Light ion fusion in deformation model

Experiments with heavy ions at moderate energies show the importance of deformation in heavy ion collisions. A deformation model which takes deformation dynamically into account is developed. Having described fusion and deep inelastic collision for a very heavy system (Xe + Bi) and a medium heavy system (Ar + Th) at various energies successfully, we turn to some comparatively lighter heavy ions where fusion is the most dominant feature. Fusion cross-sections for six pairs of lighter systems (³⁵Cl +¹¹⁶Sn,⁵⁸Ni+⁶²Ni,³⁵Cl+⁶²Ni,³²S+²⁴Mg,²⁴Mg+²⁴Mg and¹²C+²⁷Al) have been obtained using our deformation model which agree well with experiment. The two-slope-behaviour of fusion excitation function which is an important feature of light ion fusion systematics is also obtained, in our model calculations for all the systems studied.     

Influence of the entrance channel in the fusion reaction 318 MeV 74Ge+74Ge

Entrance channel effects in the fusion of heavy ions have been studied by using the ⁷⁴Ge+⁷⁴Ge reaction at 318 MeV. The population of the yrast superdeformed band in ¹⁴⁴Gd shows an increase when compared with the results obtained in the more asymmetric ⁴⁸Ti+¹⁰⁰Mo reaction at 215 MeV. The relative yields of the different evaporation residues produced in the ⁷⁴Ge+⁷⁴Ge and in the ⁴⁸Ti+¹⁰⁰Mo reactions are very similar, with the exception of the ^145,144Gd residual nuclei (3n and 4n decay channels) which are populated with a larger yield in the symmetric reaction. Statistical model calculations reproduce qualitatively such effect if a fission delay is explicitly taken into account. Effects related to fusion barrier fluctuations seem to be important in determining the spin distributions of the compound nucleus. The spectra of the high energy γ-rays emitted in the ⁷⁴Ge+⁷⁴Ge reaction have been measured as a function of the γ-ray multiplicity as well as in coincidence with selected evaporation residues. They are reproduced by standard statistical model calculations with GDR parameters taken from systematics, demonstrating that, in agreement with dynamical model prediction, the emission of γ-rays from the dinucleus formed in the earlier stage of the collision is unimportant.     

Fission accompanied by alpha particle emission in the12C(85MeV)+232Th reaction

Inclusive and coincident spectra of alpha particles and fission fragments were measured for the²³²Th+¹²C (85 MeV) reaction to study the influence of the excitation energy and the angular momentum on the fission of the compound nucleus and to separate different alpha particle emission mechanisms. At backward angles α emission can be accounted for by the evaporation processes. At forward angles the most important contribution is given by the break-up fusion process. Mass distributions for compound nuclei²⁴⁴Cm (E^*=58 MeV,ff coincidences), and²⁴⁰Pu (E^*=37 MeV,ff α coincidences) were obtained. In the case of²⁴⁰Pu mass distribution has a shape different from those obtained in light ion reactions at the same excitation energy, indicating the strong influence of the entrance channel. The dependence of the mass distribution shape on the α particle energy is also examined.     

核对称轴不同相对取向对熔合动力学的影响

计算了核对称轴不同相对取向时的熔合位垒.基于双核模型观念，考虑了熔合与准裂变的竞争，通过数值法求解主方程，计算了⁷⁶Ge+²⁰⁸Pb，⁴⁸Ca+²⁴⁴Pu核对称轴不同相对取向对熔合概率的影响，探索了最有利于超重元素合成的弹靶相对取向.取向不同时，对熔合反应的影响较大，计算结果表明弹靶碰撞为腰对腰时，更有利于发生熔合反应. 关键词： 超重元素 熔合概率 变形核 方向角度     

Comparison of the characteristics of the fusion-fission and quasifission products in reactions with heavy ions

Overlap of quasifission and fusion-fission product mass distributions measured for the ⁴⁸Ca + ¹⁵⁴Sm reaction at near-barrier energies is analyzed. As the incident ion energy increases, the angular distribution of quasifission fragments becomes narrower and they are concentrated near the forward and backward angles. The observed decrease in the yield of quasifission products with increasing collision energy is caused by noncoincidence of the product emission and detector position directions (the latter direction lags behind the beam direction by more than 15°) rather than by a decrease in the number of quasifission events. Our evaluations show that the excitation functions of evaporation residues in the ⁵⁴Cr + ²⁴⁸Cm reaction are several orders of magnitude larger than the excitation functions for the other reactions, ⁵⁸Fe + ²⁴⁴Pu and ⁶⁴Ni + ²³⁸U, used to synthesize a new superheavy element with Z = 120 (A = 302).     

Fragment mass and kinetic energy distribution in isomeric fission of 240Pu

Fragment mass and kinetic energy distributions have been measured for isomeric fission of ²⁴⁰Pu. The mass distribution is asymmetric with the average heavy fragment mass nearly equal to that found for ground state spontaneous fission of ²⁴⁰Pu, but slightly lower than for n_th + ²³⁹Pu-fission. The average total fragment kinetic energy appears to be higher in isomeric fission (179.5_?0.7^+1.5 MeV) than in spontaneous fission from the ground state (176.8 ± 1.8 MeV).     

