Dynamical Aspects of Nuclear Fission

The following sections are included:

• PREFACE

• CONTENTS

The process of fusion-fission of superheavy nuclei with Z=102-122 formed in the reactions with ²²Ne, ²⁶Mg, ⁴⁸Ca, ⁵⁸Fe and ⁸⁶Kr 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) using a time-of-fiight spectrometer of fission fragments CORSET and a neutron multi-detector DEMON. As a result of the experiments, mass and energy distributions of fission fragments, fission and quasi-fission cross sections, multiplicities of neutrons and gamma rays and their dependence on the mechanism of formation and decay of compound superheavy systems have been studied.

A two-step model is proposed for fusion of massive systems where the fusion hindrance is known to exist. It consists of a two-body collision process and a shape evolution process of the amalgamated one nucleus. Both are treated by suitable stochastic equations, respectively. An application is made to calculations of the residue cross sections for the element Z = 114 of ⁴⁸Ca + ²⁴⁴Pu reaction, combined with the calculation of the statistical decay of the compound nucleus. Preliminary results are encouraging for the purpose of predictions of the other systems leading to the superheavy elements.

In this talk an overview of the present status of the research into the structure of superheavy nuclei done by the Frankfurt/Erlangen group is presented. Both Skyrme forces and the relativistic meson-field approach are used to examine the shell structures. It is found that the spin-orbit force plays the most important role in determining shell closures in the region, and that the relativistic mean-field model appears to be more reliable in the extrapolation of that feature. It is, however, also very important to adequately treat the other ingredients of such models, such as pairing and the center-of-mass correction, in an adequate way. In addition, recent work on the point-coupling relativistic model as well as the possibilities for using TDHF to calculate the formation probabilities are also discussed.

In reactions used for the synthesis of superheavy elements the shell effects manifested themselves in the mass distribution of quasi-fission products, i.e., in an increased yield of nuclei with closed-shell, such as ²⁰⁸Pb and ^l32Sn. A model for the description of this effect is proposed.

By the analysis of some nuclear reactions (leading to the same compound nucleus or heavy and superheavy nuclei), and by the comparison of calculations with experimental excitation functions of evaporation residues, it is possible to understand the role of the nuclear shell structure of reacting nuclei on the fusion mechanism. In the framework of the dinuclear system concept, we study the effects of the mass asymmetry parameter in the entrance channel and structure effect of nuclei on the competition between quasi-fission and complete fusion processes. Such a competition is strongly related to the intrinsic fusion barrier, the B_qf quasi-fission barrier, as well as the size of the potential well. The evaporation residues cross sections for xn-channels were calculated in the frame of the combined dinuclear system concept (DNS) and advanced statistical model (ASM). We also analyze the effect of the spin distribution of the excited compound nucleus on the competition between fission and evaporation processes. Such a spin distribution affects the B_f fission barrier and the Γ_n/Γ_f ratio that determine the evaporation residue production. We present the results of reactions leading to the ²¹⁶Th*, ²²²Th*, ²²¹Pa* and ²⁸⁶112 compound nucleus.

The dinuclear system concept is used to describe the fusion of heavy ions to superheavy nuclei and the quasifission process in the same reactions. The dinuclear system can fuse by transferring nucleons between the two touching nuclei or decay by quasifission. The results agree with the present available experimental data.

The interaction potential between heavy ions around the touching point is calculated for various Skyrme forces in the extended Thomas-Fermi approximation by using the frozen Hartree-Fock-Bogoliubov densities of the individual nuclei. The potential pockets in collisions leading to superheavy elements are discussed systematically for different projectile-target combinations. An expression for the ion-ion potential well fits the semi-microscopic potentials in the wide range of both colliding ions and distances between them.

