Sub-barrier fusion of35, 37Cl+58, 64Ni evidence of effects of proton transfer channels

Sub-barrier fusion excitation functions of^35,37Cl+^58,64Ni have been measured. They are discussed in comparison to similar data of^34,36S+^58,64Ni with the aim of revealing the influence of coupled proton transfer channels. Using the simplified coupled channel code CCFUS, differences in the effects of inelastic channels are eliminated in an approximative manner to stress the differences in the effects of transfer channels. Signatures of positiveQ-value transfer channels coupled to fusion are clearly identified.     

Collective and transfer modes in the sub-barrier fusion of several A CN ∼ 100 systems

By using some simple model assumptions, the relative contribution of collective and transfer degrees of freedom to the sub-barrier fusion of several systems is investigated. Previously measured data for ³²S + ^58,64Ni, ³⁷Cl + ^58,60,62,64Ni, ³⁷Cl + ^{70,72,73,74,76}Ge, and ²⁷Al + ^{70,72,73,74,76}Ge are analyzed. Even though the effects of collective degrees of freedom are dominant, transfer effects are shown to be important for many systems in order to account for the trend of the lower energy points. Under simplified assumptions for the transfer strength, a distance for transfer is estimated for some systems. An apparent inconsistency found in previous analysis of the Cl + Ge data is solved.     

Near-barrier fusion of Sn + Ni and Te + Ni systems: examining the correlation between nucleon transfer and fusion enhancement

The fusion excitation functions for radioactive (132)Sn + (58)Ni and stable (130)Te + (58,64)Ni were measured at energies near the Coulomb barrier. The coupling of transfer channels in heavy-ion fusion was examined through a comparison of Sn + Ni and Te + Ni systems, which have large variations in the number of positive Q-value nucleon transfer channels. In contrast with previous experimental comparisons, where increased sub-barrier fusion cross sections were observed in systems with positive Q-value neutron transfer channels, the reduced excitation functions were equivalent for the different Sn + Ni and Te + Ni systems. The present results suggest a dramatically different influence of positive Q-value transfer channels on the fusion process for the Sn + Ni and Te + Ni systems.     

Spin polarization in 16O-induced heavy-ion reactions

The circular polarization P_γ of γ-rays from ¹⁶O-induced reactions on ²⁷Al, ⁴⁸Ti, ⁵⁸Ni and ⁶²Ni at 100 MeV incident energy has been measured in coincidence with projectile-like fragments detected at angles behind the grazing angle. The dependence of P_γ on the reaction Q-value, on the fragment atomic number and, for ¹⁶O+⁴⁸Ti, on the scattering angle was investigated.The results show that deep-inelastic (DI) collisions in these fairly light heavy-ion systems are associated with predominantly negative scattering angles. The average value of P_γ(DI) = 0.6 obtained for ¹⁶O + Ni at θ_lab= 35° is consistent with pure negative-angle scattering. The magnitude of P_γ(DI) is smaller for the two lighter systems which is interpreted as mainly arising from the increasing probability for orbiting trajectories leading to scattering angles beyond 180°Trajectory calculations within a classical friction model were performed, and the parameter dependence of the results was studied. Strong radial friction forces were found necessary to simulate the observed negative-angle cross sections.     

Decay of proton-rich nuclei between 39Ti and 49Ni

Decay studies of very neutron-deficient nuclei ranging from ³⁹Ti to ⁴⁹Ni have been performed during a projectile fragmentation experiment at the GANIL/LISE3 separator. For all nuclei studied in this work, ^{39, 40}Ti, ^{42, 43}Cr, ⁴⁶Mn, ^{45, 46, 47}Fe and ⁴⁹Ni, half-lives and decay spectra have been measured. In a few cases, γ coincidence measurements helped to successfully identify the initial and final states of transitions. In these cases, partial decay scheme are proposed. For the most exotic isotopes, ³⁹Ti, ⁴²Cr, ⁴⁵Fe and ⁴⁹Ni, which are candidates for two-proton radioactivity from the ground state, no clear evidence of this process is seen in our spectra and we conclude rather on a delayed particle decay. Received: 26 July 2000 / Accepted: 20 December 2000     