The role of the entrance channel in the fusion of massive nuclei

The role of the entrance channel in the fusion-fission reactions leading to nearly the same superheavy compound nucleus is studied in the framework of dynamic model. The calculations are done for ⁴⁸Ca +²⁴⁴Pu and ^74,76Ge +²⁰⁸Pb reactions which could lead to formation of superheavy element Z = 114. It is shown that for these reactions there is an energy window for the values of the bombarding energy at which a capture probability is sufficiently large. Together with the restriction coming from the intrinsic barrier for fusion, it helps to find an optimal value of the bombarding energy for a given projectile--target combination. Received: 15 July 1998     

Fusion around the barrier for 7Li+12C

Fusion cross-sections for the ⁷Li + ¹²C reaction have been measured at energies above the Coulomb barrier by the direct detection of evaporation residues. The heavy evaporation residues with energies below 3 MeV could not be separated out from the α-particles in the spectrum and hence their contribution was estimated using statistical model calculations. The present work indicates that suppression of fusion cross-sections due to the breakup of ⁷Li may not be significant for ⁷Li + ¹²C reaction at energies around the barrier.     

Synthesis of 292116 in the 248Cm + 48Ca reaction

We report on the observation of the first decay event of the new nuclide ²⁹²116 produced in an experiment devoted to the synthesis of Z=116 nuclei in the ²⁴⁸Cm + ⁴⁸Ca reaction. The implantation of a heavy recoil in the focal-plane detector was followed in 46.9 ms by an α particle with E _α=10.56MeV. The energies and decay times of the descendant nuclei are in agreement with those observed in the decay chains of the even-even isotope ²⁸⁸114, previously produced in the ²⁴⁴Pu + ⁴⁸Ca reaction. Thus, the first α decay should be attributed to the parent nuclide ²⁹²116 produced via the evaporation of four neutrons in the complete fusion of ²⁴⁸Cm and ⁴⁸Ca. The experiment is in progress at Flerov Laboratory for Nuclear Reactions (FLNR, JINR, Dubna).     

The influence of the entrance channel mass asymmetry on the reaction mechanism

We have tried to investigate the influence of the entrance channel mass asymmetry on the reaction mechanisms associated with heavy ion collisions. Two systems, one very much asymmetric (O+Mo) and the other one almost symmetric (Cr+Fe), were studied in detail by measuring evaporation residues, deep inelastic collision products and fission fragments. An important fraction of the fragments observed in the Cr+Fe system exhibits all the characteristics of fission fragments. The analysis of these data seems to indicate that these fission like products are most likely emitted by a long lived composite system having not reached full statistical equilibrium for all the degrees of freedom. As a consequence, the fusion cross section for this symmetric system is too low as compared to predictions based on a critical distance approach for fusion, whereas the asymmetric system (O+Mo) is well understood in term of the same model.     

Partial fusion of a weakly bound projectile with heavy target at energies above the Coulomb barrier

Partial-fusion cross-sections for the systems ⁶Li + ²⁰⁸Pb, ⁹Be + ²⁰⁹Bi have been determined. The effect of breakup on fusion for weakly bound projectiles ⁶Li and ⁹Be incident on ²⁰⁸Pb or ²⁰⁹Bi targets has been discussed comparing experimental fusion cross-section excitation functions to those evaluated with a semi-classical approach. It is shown that complete fusion of a weakly bound projectile with heavy target is reduced, whereas the breakup process has very little influence on the total-fusion cross-section for some of the studied systems at energies above the Coulomb barrier.     

Fusion-fission dynamics in the superheavy nucleus production

The fusion-fission reaction mechanism leading to the massive nucleus formation is studied. We investigate the superheavy nucleus formation in heavy-ion induced reactions by analysing the evaporation residue (ER) production in order to study the fusion dynamics and the decay properties of nuclei close to the stability island at Z=114. We consider the ⁶¹Ni+²⁰⁸Pb, ⁴⁸Ca+²³⁸U and ⁴⁸Ca+²⁴⁴Pu reactions that lead to the Z=110, 112 and 114 superheavy elements respectively. By using the dinuclear system (DNS) concept of the two interacting nuclei we calculate the quasifission-fusion competition in the entrance channel and the fission-evaporation competition along the de-excitation cascade of the compound nucleus. The dynamics of the entrance channel allows us to determine the beam energy window which is favorable to the fusion, while the dynamic evolution of the compound nucleus on the shell correction to the fission barrier and the dissipative effects influence the fission-evaporation competition in order to obtain the residue nuclei from the superheavy nucleus formation. We also calculate the τ _n /τ_tot ratio at each step of the de-excitation cascade of the compound nucleus and we present a systematics of τ _n /τ_tot (at first step of the cascade) for many reactions that lead to nuclei with Z=102–114.     

