acta physica slovaca

Abstract: The possibility to transmute 241Am by a high intensity thermal neutron flux depends essentially on the value of the 242gsAm capture cross section. As the recommended values given in the two most widely used nuclear data libraries ENDF-B/VI and JEF-2.2 differ by more than a factor of 20, an experiment has been carried out at the Institut Laue-Langevin in Grenoble (France) to fix this discrepancy. The obtained results and their impact on the present understanding of nuclear waste incineration systems will be discussed.

Abstract: The extension of the periodic system into various new areas is investigated. Experiments for the synthesis of superheavy elements and the predictions of magic numbers are reviewed. Different ways of nuclear decay are discussed like cluster radioactivity, cold fission and cold multifragmentation, including the recent discovery of the triple fission of 252Cf and long-living three-cluster-nuclear molecules. Furthermore, the possibility of production of clusters of hypermatter and antimatter in high energy heavy-ion collisions is envisaged. A perspective for the future is given.

Abstract: Rare cold (zero neutron emission and low excitation energy) binary fission, cold and 10Be-accompanied ternary fission and cold fission with two neutrons emitted at scission and new nuclear structure effects have been observed in the spontaneous fission of 252Cf. Evidence for stable octupole deformation in 143,144,146Ba and 145,147La are presented. The roles of rotation in both enhancing and quenching stable octupole deformation are observed. In 145La one has a new class of shape coexistence with symmetric and asymmetric rotational bands.

Abstract: Evaporation residue production in cold fusion reactions of Pb- and Bi- target nuclei with projectiles of elements between Ti (Z=22) and Zn (Z=30) leading to compound nuclei ZCN = 104-113 was investigated at SHIP. The result was the first unambiguous identification of isotopes of elements Z=110, Z=111 and Z=112 in bombardments of 208Pb ,209Bi with 62,64Ni, 70Zn. We further report on the identification of 256Db, 252Lr and on new, improved decay data of 253Lr, 255Rf, 257Db, allowing a first deeper insight in the nuclear structure of these isotopes.

Abstract: The collective friction and mass tensors are computed for the composite systems formed in the fusion of 18O+208Pb, 20Ne+249Cf and 48Ca+238U (for the temperatures MeV) in the framework of the linear response theory and locally harmonic approximation. All components of friction and inertia tensors considered as functions of deformation show the peaked structure caused by avoided crossing of single-particle levels as well as by shell and pairing effects. The numerical values of friction and mass parameters differ considerably from the ones obtained with the wall formula for the friction and the Werner-Wheeler method for the collective mass.

Abstract: In the paper, the efficient algorithms to process multiparameter -ray spectra are presented. They allow to search for peaks, to separate useful information from background and to improve resolution in 1-, 2- and 3-parameter -ray spectra. The algorithms are generalized to n-parameter data.

Abstract: The investigations of the decay properties and formation cross sections of the heaviest isotopes of the elements 110 ( 276-277110) and 112 ( 282-283112) were performed at the Flerov Laboratory of Nuclear Reactions at Dubna with the use of the internal and extracted high intensity 48Ca beams. 232Th and 238U targets were used in the experiments. The experiments with internal beam probe include the use of fast chemistry for the extraction of the fraction containing 106 element nuclei from the 232Th target material and the off line measurements of the possible correlation chains from the 268,269106 decay and 264,265104 spontaneous fission. The 48Ca+238U reaction was investigated at the VASSILISSA recoil separator. The experiments were performed at two beam energies leading to excitation energies of 33 and 39 MeV. The collected beam doses were 3.5 1018 and 2.2 1018, respectively. Two spontaneous fission events were observed at the lower beam energy, which tentatively were assigned to the new neutron rich isotope 283112 produced by the reaction 238U(48Ca,3n)283112. The measured cross-section is (5.0 +6.3-3.2) pb and the half-life is (81 +147- 32) s. No event was observed at the higher beam energy resulting in an upper cross-section limit of 7.3 pb for the 4n evaporation channel.

Abstract: One of the long-term programs of nuclear physics is to investigate the properties of the heaviest atomic nuclei. The synthesis of nuclei with Z >100 always belonged to experiments at laboratory detection limits. The last large scale upgrade of experimental technique enabled to synthesize new nuclei with Z = 110, 111, 112. At these experiments no such a significant shell stabilizing or isospin effect was observed, which could eliminate the continuous decrease of cross sections of both "cold" and "hot" fusion reactions. For the synthesis of nuclei with Z 113 cross sections below 1 pb are expected. To synthesize new superheavy nuclei with higher nuclear charge, the laboratory technique, to be used at the proposed reactions, should be again considerably upgraded. New target systems are considered, able to accept higher beam intensities. Some of the new superheavy evaporation residues will probably decay by fission. At present no adequate detection methods are used at superheavy element research, enabling an unambiguous identification of single fissionable evaporation residues. Possibilities to develop such detector systems are presented.