The fission barriers of sixteen isotopes of Californium, Fermium and Nobelium have been microscopically calculated up to and beyond the second saddle point within Hartree-Fock plus pairing approaches. The Skyrme density-dependent effective force in its SkM* parametrization – rather well suited to the description of fission barriers – was used in the particle-hole channel. As for the treatment of pairing correlations, two approaches have been followed: the first is the traditional HF plus BCS treatment with a seniority force, while the second makes use of a recently proposed method explicitly conserving the particle number and using a delta force for the particle-particle and hole-hole matrix elements. The energy correction due to the rotational zero-point motion has been approximately taken into account.

A stochastic approach to fission dynamics based on three-dimensional Langevin equations was applied to calculate a fission fragment mass-energy distribution from a number of excited compound nuclei formed in reactions induced by heavy ions. The evaporation of prescission light particles along Langevin fission trajectories from the ground state of the compound nucleus to its scission have been taken into account through a Monte-Carlo simulation technique. The inclusion of the third collective coordinate in Langevin dynamics leads to a considerable increase of the variance of the mass and the kinetic-energy distributions of fission fragments in comparison with two-dimensional Langevin calculations. The liquid drop model with finite-range nuclear forces and a modified one-body mechanism for nuclear dissipation have been used in the calculations. The results of the calculations are compared with the available experimental data. The performed calculations using three-dimensional Langevin dynamics reproduce sufficiently well all the parameters of the two-dimensional fission fragment mass-energy distribution and their dependence on the various parameters of the compound nucleus. The mean prescission neutron multiplicities are also reproduced with good accuracy. In order to reproduce simultaneously the measured prescission neutron multiplicities and the variance of the fission fragment mass-energy distribution, the reduction coefficient of the contribution from a wall formula has to be decreased at least half of the one-body dissipation strength (0.25 ≤ k_s ≤ 0.5).

The fission time distributions have been analysed using the comparison between measured and calculated characteristics of neutrons, protons and α-particles for compound nuclei near ¹⁸²Pt, ²²⁰Th and ²³⁹Np, formed in the heavy ion and proton induced fission. Theoretical calculations were carried out taking into account effects of nuclear friction which increases with nuclear temperature. The form of the extracted fission time distributions consits of short and long time components. The center position of both components decreases slowly with increasing of excitation energy of compound nucleus.

In the frame of the stationary scattering formalism we propose a new description of the excitation of rotational states of fragments emerging in cold fission. The decay of the mother nucleus, ²⁵²Cf, consists in the tunneling of a metastable state from an internal region, where the nucleus-nucleus potential produces a quasi-molecular pocket, to the asymptotic region.

We present a statistical model of fragment excitation in nuclear fission. The assumptions of the model are twofold. First we assume that the excitation of the single fragment is random, and can be described by a density distribution which is an exponential with a mean excitation energy of 〈E*〉= a(cQ)². Here a is the level density parameter, and c is a constant which determines how much of the available Q-value is taken over to excitation. Secondly, the model implies that both fragments are excited almost independently during fission. We show that on this basis a full description of the energetics in the fission process emerges. The theory for spontaneous fission is easily extended to fission following nuclear reactions at higher energies. Calculation of mean values of single fragment excitation, total fragment excitation TXE, total kinetic energy TKE and single fragment kinetic energies are in quantitative agreement with experimental data.

The paper deals with a study of the formation and decay of Ra compound-nuclei and continues the work started in 1998. The present investigation was aimed at obtaining experimental data on the excitation functions for the ¹²C + ^204,206,208Pb reactions and studying of fragments mass and energy distributions (MED) in order to understand a nature of the fission process in At–Th region. The evaporation residues cross-sections (σ_ER) as well as the fission cross-sections (σ_tis) were measured simultaneously. It was found that the σ_ER/σ_tis ratios for all the measured reactions are about two orders of magnitude higher than one obtained for ¹⁶O + ²⁰⁸Pb reaction.

