Isotope Dependence of Superheavy Nucleus Formation Cross Section

The dynamical process in the superheavy nucleus synthesis is studied on the basis of the two-dimensional Smoluchowski equation. Special attention is paid to the isotope dependence of the cross section for the superheavy nucleus formation by means of making a comparison among the reaction systems of ^54Re＋204pb, ^56Re ＋206Pb, and ^58Fe＋^208Pb. It is found by this comparison that the formation cross section is very sensitive to the conditional saddle-point height and the neutron separation energy of the compound nucleus. Reaction systems with lower height of conditional saddle-point and smaller neutron separation energy are more favourable for the synthesis of the superheavy nucleus.     

Study of the fusion-fission process in the 35Cl +24Mg reaction

Fusion-fission and fully energy-damped binary processes of the ³⁵Cl+²⁴Mg reaction were investigated using particle-particle coincidence techniques at a ³⁵Cl bombarding energy of E_lab≈ 8 MeV/nucleon. Inclusive data were also taken in order to determine the partial wave distribution of the fusion process. The fragment-fragment correlation data show that the majority of events arises from a binary-decay process with a relatively large multiplicity of secondary light-charged particles emitted by the two primary excited fragments in the exit channel. No evidence is observed for ternary-breakup processes, as expected from the systematics recently established for incident energies below 15 MeV/nucleon and for a large number of reactions. The binary-process results are compared with predictions of statistical-model calculations. The calculations were performed using the Hauser-Feshbach method, based on the available phase space at the scission point of the compound nucleus. This new method uses temperature-dependent level densities and its predictions are in good agreement with the presented experimental data, thus consistent with the fusion-fission origin of the binary fully-damped yields. Received: 12 March 1998     

Variation of total kinetic energy and mass distributions in the fusion-fission reaction of 16O,19F + 209Bi at near barrier energies

Fission fragment mass and energy distributions and their correlations have been measured for the ¹⁶O and ¹⁹F + ²⁰⁹Bi reactions over a wide range of excitation energies ( E ^* = 30-50 MeV). It is observed that in the case of ¹⁶O + ²⁰⁹Bi reaction, the average total fragment kinetic energy, <TKE> is nearly independent of the bombarding energy. However, in the case of ¹⁹F + ²⁰⁹Bi reaction, the average total kinetic energy of the fission fragments shows a peaking behaviour near the barrier. The variation in <TKE> at near barrier energies in the ¹⁹F + ²⁰⁹Bi system seems to be correlated with corresponding strong variation in the variance of the fragment mass distribution. The present results may imply certain dynamical effects leading to compact scission configurations in the fission of ¹⁹F + ²⁰⁹Bi system at near barrier bombarding energies. Received: 9 April 2001 / Accepted: 26 May 2001     

Synthesis of superheavy nuclei in the reactions of 244Pu and 248Cm with 48Ca

This paper presents results of the experiments aimed at producing long-lived superheavy elements located near the spherical-shell closures with Z ⩾ 114 and N ⩾ 172 in the ²⁴⁴Pu + ⁴⁸Ca and ²⁴⁸Cm + ⁴⁸Ca reactions. The large measured α-particle energies of the newly observed nuclei, together with the long decay times and spontaneous fission terminating the chains, offer evidence of the decay of nuclei with high atomic numbers. The decay properties of the synthesized nuclei are consistent with the consecutive α-decays originating from the parent nuclides ^{288, 289}114 and ²⁹²116, produced in the 3n and 4n evaporation channels with cross-sections of about a picobarn. The present observations can be considered as experimental evidence of the existence of the “island of stability” of superheavy elements. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: utyonkov@sungns.jinr.ru     