Abstract: Recent theoretical studies of stability and properties of heavy and superheavy nuclei are described. Such properties as deformations, rotational energies and half-lives with respect to main decay modes are discussed. Even-even nuclei with proton number Z=82-120 and neutron number N=126-190 are considered.

Abstract: New data on neutron, proton and alpha particle multiplicity measured for the reactions 58Ni (340 MeV) + 112,118,124Sn, 20Ne (160 MeV) + 162Er, 40Ar (243 MeV) + 142Nd, and 58Ni (320 MeV) + 124Sn are presented. The results are compared with new model calculations. New results on proton induced very asymmetric division of 238U at intermediate energies are discussed. A short description of experimental set-up HENDES for investigation of fusion-fission processes is given.

Abstract: The available production cross sections of the Z 102 nuclei are compared to the results of a statistical model using the parameters taken from the region. The hot fusion data are reproduced using a summary fission barrier reduced by 15 - 20 %. For the cold fusion reactions the parameters of the fusion channel are found, nevertheless the cross sections are not reproduced for the nuclei above Z=104 where the fusion probabilities are determined. A simple statistical approach, based on the neutron multiplicity data from the spontaneous fission as a unique information on the scission configuration, is proposed and the fusion probabilities of a cold fusion reactions are reproduced using one set of parameters. With these parameters, the estimate of the production cross sections in reactions for the synthesis of superheavy nuclei is calculated. The steep decrease of estimated cross sections is predicted for the cold fusion reactions. In the case of the 48Ca-induced reactions the overall decrease of production cross sections is less significant.

Abstract: In Garching near Munich the new research reactor FRM II is under construction. It is supposed to be commissioned in 2001. The reactor will have a power of 20MW, a rather small core diameter of 25cm and a maximum neutron flux of . Many new powerful instruments and novel experiments are planned in material science, solid state physics, nuclear physics, fundamental physics, chemistry, biology, medicine and technology. Consequently, the applications will range from material science diffractometers and inelastic neutron scattering over neutron tomography and cancer therapy to fusion producing the heaviest elements using accelerated fission fragments. The acceleration of radioactive isotope beams (RIB) is an important new field in nuclear physics and many new facilities are planned or under construction all over the world. Nuclear fission is the most suitable method to produce neutron-rich isotopes due to the large fission cross sections of thermal neutrons and the high thermal neutron flux in modern research reactors. Consequently, the new facility MAFF (Munich Accelerator for Fission Fragments) is foreseen for the new FRM II reactor. Beams of about 1011atoms/s are expected for 91Kr, 132Sn,144Cs from a 235U source in a graphite cylinder close to the reactor core placed in a through-going beam-tube. Laser ionisation and a linear accelerator with IH and RFQ structures will provide the possibility to accelerate many different isotopes up to about 6MeV/nucleon. Neutron-rich isotope beams are most interesting, because they will allow the investigation of several exciting new areas: i)Production of heavy and superheavy elements. ii) Atomic physics and chemistry of the heaviest elements. iii) Nuclear structure of astrophysically relevant nuclides and data for the r-process nucleosynthesis. iv) Isospin dependence of nuclear reactions. v) Application of radioactive isotope beams for solid state physics, medicine and radioactive waste management.

Abstract: Ternary particles (TP's) from the thermal neutron induced fission of 229Th, 233U and 239Pu have been studied. Their energy distributions and yields have been determined using the mass separator Lohengrin at the high flux reactor of the Institut Laue-Langevin (ILL). Special emphasis was put on the search of the heaviest ternary particles. As a new result it has been found that the mass and/or charge spectra of the heaviest TP's at the limit of detectability of present techniques are strongly correlated with the mass and/or charge of the fissioning compound nucleus. The actual limit of detection reached is about 1 TP in 1010 fission reactions. At this limit, in Th-fission the heaviest TP's are Ne-isotopes, while for 239Pu-fission the observable TP's are shifted to Si-isotopes. The energy distributions and yields of several TP's having so far not been reported from the reactions 229Th(n ,f), 233U(n ,f) and 239Pu(n ,f) are given. The dependence of the yields of the TP's on the fissility of the fissioning compound nucleus for actinides ranging from Th to Cf is shown. Experimental yields are compared to predictions in the framework of the Halpern model.