The analysis of the MED of the fission fragments has been performed in order to separate symmetric and asymmetric components of the distributions. The analysis shown that there are three components present in fission of Ra compound-nuclei. The symmetric component dominates but two asymmetric components connected with the influence of the nuclear shells Z = 28, N = 50, Z = 50 and N = 82 were also observed. Thus the statement that the multimodal nature of the fission process is an inherent characteristic of the whole At–Th region has been reinforced. The ratios of the symmetric component yield to the asymmetric one (Y_s/Y_a) have been obtained. The dependence of the Y_s/Y_a ratio on the excitation energy of the compound-nucleus showed that Y_s/Y_a does not depend on the neutron number in compound-nucleus when the excitation energy is higher than 50 MeV. On the contrary, the behavior of the Y_s/Y_a dependences becomes strongly dependent on the compound-nucleus neutron number at lower excitations. The values of the excitation energy where the asymmetric component has its maximum yield are different for different isotopes and increases as the neutron number in compound-nucleus decreases. Thus, the statement that the more neutron-deficient compound-system has the higher asymmetric fission barrier has been reinforced.

A series of experiments on the fission of superheavy nuclei formed in reactions with ⁵⁸Fe ions, namely, the ⁵⁸Fe+²⁰⁸Pb, ⁵⁸Fe+²³²Th, ⁵⁸Fe+²⁴⁴Pu and ⁵⁸Fe+²⁴⁸Cm reactions, were carried out during the past two years at the Flerov Laboratory of Nuclear Reactions (JINR, Dubna) using the CORSET+DEMON setup. Experimental data were obtained on the mass and energy distributions of fission fragments of compound nuclei 108-122 in the excitation energy range of 14-53 MeV. As a result of the experiments it was found that the mass distribution of fission fragments for the compound nuclei ²⁹⁰116, ³⁰²120 and ³⁰⁶122 is asymmetric. Its nature, in contrast to the asymmetric fission of actinides, is determined by the shell structure of the light fragment with the average mass 132-134. It was established that the TKE and neutron multiplicity for fission and those for quasi-fission of superheavy compound nuclei are significantly different. The dependences 〈TKE〉/mass and mass yields for different excitation energies were analyzed. It was found that the TKE and mass distributions become more symmetric with an increase in the excitation energy.

The γ-ray multiplicities 〈M_γ〉 and total energy emitted by γ-rays in coincidence with fission fragments in a variety of ²⁶Mg, ⁴⁸Ca, and ⁵⁸Fe -induced reactions were investigated. For several reactions studied in this work, local minima are observed in 〈M_γ〉 as a function of mass suggesting the great influence of nuclear structure of fission fragments on 〈M_γ〉. A weak dependence of 〈M_γ〉 as a function of the excitation energy E* for quasifission in comparison with fusion-fission was found. The different dependence of 〈M_γ〉 as a function of fission fragment mass or total kinetic energy for fusion-fission and quasifission processes was observed.

The essence of our new way to analyse TKE-M matrices is presented. This analysis is applied to 3 different sets of data obtained in the past by HENDES collaboration: ²³⁸U(d,f) at E_d = 65 MeV, ²³⁸U(p,f) at E_p = 60 MeV and ²³⁸U +⁴⁰Ar (275 MeV). The chosen examples are representative of the power of the new method and reveal big diversity of the physical mechanisms responsible for the appearance of fine structure patterns in TKE-M matrices. In the studied reactions we find strong indications of the importance of ⁷⁸Ni and ¹⁰⁸Mo clusters marking distinct exit channels in fission and fast fission. Strong proton odd-even staggering that persists up to very high excitation energies is observed as well.

A method for studying the partition of excitation energy between binary fission-like reaction products from the mass-asymmetry distributions is proposed. It is based on the deviation of the fragment mass-asymmetry as registered by detectors from that formed at the scission point, which is caused by the emission of post-scission light particles. The excitation energy sharing parameter ζ for the event-by-event comparison of the temperatures of the fragments is introduced. This parameterization appears to be a useful tool for an advanced analysis of data from heavy-ion induced reactions. Its consistency is demonstrated with experimental data obtained for the reactions ¹⁴N (53 AMeV)+¹⁹⁷Au and ¹⁴N (53 AMeV)+²³²Th.

