Fusion-fission of light nuclear systems

Considerable interest has been devoted to fusion reactions between light heavy ions specially between weakly bound ones, due to the anomalous decrease of the fusion cross sections when compared to the total reaction cross section in the energy region around the barrier [1–4]. While the exact nature of the process responsible for the fusion cross section limitation at barrier energies is still unclear, this study shows an inhibition of the yield as the system mass decreases, resulting from the progressive increase of the barrier height and decrease of the effective barrier radius [3]. Furthermore, extensive efforts have been made recently in the study of energy-damped binary yields from light heavy-ion collisions [2,4]. Based on the substantial amount of data accumulated so far, it is now generally accepted and supported by the transition state model [4], that the observed yields arise mostly from a fusion-fission process. Data on complete fusion, fusion-fission and ‘elastic fission’ for the ⁹Be, ^10,11B+^10,11B; ^16,17,18O + ^10,11B; ¹⁹F+¹²C; ^6,7Li+⁹Be, ¹²C reactions among others, are presented. For the loosely bound nuclei it was found that the severe fusion cross section limitation is due to a low survival probability of the weakly bound nuclei until the instant of the collision [1].     

Effect of lone-pair orientation on nuclear spin couplings

The ² J _nh and ¹ J _ch couplings involving ¹⁵N and ¹³C in oximes have been studied theoretically by the finite perturbation method employing CNDO/2 and INDO wave functions. The couplings are found to be significantly influenced by the orientation of the nitrogen lone-pair orbital. It has been shown that the dihedral-angle dependence of vicinal H-H coupling in methylamine is also influenced by the orientation of the lone-pair on nitrogen, leading to deviations in the couplings predicted by a Karplus-type equation. The localized molecular orbitals and the hybridization and orientation of the nitrogen lone-pair in these molecules have been obtained by the Edmiston-Ruedenberg transformation of the semi-empirical SCF molecular orbitals.     

Fusion-fission of heavy systems

The influence of the entrance channel on fission processes was studied by forming the same composite system by two different target-projectile combinations (⁴⁰Ar+²⁰⁹Bi and⁵⁶Fe+¹⁸⁷Re, respectively). Compound nucleus fission and quasi fission were observed and the analysis was performed in the framework of the extra-extra-push model, which provides a qualitative interpretation of the results; limits for the extra-extra-push threshold are given, but problems with quantitative predictions for the extra-push are noted.     

Nuclear orientation and nuclear structure

The purpose of this paper is to demonstrate how recent experimental results of¹⁵¹Eu and⁵⁷Fe high pressure Mössbauer studies in 4f and 3d metallic magnetic systems can contribute to a deeper understanding of the nature of local moment (4f) and itinerant (3d) magnetism in these systems. Special emphasis is given on the comparison of the experimental results with related theoretical models.     

On-line nuclear orientation at KOOL

Since the previous conference we extended our series of on-line -anisotropy measurements in the Z=50 and Z=82 region leading a.o. to the identification of the phase transition in the Au isotopes. Besides conventional on-line low temperature nuclear orientation, we introduced the first on-line applications of NMR/ON, on-line -anisotropy measurements and -decay asymmetry experiments. The intriguing information from these new developments both in the field of nuclear structure and solid state physics is briefly discussed.     

Relaxation phenomena in nuclear orientation

A survey is given of relaxation phenomena relevant to nuclear orientation at temperatures below 1 K. Relaxation mechanisms, time dependence of the angular distribution of nuclear radiation and experimental methods for relaxation measurements are reviewed. Finally, some open questions related to simultaneous influence of rf fields and relaxation, the existence of a spin temperature and the role of domain walls are discussed shortly.     

Applications of low temperature nuclear orientation

The techniques of low temperature nuclear orientation (LTNO) and nuclear magnetic resonance on oriented nuclei (NMRON) are versatile tools used across a range of applications. Such applications include nuclear moment and hyperfine field measurements and testing of nuclear decay models, as well as condensed matter studies, particularly applied to magnetism. Following the tradition of such presentations, the techniques LTNO and NMRON, are briefly outlined along with the principal applications with a focus on some recent works.     

On-line nuclear orientation of70As

On-line nuclear orientation was performed on⁷⁰As. Anisotropy data have been obtained at 8.7mK for 29 gamma transitions. Multipole mixing ratios have been extracted for 17 transitions. Our results are in good agreement with existing data for the ten previously measured transitions. Our first time data for the 1496 keV, 2⁺→2⁺ γ-ray suggest the 2536 keV level is a good candidate for a mixed symmetry level.     

On line nuclear orientation: A rapidly developing technique

Nuclear orientation facilities are operating fully on-line to acclerators/isotope separators capable of producing wide ranges of radioisotopes. The lowest on-line base temperatures are close to 7 mK and the shortest half-life yet studied is 0·9 s. Experimental results yield magnetic dipole interaction strengths, nuclear spin-lattice relaxation times, directional distributions of alpha, beta and gamma emissions and gamma ray linear polarization. Nuclear magnetic resonance has been observed on-line.The broad range of the technique in opening up systematic measurements of nuclear magnetic moments and excited state structures is stressed, illustrated by examples from current work.     