Effect of shell structure on energy dissipation in heavy-ion collisions

The effect of shell structure on the distribution of the excitation energy between fragments of the deep inelastic collisions is analysed in the microscopic approach. It is shown that the density of the single-particle levels of the proton and neutron subsystems near the Fermi surface determines the ratio between the excitation energies of fragments at the initial stage of the collision. It is shown also that the shell structure strongly influences the correlations between the width of the charge distributions and the total kinetic energy losses. Calculations are performed for the ^40,48Ca+²⁴⁸Cm reactions. The results obtained suggest a possible interpretation for the observed concentration of the excitation energy in the light fragment in deep inelastic collisions for a wide range of the total kinetic energy losses. Received: 27 July 1999 / Accepted: 29 March 2000     

Weak e+e− lines from internal pair conversion observed in collisions of 238U with heavy nuclei

We present the results of a Doppler-shift correction to the measured e⁺e⁻–sum-energy spectra obtained from e⁺e⁻–coincidence measurements in ²³⁸U +²⁰⁶Pb and ²³⁸U +¹⁸¹Ta collisions at beam energies close to the Coulomb barrier, using an improved experimental setup at the double-Orange spectrometer of GSI. Internal-Pair-Conversion (IPC) e⁺e⁻ pairs from discrete nuclear transitions of a moving emitter have been observed following Coulomb excitation of the 1.844 MeV (E1) transition in ²⁰⁶Pb and neutron transfer to the 1.770 MeV (M1) transition in ²⁰⁷Pb. In the collision system ²³⁸U +¹⁸¹Ta, IPC transitions were observed from the Ta-like as well as from the U-like nuclei. In all systems the Doppler-shift corrected e⁺e⁻–sum-energy spectra show weak lines at the energies expected from the corresponding γ–ray spectra with cross sections being consistent with the measured excitation cross sections of the γ lines and the theoretically predicted IPC coefficients. No other than IPC e⁺e⁻–sum-energy lines were found in the measured spectra. The transfer cross sections show a strong dependence on the distance of closest approach (R_min), thus signaling also a strong dependence on the bombarding energy close to the Coulomb barrier. Received: 22 July 1997 / Revised version: 15 October 1997     

Production of medium-weight isotopes by fragmentation in 750 A MeV 238U on 208Pb collisions

Projectile fragmentation of ²³⁸U in a lead target was investigated at a bombarding energy of 750 A MeV. Isotopic production cross sections of about 250 different projectile fragments in the element range Z= 30–53 were measured with the FRagment Separator (FRS). The magnetic selection and the kinematical analysis of the measured isotopes allowed to disentangle fission and fragmentation residues. The mass loss of these residues indicates a violent collision where a large amount of energy is dissipated. The position of the fragmentation corridor defined by the measured residues was used to determine an effective proton-evaporation barrier. Received: 3 October 1997 / Revised version: 27 February 1998     

Prompt dipole γ-ray emission in fusionlike heavy-ion reactions

The ³²S + ¹⁰⁰Mo and ³⁶S + ⁹⁶Mo fusionlike reactions were studied at incident energy of E _lab = 298 MeV and 320 MeV, respectively, with the aim of probing the influence of the entrance channel charge asymmetry on the dipole γ-ray emission. The excitation energy and spin distribution of the compound nucleus created in these reactions were identical, the only difference being associated with the unequal charge asymmetry of the two entrance channels. High-energy γ-rays were detected in an array of 9 seven-pack BaF₂ clusters. Coincidence with fusionlike residues detected in four PPAC ensured the selection of central reaction events. By studying the differential γ-ray multiplicity associated with the two reactions it was shown that the dipole strength excited in the compound nucleus increases with the entrance channel charge asymmetry. From the linearized spectra, the increase of the GDR γ-ray intensity was found to be ∼ 25% for the more charge asymmetric system. The results are discussed and compared with those of previous data obtained at different incident energies. Received: 21 October 2002 / Accepted: 23 December 2002 / Published online: 1 April 2003     

Synthesis and decay properties of superheavy atoms in nuclear reactions induced by stable and radioactive ion beams