Near-barrier transfer and fusion of the systems33S +90,91,92Zr

Double differential cross sections of all prominent transfer channels have been measured in the systems³³S +^99,91,92Zr at two energies close to the nominal Coulomb barrier. In addition the fusion excitation functions of these systems have been measured below and around the barrier. The angular- andQ-distributions of the most important transfer reactions have been analysed in the framework of a simple semiclassical formalism. Particularly the two-nucleon transfer angular distributions exhibit strong multi step coupling effects which manifest themselves in reduced cross sections at large angles corresponding to close distances. From the angular distributions at forward angles, where a single step character of the transfer reaction can be assumed, approximate form factors have been extracted employing a first order perturbation theory. Within the uncertainties of a schematic coupled channels calculation the isotopic differences of the sub-barrier fusion enhancement can be understood on the basis of the isotopic differences of the transfer form factors andQ-values.     

Exact finite-range DWBA description of the60Ni(160,12C)64Zn reaction at 60 MeV incident energy

Cross section angular distributions of⁶⁰Ni(¹⁶O,¹²C)⁶⁴Zn reactions leading to three strongly excited states at 60 MeV incident energy and the¹⁶O+⁶⁰Ni and¹²C+⁶⁴Zn elastic scattering at 60 MeV respectively 45 MeV and 54 MeV have been measured using aQ3D magnetic spectrograph. EFR-DWBA calculations assuming the transfer of anα-cluster in its 0s ground state are able to describe the general features of the strongly oscillating experimental angular distributions using a surface transparent optical model potential. The optical model parameters used in the DWBA calculations are obtained from fits of the elastic scattering data of incident and exit channels. The importance of “correct” optical model parameters in the exit channels for relative spectroscopic factors will be discussed and the extracted relative spectroscopic factors will be compared to previous (⁶Li,d) results.     

Pre-equilibrium γ ray emission in different reaction mechanisms at 8 MeV/nucleon

Recent experimental results on particles-γ coincidence measurements on the systems ¹²C +⁶⁴Ni and ³⁵Cl +⁶⁴Ni at about 8 MeV/nucleon are interpreted as due to the effect of a Dipole pre-equilibrium emission produced during the damping of the proton-neutron relative collective motion in very deformed intermediate systems formed in the first instants of the collision. The pre-equilibrium effects are evaluated through semiclassic kinetic theories and through modified statistical approach to include non-stationary effects in Fusion processes, Massive Transfer reactions (in the ¹²C +⁶⁴Ni system) and in Binary Dissipative reactions (in the ³⁵Cl +⁶⁴Ni system). In particular the study performed on the dipole molecular component allows to establish a link between the above phenomenon and the charge and mass transfer process in quasi-peripheral reactions. Received: 15 April 1998 / Revised version: 8 June 1998     

Strong enhancement of two-neutron transfer in the system 206Pb+118Sn

One and two neutron transfer has been measured in the heaviest asymmetric nuclear system with semi magic nuclei showing superfluid properties, in ²⁰⁶Pb+¹¹⁸Sn collisions at an energy well below the Coulomb barrier with scattering orbits covering the largest angles. Particle-γ coincidence techniques using 5 Euroball-Cluster detectors (EB) combined in a set-up with the Heidelberg-Darmstadt NaI-Crystal Ball (CB) have been used. Transfer channels are identified with EB via their known γ-decays of the lowest excited states. Using the unique feature of the set-up with the CB, transfer to well defined final states with known quantum numbers (without feeding) are selected using the high efficiency multiplicity filter of the CB (no second γ-ray). The data are analysed using the semiclassical approach and transfer probabilities are obtained. The enhancement for the two-neutron transfer populating the low lying superfluid 2⁺ state in ¹²⁰Sn (and ¹¹⁶Sn), while the Pb-branch is in the groundstate is deduced by comparison with the strongest single neutron transfer transition. Large enhancements (EF ≃ 10³) are observed. This is the first direct measurement of enhancement for a heavy nuclear binary system with experimentally separated levels suggesting a strong contribution from superfluid pair transfer. Received: 18 December 1998 / Revised version: 23 February 1999     