Dynamic nuclear polarization and nuclear orientation in an antiferromagnet

Dynamic nuclear polarization is a well established technique which has been used to produce polarized targets for experiments in nuclear physics. This paper suggests experiments of a similar type but involving the nuclear magnetic resonance of two isotopes, one stable and the other radioactive. The substance is an antiferromagnet, dysprosium phosphate, at temperatures below the Néel point, where line widths are comparatively small. The effect may be detected through changes in the rate of gamma ray emission observed by a nuclear orientation experiment.     

Dynamic nuclear polarization and nuclear orientation in terbium vanadate

Dynamic nuclear polarization is a well established technique, which has been used to produce polarized targets for experiments in nuclear physics. This paper suggests new experiments, involving the nuclear magnetic resonance of two isotopes, one stable, the other radioactive, in an antiferromagnet, terbium vanadate. At temperatures well below the Néel point, the line widths should be comparatively small. Resonance may be detected through changes in the rate of gamma-ray emission observed by a nuclear orientation experiment.     

Fusion-fission of heavy and superheavy nuclei

The process of fusion-fission of heavy and superheavy nuclei (SHE) with Z=82–122 formed in the reactions with ⁴⁸Ca and ⁵⁸Fe 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 multidetector DEMON. As a result of the experiments, mass and energy distributions (MED) of fission fragments; fission, quasifission, and evaporation residue cross sections; and multiplicities of neutrons and γ-quanta and their dependences on the mechanism of formation and decay of compound systems have been studied.     

Magnetic moment measurement by nuclear orientation

This review deals with problems concerning ground and long-lived metastable states. The interest of magnetic moments in the knowledge of the nuclear configurations and the couplings is shown. Static, dynamic and transient orientations, and the angular distribution of the emitted radiation by oriented nuclei are discussed.Some problems involved in the extraction of magnetic moment values from low temperature nuclear orientation measurements will be discussed: combined magnetic and electric interactions, intermediate state perturbation, Knight shift, hyperfine anomaly, and thermal reorientation. Methods allowing to find the sign of the magnetic moment are also outlined.     

Magnetic structure determined by nuclear orientation

Conclusion In conclusion we suggest that nuclear orientation is a useful technique for determining nuclear spin structures. For a known hyperfine interaction atomic magnetic structures may also be deduced. Measurements on antiferromagnets and the rare earth magnet holmium demonstrate the method, although in the latter case the turn angle cannot be determined because of the limitation of L2 radiation. These experiments show that nuclear orientation can at least complement neutron diffraction and at best furnish information about magnetic structure when the latter technique is not applicable. We intend to study other rare earth magnets and more complicated antiferromagnetic structures.     

Fusion-fission dynamics and perspectives of future experiments

The paper is focused on reaction dynamics of superheavy-nucleus formation and decay at beam energies near the Coulomb barrier. The aim is to review the things we have learned from recent experiments on fusion-fission reactions leading to the formation of compound nuclei with Z≥102 and from their extensive theoretical analysis. Major attention is paid to the dynamics of formation of very heavy compound nuclei taking place in strong competition with the process of fast fission (quasifission). The choice of collective degrees of freedom playing a fundamental role and finding the multidimensional driving potential and the corresponding dynamic equation regulating the whole process are discussed. A possibility of deriving the fission barriers of superheavy nuclei directly from performed experiments is of particular interest here. In conclusion, the results of a detailed theoretical analysis of available experimental data on the “cold” and “hot” fusion-fission reactions are presented. Perspectives of future experiments are discussed along with additional theoretical studies in this field needed for deeper understanding of the fusion-fission processes of very heavy nuclear systems.     

Low temperature nuclear orientation of238NpGd

The decay of²³⁸Np oriented at low temperatures in gadolinium host was investigated. Multipole mixing ratios for beta-transition to the level 1028.6 keV and nine gamma-transitions in²³⁸Pu were calculated from the anisotropies of gamma radiation. The attenuation factor for the isomeric state of²³⁸Pu at 1082.6 KeV and orientation parameters for²³⁸NpGd were deduced from the experimental results.     

On-line nuclear orientation of odd-odd120I

The use of Low Temperature Nuclear Orientation (LTNO) in nuclear structure studies is discussed. LTNO has been employed, together with γ-γ and e-γ spectroscopy, to study the nuclear structure of¹²⁰Te following the beta decay of¹²⁰ I ^g . The strengths and limitations of LTNO as a probe of nuclear structure are discussed with particular reference to this experiment.     

On-line nuclear orientation study of184Au

The¹⁸⁴Au→¹⁸⁴Pt decay, studied on-line with the UNISOR facility at HHIRF, is discussed. Gamma-ray and conversion-electron spectroscopy of¹⁸⁴Pt as well as on-line nuclear orientation measurements of¹⁸⁴Au were done. A new low-lying level scheme of¹⁸⁴Pt is proposed. Two coexisting bands with different deformations and their respective γ-vibrational bands are established. Internal conversion coefficients for interband transitions between states with the same spin are extracted from the spectroscopy measurements. The relative E0 contents of the transitions are determined by combining internal conversion coefficients with E2/M1 mixing ratios deduced from gamma-ray anisotropies measured from oriented nuclei.