This talk consists of two parts. The first one presents the results of investigations performed in 1998-2000 in Dubna on the synthesis of superheavy nuclei in reactions induced by ⁴⁸Ca ions. The radioactive decay properties of the nuclei, indicating a considerable increase in the α-decay and spontaneous fission half-lives of the isotopes of elements 110-116 when approaching the closed neutron shell at N = 184, are given. In the second part the possible ways of advancing into the region of more neutron-rich nuclei, using stable and radioactive ion beams, are discussed. Since so far no intense radioactive ion beams are available, some experiments with stable beams are considered as a test for the suggested ideas. Received: 1 May 2001 / Accepted: 4 December 2001     

Incomplete fusion reactions: Analysis of excitation functions and recoil range distributions in 16O + 51V

Incomplete fusion reactions were investigated by measuring the excitation functions of nine evaporation residues in ¹⁶O + ⁵¹V reaction in the beam energy 4-6 MeV/amu, using the well-known recoil catcher technique and gamma-ray spectrometry. The experimental data were compared with that obtained from Monte Carlo simulation calculations using the PACE2 code. The results indicate the presence of incomplete fusion process in the production of two alpha emission products. This was further confirmed by the measurement of recoil range distribution of these isotopes at 96 MeV beam energy. Calculations of the average angular momentum associated with these products revealed the peripheral nature of these ICF reactions. Received: 20 June 2001 / Accepted: 11 September 2001     

Measurement of fusion excitation functions of 27, 29, 31Al + 197Au

A systematic study of the sub-barrier fusion reactions with neutron-rich projectiles has been carried out for three isotopes ^27,29,31Al bombarding a ¹⁹⁷Au target. A target chamber equipped with a target stack and sets of MWPC was employed in order to enhance the efficiency of the radioactive beam experiment. Coupled-channel calculations including the quadrupole excitations do not well fit the measured fusion excitation functions, whereas flat barrier distributions to represent the coupling to the neutron transfer largely account for the observed enhancement of the sub-barrier fusion cross-sections. Received: 13 March 2001 / Accepted: 27 April 2001     

Peculiarities in fragment mass distribution in the 238U + 40Ar (243 MeV) reaction

A pronounced fine structure (FS) in the form of distinct peaks was observed in neutron gated mass spectra from the decay of the ²⁷⁸110 composite system produced in the reaction ²³⁸U + ⁴⁰Ar (243 MeV) at an initial excitation energy E ^* > 70 MeV. The FS peaks are located in the vicinity of mass numbers 70-80, 100, and 130, which correspond to those of magic nuclei (clusters). In the data there is also evidence for a new type of decay -- collinear cluster tripartition of an excited nucleus. Received: 8 August 2000 / Accepted: 2 February 2001     

Modified Glauber model for the total reaction cross-section of 12C + 12C collisions

Glauber's theory has been adopted to calculate the total heavy-ion reaction cross-sections at high energies. At relatively low energies, Glauber's total reaction cross-section has been modified in order to take into account the Coulomb field effect and is called modified Glauber model I. In addition to the Coulomb field effect, the nuclear effect has also been taken into account in the Glauber model and is called modified Glauber model II. An analytical expression for the transparency function for heavy-ion reactions, involving the nuclear densities of the colliding ions and the nucleon-nucleon cross- section, has been obtained within the framework of the modified Glauber models I and II. The transparency and the total reaction cross-sections of the ¹²C + ¹²C collisions are calculated at different bombarding energies. The obtained results are in good agreement with the experimental data and with previous theoretical calculations. Received: 26 January 2001 / Accepted: 15 October 2001     

On the quest of production of superheavy nuclei in reactions of 48Ca with the heaviest cctinide targets