The following sections are included:

• Introduction

• Experiment

• Results and discussion

• References

Multifragmentation is the emission of several intermediate mass fragments from a hot nucleus. This phenomenon is observed in nuclear reactions, induced by light and heavy projectiles over a wide range of incident energies. One of the hopes of these studies is to gain knowledge about the topology of the nuclear phase diagram, in particular the liquid-to-gas transition. Multifragment decays are studied at GSI with two detectors : the ALADIN forward spectrometer and the 4π detector FOPI. Moreover, for a period of one and a half years, the 4π multidetector INDRA was available for experiments at GSI with beams from the heavy-ion synchrotron SIS. Some of the recent data obtained with these instruments in the energy range 0.1-1.8 AGeV will be presented.

The number of prompt neutrons emitted in the fission event have been measured separately for each complementary fragment in coincidence with fragment mass and kinetic energies in spontaneous fission of ²⁵²Cf, ²⁴⁴Cm and 248Cm. Two high efficient Gd-loaded liquid scintillator tanks were used for the neutron registration. Approximately 3 · 10⁶ fission events coincident with prompt neutron emission have been accumulated for each isotope. The mean neutron multiplicity, the dispersion and the covariance of the multiplicity distributions have been obtained as a function of fission fragment mass and kinetic energy. Dependencies of the moments of the multiplicity distributions on the fragment mass and total kinetic energy for different mass bins, as well as mass and total kinetic energy distributions of the fission fragments are presented, discussed and compared for the different isotopes investigated. The results showed a different behavior of the moments of the multiplicity distribution depending on the fragment mass asymmetry that reflects changes in the dynamical effects for different fission modes. Possible reasons for the decrease of the neutron yield at low TKE of fission fragments are discussed.

Mass yields and kinetic energy distributions of fission fragments from thermal neutron induced fission of ²³⁵U have been measured in the symmetry region with the mass separator Lohengrin of the Institute Laue-Langevin (Grenoble). The kinetic energy dip between asymmetric and symmetric fission amounts to 29 MeV, which agrees with previous data. A strong peak is observed in the widths of the kinetic energy distributions around the mass A = 112. A presence of the second peak in the rms distribution for the heavy mass group does not unequivocally follow from the data measured. A comparison of the data to the libraries and to previous experimental data has been done. A strong deviation in the yields from the predicted values is found for the masses from A = 113 to A = 127. The origin of this deviation (asymmetry in the mass distribution) is discussed.

The fission fragment mass, charge, and kinetic energy distributions for the thermal neutron induced fission of ²⁴⁵Cm were measured using the LOHENGRIN mass separator at ILL Grenoble associated with a big ionization chamber (Z-identification) with a split anode (energy loss-remaining energy simultaneously measured, ΔE - E_R technique). The ionization chamber was combined with a passive absorber (Stack of Parylene C foils) to further improve the Z-resolution for heavier products (A > 96) up to 4 nuclear charges per mass. Considering the available experimental data prior to this work, the range of measured mass yields was extended from A = 76-132 to A = 67-167, and isotopic yields from A = 76-96 to A = 67-119, i.e. for corresponding nuclear charges ranging from Z = 26 to 48. A comparison of these new data with the existing evaluations of fission product yields (JEF2.2 and ENDF/B-VI) and systematics (by Wahl) was also performed. A study of the global odd-even effect for protons was done and the results were compared with the data for the fission of other transuranium elements. The global δ_Z value of proton odd-even effect calculated is 10.5 ± 0.5 %, which fits well to the known systematics for the δ_Z as a function of the fissility.

