatic way to study fusion behavior in the energy regime of interest. It is shown that the steep falloff in cross section observed in several heavy-ion systems translates into a maximum of the S factor. The energies where the maximum occurs can be parameterized with a simple empirical formula. The parameterization, which is derived here for rather stiff heavy-ion systems, provides also an upper limit for reactions involving softer nuclei.

Peculiarities of Nuclear Fusion in Synthesis of Superheavy Elements

Journal of Nuclear and Radiochemical Sciences, 2007

The role of entrance channel for reactions with massive nuclei is discussed by comparison of experimental data of reactions with different mass (charge) asymmetry and leading to different isotopes of the superheavy element Z = 112. Three stages (capture, complete fusion, and formation of evaporation residues) of the so called "cold" and "hot" fusion reactions are considered for 70 Zn + 208 Pb and 48 Ca + 238 U reactions which were used to synthesize superheavy elements Z = 112. It is shown that the collisions by the orientation angles 30° < α P < 60° for projectile and 30° < α T < 45° for the target nucleus lead to the largest cross section in synthesis of superheavy element in the 48 Ca + 238 U reaction at E* CN = 35 MeV.

Heavy element formation in compound nucleus reactions with 238U targets

2007

Cross sections for heavy element formation by compound-nucleus reactions have been measured for reactions with 22 Ne, 23 Na, 26 Mg, 27 Al, 30 Si, 31 P, and 37 Cl projectiles and 238 U targets. Together with previously published results for reactions of 18 O, 19 F, 34 S, 40 Ar, and 48 Ca projectiles with 238 U targets, the systematics of heavy element formation cross sections are analyzed. Conclusions about critical angular momentum for fusion and neutron evaporation in competition with fission during de-excitation are drawn.

Fusion-fission of superheavy compound nuclei produced in reactions with heavy ions beyond Ca

EPJ Web of Conferences, 2011

Total Kinetic Energy -Mass distributions of fission-like fragments for the reactions of 22 Ne, 26 Mg, 36 S, 48 Ca, 58 Fe and 64 Ni ions with actinides leading to the formation of superheavy compound systems with Z=108-120 at energies near the Coulomb barrier have been measured. Fusion-fission cross sections were estimated from the analysis of mass and total kinetic energy distributions. It was found that the fusion probability drops by three orders of magnitude for the formation of the compound nucleus with Z=120 obtained in the reaction 64 Ni+ 238 U compared to the formation of the compound nucleus with Z=112 obtained in the reaction 48 Ca+ 238 U at the excitation energy of the compound nucleus of about 45 MeV. From our analysis it turns out that the reaction 64 Ni+ 238 U is not suitable for the synthesis of element Z=120.

Fusion reactions and synthesis of some superheavy nuclei

NPA 1000 (2020)121811, 2020

The potential barriers standing against the formation of some superheavy nuclei have been studied within a Generalized Liquid Drop Model taking into account the proximity forces acting between surfaces in regard, the charge and mass asymmetry, the shell and pairing effects and an adjustment to reproduce the experimental or predicted Q value. Quasimolecular shapes leading from two spheres to one sphere have been used. In very asymmetric entrance channels one hump potential barrier stands and might lead to very excited nuclei cooling by neutron or α particle evaporation. For less asymmetric reactions double hump potential barriers appear and quasifission or alpha decay of compact and creviced one-body shapes may occur in the external well beside fusion-fission events in the first inner well. For almost symmetric reactions there is no more external barrier but rather a continuous climb till the sphere. Predictions of alpha decay half-lives of the studied superheavy nuclei are given using a recently proposed formula.

Formation of superheavy elements in the capture of very heavy ions at high excitation energies

Physical Review C, 2013

The potential barriers governing the reactions 58 Fe+ 244 Pu, 238 U+ 64 Ni and 238 U+ 72 Ge have been determined from a liquid drop model taking into account the proximity energy, the shell energies, the rotational energy, and the deformation of the incoming nuclei in the quasimolecular shape valley. Double-humped potential barriers appear in these entrance channels. The external saddle-point corresponds to two touching ellipsoidal nuclei when the shell and pairing effects are taken into account while the inner barrier is due to the shell effects at the vicinity of the spherical shape of the composite system. Between them, a large potential pocket exists and persists at very high angular momenta allowing the capture of very heavy ions at high excitation energies.

STUDY OF FUSION REACTIONS INDUCED BY WEAKLY BOUND NUCLEI

Fission and evaporation cross sections were measured in a broad range of energies near the Coulomb barrier and up to 180 MeV in the reactions 46He+U'9Bi and 'Li+2"8Pb. The secondary 4.hHe beams were produced using the beam-transport line of the U400M accelerator at FLNR, JINR. The experimental fusion and fission excitation functions obtained for the different reactions 4He+2"9Bi, 'He+*"'Bi and 7Li+Z'1XPb and leading to the same composite nuclei 2".2'5At were analyzed using the PACE-4 and CC codes from the point of view of the fusion reaction mechanism induced by weakly bound nuclei. The comparison between the fission and fusion excitation functions for the three reactions 46He+211yBi and 7Li+2"8Pb has shown that they are the same within the experimental error for a broad range of energy.

Shell Effects in Fusion–Fission of Heavy and Superheavy Nuclei

2004

The process of fusion-fission of heavy and superheavy nuclei (SHE) with Z = 82-122 formed in the reactions with 48 Ca and 58 Fe ions at energies near and below the Coulomb barrier has been studied. The experiments were carried out at the U-400 accelerator of the Flerov Laboratory of Nuclear Reactions (JINR) and at the XTU Tandem accelerator of the National Laboratory of Legnaro (LNL) using the time-of-flight spectrometer of fission fragments CORSET and the neutron multi-detector DEMON. As a result of the experiments, mass and energy distributions (MED) of fission fragments, fission, quasi-fission and evaporation residues cross sections, multiplicities of neutrons and γ quanta and their dependence on the mechanism of formation and decay of compound systems have been studied.