The sequence of radioactive decays of an unknown isotope produced in a rare fusion reaction to known lighter isotopes is used to identify mass and atomic number of the mother isotope, which has been separated before from the bulk of other reaction products by an in-flight recoil separator. By this technique the elements 107 to 112 were produced by single atom decay-chain analysis. Such a correlation technique reaches its limit by the occurrence of accidental sequences and it collapses beyond a maximum possible correlation time, at which a true event cannot be distinguished anymore from a random event. ⁴⁸Ca-induced fusion reactions with actinides are discussed. In 1983 at GSI, Darmstadt and LBL, Berkeley, ⁴⁸Ca/²⁴⁸Cm-experiments (II) were performed, which are compared to recent ⁴⁸Ca-experiments at FLNR-Dubna (I) irradiating ²⁴⁴Pu, ²⁴²Pu, and ²³⁸U. In these experiments production of isotopes of superheavy elements 112 and 114 is claimed. Our analysis of accidental sequences in ⁴⁸Ca-induced reactions is presented, which is at variance with the published analysis from FLNR-Dubna. We find that the maximum correlation time using continuous beams at today existing separation systems is not in the one-hour regime, but in the few-minute regime. The five spontaneous fission events observed in the FLNR experiments are preceded by signals in the (1–16)-minute range. These times are shown to be longer than the maximum possible correlation times. The preceding signals are decoupled from the spontaneous fission signal and carry no information on the spontaneous fission events observed. Moreover, random probabilities of 0.2 to 0.6 for the signals preceding the fission events indicate that the correlations are of random origin. The evidence to have discovered element 114 in the reported experiments is classified “very weak”. Received: 13 October 1999     

Isospin effects on squeeze-out flow in heavy-ion collisions

The squeeze-out flow in reactions of ¹²⁴Sn + ¹²⁴Sn and ¹²⁴Ba + ¹²⁴Ba at different incident energies for different impact parameters is investigated by means of an isospin-dependent quantum molecular dynamics model. For the first time, it is found that the more neutron-rich system (¹²⁴Sn + ¹²⁴Sn) exhibits weaker squeeze-out flow. This isospin dependence of the squeeze-out flow is shown to mainly result from the isospin dependence of nucleon-nucleon cross-section and the symmetry energy. Received: 14 March 2000 / Accepted: 29 September 2000     

The competition between fusion-fission and deeply inelastic reactions in the medium mass systems

The competition between fusion-fission and deeply inelastic reactions in the Cl + Ni system has been studied by investigating the exit channelQ-value dependence of theZ-distributions of fission-like fragments. TheZ-, kinetic energy- and angular distributions of the fission-like fragments produced in the³⁷Cl +⁶⁴Ni system atE _lab=170, 186 MeV and in the²⁸Si +⁷⁴Ge system atE _lab=176 MeV have been measured by counter telescopes. TheZ-distribution of the³⁷Cl +⁶⁴Ni system was found to be essentially the same as that of the³⁵Cl +⁶²Ni system at the same bombarding energy. It is shown that this result can not be explained by the statistical fusion fission model but is consistent with a deeply inelastic model. A systematic study of the fission-like phenomenon in this mass region indicates that the maximum angular momenta for fusion reactions as well as the minimum angular momenta for DIC can be reproduced by the Bass model in the sliding limit (f=1.0) while the maximum angular momenta for fully energy-damped deeply inelastic reactions are governed by the strong interaction radius as predicted by the successive critical distance fusion model. The significance of these results are discussed.Communicated by V. Metag     

Alpha particle emission from the 198Pb compound nucleus: comparison between the fusion-evaporation and the pre-scission channels

The α-particle emission from the ²⁸Si+¹⁷⁰Er fusion-evaporation reaction at 165 and 185 MeV has been measured and compared with published data relative to the pre-scission emission in the same reaction. The spectra in the two channels exhibit nearly the same shape in the low energy region, whereas in the high energy region the spectrum for the fusion-evaporation case features a larger apparent temperature. The interpretation of the difference in shape between the two decay channels is based on statistical model calculations which account for the effects due to the different length of the decay chain. Statistical model calculations with standard parameters describe well the gross features of the alpha particle spectra in the fusion-evaporation channel and the proton spectra in the pre-scission channel. On the contrary, the model predictions seem to overestimate by ≃2 MeV the emission barrier for the alpha particles in the pre-fission channel. This effect is ascribed to the larger elongation of the nucleus during the fission process. An average axis ratio b/a∼ 2 for the emitter is suggested. Received 29 April 1997 / Revised version 10 September 1997