The mass and energy distributions of fragments from the proton induced fission of compound nuclei ^234,237,239Np, ^{239,240,241,243}Am, ²⁴⁵Bk at proton energy E_p = 10.3 MeV, ²³³Pa, and ²³⁶Np at E_p = 7.4 – 30.0 MeV have been measured and analyzed in terms of the multi-modal fission on the basis of a new-developed method for the multi-modal deconvolution that is free from any parametrization of the distinct fission mode mass distribution shapes. At excitation energies above 10 MeV the strong correlations have been revealed in the behaviour of the characteristics of two predominant asymmetric modes Standard 1 and Standard 2. This effect could be interpreted as an indication of that mode Standard 2 is mainly formed under the influence of spherical shell closures Z = 50 and N = 82, as well as Standard 1, and several nucleons attached from the neck. The characteristics of the asymmetric fission mass distributions are shown to be mainly defined by the proton numbers of compound actinide nuclei and demonstrate only weak dependence on the neutron ones. At this, the sharply expressed grouping of the asymmetric fission mass yields at Z ≤ 51 in heavy fragments and at Z ≤ 31 in light ones has been observed.

In the first part of this paper, the fundamental and applied interest of ternary particle emission is briefly outlined. The main emission characteristics of ternary α's and tritons (angular and energy distributions, yields) are compared and discussed. Special attention is given to observed differences between tritons and α's, such as the strong difference in yield, the (apparent?) difference in energy distribution and the fluctuations of the t/α ratio.

In a second part we report on recent progress in our systematic study of the triton- and α-emission characteristics in spontaneous and thermal neutron induced fission. In the third part of the talk, a detailed discussion and interpretation of the observed phenomena is given. Concerning ternary α-emission, we highlight the strong influence on the ternary α-emission probability of both the fission modes followed by the fissioning nucleus and the α-cluster preformation probability. Concerning triton emission, we establish a new correlation between the triton emission probability (t/B) extrapolated to zero excitation energy with the average number of prompt neutrons (〈ν〉) emitted by the same compound nucleus at zero excitation energy. This correlation allows to treat simultaneously all ternary fission data coming from spontaneous as well as thermal neutron induced fission. Lastly, we discuss the impact of the fissioning nucleus excitation energy on the ternary emission probability: the different behaviour of tritons and α's suggests a possible difference in their emission mechanism.

The energy distributions and yields of light charged particles emitted during thermal neutron induced fission of ²⁴⁵Cm have been measured at the high flux reactor of the Institute Laue-Langevin in Grenoble (France).

The detection of the ternary particles was done using a ΔE-E telescope, permitting a good separation of the ternary particles. In this way, the characteristics of the energy distribution (average energy and full width at half maximum) for ⁴He, ³H and ⁶He particles as well as their emission probabilities could be determined. For the emission probabilities per fission, the following values were obtained: LRA/B = (2.15 ± 0.05) × 10^-3; ³H/B = (1.85 ± 0.10) × 10^-4 and ⁶He/B = (4.95 ± 1.25) × 10^-5.

In the past, the rare ternary fission (TF) process was mainly studied either by inclusive measurements of the energies and fractional yields of the light charged particles (LCPs) from fission, or by experiments on the angular and energy correlation between LCPs and fission fragments (FF). These previous studies, although having revealed valuable insight into many aspects of the TF process, are inherently limited to the emission of stable or β-radioactive LCP species. The present article describes a number of recent more elaborate correlation measurements that include either the registration of neutrons and γ-rays with LCPs and FFs, or the coincident registration of two LCPs. These experimental approaches have permitted to identify the population of excited states in LCPs, the formation of neutron-unstable nuclei as short-lived intermediated LCPs, as well as the sequential decay of particle-unstable LCP species into charged particle pairs. "Quaternary" fission with an apparently independent emission of two charged particles has also been observed. Details of these studies performed on either ²⁵²Cf(sf) or ²³⁵U(n_th,f) are presented, and the results obtained at the present stage of the analysis are discussed.