Exploiting barrier distributions to investigate breakup effects in the fusion of 9Be+208Pb

The availability of precisely measured fusion excitation functions have allowed the determination of experimental fusion barrier distributions. This concept is utilised in ⁹Be+²⁰⁸Pb reaction, to reliably predict the expected complete fusion cross-sections. However, the measured cross-sections are found to be only 68% of those predicted. The large cross-sections observed for incomplete fusion products support the interpretation that this suppression of fusion is caused by ⁹Be breaking up into charged fragments before reaching the fusion barrier.     

Elastic scattering and fusion cross-sections in 7Li + 27Al reaction

With an aim to understand the effects of breakup and transfer channels on elastic scattering and fusion cross-sections in the ⁷Li + ²⁷Al reaction, simultaneous measurement of elastic scattering angular distributions and fusion cross-sections have been carried out at various energies (E _lab?=?8.0–16.0 MeV) around the Coulomb barrier. Optical model (OM) analysis of the elastic scattering data does not show any threshold anomaly or breakup threshold anomaly behaviour in the energy dependence of the real and imaginary parts of the OM potential. Fusion cross-section at each bombarding energy is extracted from the measured α-particle evaporation energy spectra at backward angles by comparing with the statistical model prediction. Results on fusion cross-sections from the present measurements along with data from the literature have been compared with the coupled-channels predictions. Detailed coupled-channels calculations have been carried out to study the effect of coupling of breakup, inelastic and transfer, channels on elastic scattering and fusion. The effect of 1n-stripping transfer coupling was found to be significant compared to that of the projectile breakup couplings in the present system.     

Yields of evaporation residues and average angular momentum in heavy ion induced fusion reactions leading to compound nucleus96Ru

Cross-sections for production of evaporation residues from the compound nucleus⁹⁶Ru* formed by fusion reactions²⁸Si+⁶⁸Zn,³²S+⁶⁴Ni,³⁷Cl+⁵⁹Co and⁴⁵Sc+⁵¹V have been obtained from the yields of their characteristicγ-rays. The measurements span an excitation energy range of 55 MeV to 70 MeV of the compound nucleus. The evaporation residue (ER) cross-sections have been analysed in terms of statistical model for the decay of the compound nucleus. A good agreement is found between statistical model calculation and the experimental evaporation residue cross-sections in all the four cases. It is shown that the average angular momentum of the compound nucleus can be deduced from a comparison of the experimentally measured and the statistical model predictions for the ER cross-sections. The validity of this method of deriving has been discussed for the case of¹⁶O+¹⁵⁴Sm system.     

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     

Evaporation and fission decay of 132Ce compound nuclei at Ex=122 MeV: some limitations of the statistical model

Light charged particle (LCP) emission in the evaporation residue (ER) and fusion fission (FF) channels have been studied for the 200 MeV ³²S + ¹⁰⁰Mo reaction, leading to ¹³²Ce composite nuclei at E _x =122 MeV. The main goal was to study the decay of ¹³² Ce on the basis of an extended set of observables, to get insights on the fission dynamics. The proton and alpha particle energy spectra, their multiplicities, ER-LCP angular correlations, ER and FF angular distributions, and ER and FF cross-sections were measured. The measured observables were compared with the Statistical Model (SM). Using standard parameters, the model was able to reproduce only the pre-scission multiplicities and the FF and ER cross-sections. The calculation was observed to strongly overestimate the proton and alpha particle multiplicities in the ER channel. Disagreements were also observed for the ER-LCP correlations, the LCP energy spectra and the ER angular distribution. By varying the SM input parameters over a wide range of values, it is shown that it is not possible to reproduce all the observables simultaneously with a unique set of parameters. The inadequacy of the model in reproducing the ER particle multiplicities is also observed analysing data from the literature for other systems in the A ≈ 150 and E _x ≈ 100?200 MeV region. These results indicate serious limitations about the use of the SM in extracting information on fission dynamics.     

Fusion-evaporation cross-sections for 48Ca + 154Sm near the Coulomb barrier

Measurements of fusion-evaporation cross-sections for the system ⁴⁸Ca + ¹⁵⁴Sm have been performed in the sub- and near-barrier energy range. Barrier-passing cross-sections have been obtained by adding recently measured capture-fission cross-sections at the same energies, and the barrier distribution for capture has been extracted. The data have been analyzed within a coupled-channel model, and a large subbarrier cross-section enhancement is observed, due to the ground-state prolate deformation of ¹⁵⁴Sm. The ⁴⁸Ca + ¹⁵⁴Sm capture cross-sections are compared to existing data on ¹⁶O + ¹⁸⁶W fusion, leading to the same CN, where a few higher-energy points have also been measured. The evaporation residue cross-sections for the two systems above the barrier indicate that complete fusion is inhibited for ⁴⁸Ca + ¹⁵⁴Sm by 40% in that energy region, with respect to ¹⁶O + ¹⁸⁶W.