Ternary spontaneous fission of ²⁵²Cf has been studied in two experiments with the Gammasphere detector array with light charged particle detectors surrounding the source. From α-γ double gated spectra, neutron multiplicity distribution were determined for related α ternary fission pairs. In going from binary to α ternary SF for approximately the same mass splittings (A≈104-146) the average neutron multiplicity decreases about 0.7 AMU. The data further indicate an enhancement of the octupole band in ¹⁴⁶Ba in the α ternary SF. In the first LCP-γ-γ experiment, the ¹⁰Be spectrum was cutoff below 27 MeV and in the recent experiment, below 18 MeV. For high energy (E > 27 MeV) ¹⁰Be ternary fission, the data indicate that the largest yields go via the cold process (zero neutron evaporation). This results is different from that observed for α ternary SF. In the recent experiment with E cutoff of 18 MeV, the ¹⁰Be ternary SF was observed for zero to 4n emissions. In some cases the zero channel appears to dominate and in others the 2n channel.

From multiparameter data obtained on GAMMASPHERE, the integral characteristics of the prompt γ-ray emission were extracted for ternary fission of ²⁵²Cf with He, Be and C being the third light charged particle (LCP). The multiplicity distribution characteristics i.e. mean multiplicity and its dispersion were obtained by minimizing with respect to the calculated values of probabilities of multifold γ-ray coincidences using combinatoric method. Further, we investigated dependencies of the mean γ-ray multiplicity on the kinetic energy of LCP. Behavior of mean γ-ray multiplicity shows anomalous character for He when studying its dependence on kinetic energy of He. In the low energy of ⁴He the colder region of ternary fission appears, which is also indicated by the enhanced yield of ¹³²Sn. The mean γ-ray multiplicity was determined for the first time for Be and C ternary fission. For Be, the γ-ray multiplicity as a function of kinetic energy was obtained as well. Based on the analysis of the ¹⁰Be yield observed through its γ-ray transition with the energy 3364 keV, we determined the upper limit of its possible participation on a quasibound molecular structure.

Large number of μs isomers have been observed near ¹³²Sn. These nuclei are produced by thermal induced fission of ^233,235U and ^239,241Pu. The detection is based on time correlation between fission fragments selected by the LOHENGRIN spectrometer at ILL (Grenoble) and the γ rays or conversion electrons from isomers. The interpretation of the observed levels schemes is mainly based on shell model calculations. Several new B(E2)values were measured and are discussed in the paper.

Spectroscopic studies in the superdeformed shape isomer of ²⁴⁰Pu using γ-, conversion electron and transmission resonance spectroscopy have been performed. In a γ spectroscopy experiment the out-of-band decays of several excited superdeformed rotational sequences with K = 2^- and 1^- could be identified together with evidence for a weakly populated 0^- octupole band. Complementary information could be obtained in conversion electron measurements in coincidence with isomeric fission, resulting in the first identification of the lowest β vibrational K = 0⁺ band. For all rotational bands the variation of the moment of inertia could be studied. A predominant population of negative parity states in the second minimum could be observed, which can be explained by a filtering action of the inner and outer fission barrier. From a systematical behaviour of the β and γ band head energies in actinide isotopes predictions for phonon energies in the second well can be derived and new magic numbers can be experimentally determined. Complementary transmission resonance experiments have been performed, resulting in new information on the rotational structure of multi phonon β vibrational states. A new method could be established to determine the excitation energy of the ground state of the fission isomer from measured level densities. This method was applied to a study of the hyperdeformed third minimum in ²³⁴U, resulting in the first experimental determination of the depth of the third well.

The new isotopes ²³³Cm and ²³⁴Cm were produced using the reaction ¹⁹⁸Pt(⁴⁰Ar,xn)^238-xCm(x=4,5). Five different bombarding energies of E_lab=184, 192, 196, 200 and 208 MeV respectively, were used to measure the excitation function for the 4n evaporation channel. A maximum production cross section of σ_4n ≈1 nb was measured. The α-decay branch of ²³⁴Cm was identified by α-α -correlations to known daughter products ²³⁰Pu, ²²⁶U, ²²²Th, ²¹⁸Ra and ²¹⁴Rn. The measured α decay energy is E_α = (7239 ± 10) keV, the half-life was determined to be T_1/2 = (51 ± 12) s. Also 26 fission events, tentatively assigned to a fission branch of ²³⁴Cm, were observed. The technique of α-α - correlations allowed to determine the half-life of ²³⁰Pu to be T_1/2 = (102 ± 10) s. The EC branch of ²³⁰Pu was estimated to be lower than 27 %. The identification of ²³³Cm was also based on the α-α - correlations to its known daughter products ²²⁹Pu, ²²⁵U, ²²¹Th, ²¹⁷Ra and ²¹³Rn. Five such decay chains were observed at the bombarding energy of 208 MeV. In addition 7 decay chains starting with the same α energy but followed by decays of ²²⁹Np, ²²⁵Pa, ²²¹Ac, ²¹⁷Fr and ²¹³At were detected. The α energy of E_α = (7340 ± 10) keV was determined for ²³³Cm, the half-life could not be measured. The half-life of ²²⁹Pu was measured to be T_1/2 = (90 ± 10) s.

A review of recent applications of the relativistic mean-field theory to the structure of exotic nuclei is presented. Models based on the relativistic mean-field approximation provide a microscopically consistent description of the nuclear many-body problem, not only in nuclei along the valley of β-stability, but also in exotic nuclei with extreme isospin values and close to the particle drip lines.

For systems with extreme isospin values, the relativistic Hartree-Bogoliubov model provides a unified and self-consistent description of mean-field and pairing correlations. The model has been applied in studies of structure phenomena that include: formation of neutron skin and of neutron halos in light nuclei, the strong isospin dependence of the effective spin-orbit interaction and the resulting modification of surface properties, the suppression of the spherical N = 28 shell gap for neutron-rich nuclei and the related phenomenon of deformation and shape coexistence. The relativistic random phase approximation, based on effective mean-field Lagrangians with nonlinear meson self-interaction terms, has been applied in the analysis of the evolution of the isovector dipole response in nuclei with a large neutron excess. In particular, the onset of dipole pygmy resonances in medium-heavy neutron-rich nuclei has been studied.

Continuous gamma spectra from reactions ⁴⁰Ca+^40,48Ca at 25 MeV/A were originally analyzed using the CASCADE code with provision for energy dependence of the GDR parameters. Strong indications of incomplete fusion of the interaction ions were obtained there. The present analysis using the pre-equilibrium exciton model code PEQAG returns to that experiment and questions the need for incomplete fusion.

Ground–state properties of even–even oxygen isotopes are calculated using the framework of the relativistic mean–field approach (RMFA) with several global parameterizations. The particle stability of very neutron–rich O isotopes id discussed. This approach fails to describe supposed particle instabilities of ^26,28O and the confirmed particle stability of ³¹F. The local optimization of the RMFA functional is performed and the isovector properties of the RMFA effective interaction are discussed.

The far reaching shape of the matter distributions (halo) of exotic nuclei with high isospin components is calculated in the Dynamic-Correlation Model (DCM) which is based on the interaction of valence- and core-particles. In this model, one, two, or more valence particles and intrinsic vacuum-clusters (collective-excitations of reference vacuum) are treated within the same formalism. Matter and charge distributions of lithium and beryllium isotopes are strongly modulated by the coupling of the valence particles with the core, and the matter radii extracted from the theoretical distributions are in good agreement with experimental results. Within the model the core-protons are contributing de facto to the halo formation. The effect of the core excitation mechanism on the calculated cross sections for proton scattering on helium and lithium isotopes is analyzed. For the charge radii new experiments based on the determination of the volume-shift are discussed.

An overview of recent results on reaction dynamics in the energy region 20 - 50 A.MeV is given. The results of the study of projectile multifragmentation using the detector array FAUST are presented. Reaction mechanism is determined and thermodynamical properties of the hot quasiprojectile are investigated. Preliminary results on fragment isospin asymmetry obtained using the 4π detector array NIMROD are given. Procedure for selecting centrality in two-dimensional multiplicity histograms is described. Possibility to extract thermodynamical temperature from systematics of isotope ratios is investigated. Reaction mechanism leading to production of hot sources is discussed. Furthermore, the possibilities for production of rare isotopes are discussed and recent experimental results obtained using recoil separator MARS are presented.

The experimentally measured total reaction cross section for the reaction ⁶He+Si, and the excitation functions for fission and 4n-evaporation channels for the ⁶He + ²⁰⁹Bi-reaction are presented. The secondary ⁶He beam was produced using the beam-transport line of the U400M accelerator at FLNR, JINR.

The comparison of the obtained experimental data with similar results for the ⁴He + Si-reaction shows that at energies below 17 MeV/A, a strong increase in the values of σ_R takes place. In the case of the ⁶He + ²⁰⁹Bi-reaction, a significant enhancement of the cross section is observed for energies above the barrier. An agreement between the experimental data and the theoretical calculations is obtained when the Coulomb barrier is reduced by 15-20 %, which corresponds to an increase of the parameter r₀ of the nuclear potential up to 1.5-1.6 fm.

After a short discussion of ISOL-facilities in Europe we focus on the present status of the REX-ISOLDE facility at CERN and the Munich Accelerator for Fission Fragments MAFF. REX-ISOLDE, which post-accelerates reaction products of ISOLDE/CERN to energies between 0.8 and 2.4 MeV/u, has been installed and all parts have been tested successfully. First radioactive beam were available in October/November 2001. At the new Munich FRM-II reactor MAFF is under design. Its target/ion source of MAFF with deliver 10¹⁴ fissions/s. Probably in 2004 intense low-energy beams (≈ 10¹¹/s) of very neutron-rich fission fragments will be available. For MAFF a linac similar to the REX-ISOLDE type is being developed, which will accelerate the ions after charge breeding to energies between 3.7 and 5.9 MeV/u.

The facility for neutron–rich nuclei production, as a specific part of the project DRIBS – phase II, based on the use of photofission and the characteristics of microtron MT–25 as a source of bremsstrahlung photons are described. Target selection with regard to maximum yield production of strong neutron–rich fission products and with respect to its safety and availability is discussed. Geant 4 simulation toolkit was used to obtain bremsstrahlung γ-beam characteristics and its angular spred was compared with measurements on MT–25 microtron. Next, calculation of fission–rate and fission density in ²³⁸U target at nominal values of MT–25 microtron are presented. Calculations of the yield from photofission based on the Wahl's Z_P model for charge distribution of fission fragments are compared with experimental data for independent yield of xenon isotopes measured. As result the independent and cummulative yields from photofission of ²³⁸U for mictrotron MT–25, as a driver accelerator for DRIBS II, for up to 890 isotopes and their isomers were obtained. Mean characteristics of this compact facility with some others RIB projects are compared.

Electrostatic separator VASSILISSA is used for exploring complete fussion nuclear reactions. The magnetic analyzer, based on D37 dipole magnet, was installed after the second triplet of quadrupole lenses of the separator for the mass identification of evaporation residues. Mass identification is an powerful tool for identification of recoil atoms of super-heavy elements. The new detection system consisting of the time-of-fiight system and 32-strips position-sensitive detector array was installed in the focal plane of the separator. The mass resolution of the separator after upgrade was found to be about 2.5 %.

In this work the possibilities of direct mass measurement based on the evaporation residue's energy measurement and the velocity measurement are described. In particular the influence of energy straggling, pulse height defect, time of flight measurement and degrader foils thickness is evaluated. For higher or lower energetic evaporation residues the mass measurement uncertainties are calculated. It was found that using the described method the mass measurement uncertainty could be not better than 20%.

In the paper the efficient algorithms to analyze multiparameter γ-ray spectra are presented. They allow to search for peaks, to separate peaks from background, to improve the resolution and to fit 1 to 3-parameter γ-ray spectra. The algorithms presented are implemented in the system for nuclear data acquisition, analysis, processing and visualization - DAQPROVIS.

The following sections are included:

• LIST OF PARTICIPANTS

• AUTHOR INDEX