Gap levels of Ti<Superscript>3+</Superscript> on Nb or Li sites in LiNbO<Subscript>3</Subscript>:(Mg):Ti crystals and their effect on charge transfer processes

The Ti dopant occupying Li or Nb sites and the charge transfer processes induced by thermochemical reduction and optical bleaching treatments have been investigated in LiNbO₃ systems using optical absorption and EPR. The Ti³⁺ centers, built preferentially at Nb sites in heavily Mg-codoped crystals, are shown to have absorption bands at 1.62±0.08 eV and 2.65±0.25 eV, which are similar or slightly redshifted compared to Ti³⁺ centers at Li sites in LiNbO₃:Ti crystals. The Ti_Nb^4+/3+ gap level plays an important role in the trapping of electron-polarons in LiNbO₃, double-doped with Mg and Ti; in particular, an enhanced optical detrapping sensitivity for pumping in the 1.3–2.8 eV range is observed, which may be relevant for applications in integrated optics. Evidence indicating the possible existence of bipolarons involving the Ti dopant is presented. PACS 77.84.Dy; 71.55.Ht; 71.38.-Mx; 78.40.Ha; 76.30.Fc     

Fusion excitation function measurements for the 16O+58Ni and 16O+62Ni systems

High precision fusion excitation functions have been measured for the ¹⁶O+⁵⁸Ni and ¹⁶O+⁶²Ni systems from which fusion barrier distributions have been evaluated. Coupled-reaction-channels (CRC) calculations, which describe elastic and quasi-elastic scattering, also satisfactorily reproduce the fusion cross sections and barrier distributions. The small value of Z₁Z₂ in this case leads to barrier distributions with relatively little structure. However, in conjunction with the detailed elastic scattering data for these systems, this allows us to elucidate the role of previously ignored states in ¹⁶O in pushing the entire distribution to lower energies. These shifts are consistent with derived magnitudes of polarization potentials for both systems.     

Competition between the fusion-fission and deep-inelastic orbiting mechanisms in the35Cl +12C reaction

The binary decay properties of the⁴⁷V nucleus, produced in the³⁵Cl +¹²C reaction, have been investigated at the³⁵Cl bombarding energiesE _lab = 180 and 200 MeV by means of a kinematical coincidence technique. Binary reaction products show full energy equilibration and a characteristic 1/sin(θ _cm) angular distribution. The elemental distribution of the fully-damped products is asymmetric, similar to what has previously been observed in the decay of the⁵⁶Ni nucleus. Comparison with theoretical model predictions suggests the occurrence of a fusion-fission rather than orbiting process. Moreover the calculations performed using the Extended Hauser-Feshbach Method reproduce well the experimental fission yields. A general discussion of orbiting and fusionfission experimental data of light heavy-ion systems is presented in the framework of the calculated number of available open channels for these systems.     

Enhanced neutron pair transfer and collective excitations in the system 206Pb + 118Sn at barrier energies

At energies below the Coulomb barrier, neutron transfer and Coulomb excitation have been measured in a very heavy asymmetric nuclear system, in ²⁰⁶Pb + ¹¹⁸Sn. These are semi-magic nuclei showing super-fluid properties. Particle-γ coincidence techniques using 5 Euroball Cluster detectors (EB), combined in a set-up with the Heidelberg-Darmstadt NaI Crystal Ball (CB), have been used. Position-sensitive detectors allowed the observation of scattering processes covering angles from 110 up to 150 degrees. The fragments are identified via the known γ-decays of the lowest excited states using the high resolution of EB. Using the unique feature of the set-up with the CB, transfer to well-defined final channels with known quantum numbers is selected using the high-efficiency multiplicity filter of the CB with no second γ-ray, i.e. without feeding. The data are analysed using the semi-classical approach and transfer probabilities are obtained. Coulomb excitation has been analysed using known transition probabilities. The enhancement is deduced for the two-neutron transfer populating the low-lying super-fluid 2⁺ states in ¹²⁰Sn and ¹¹⁶Sn, while the 2n transition remains in the ground state for the ^20NPb nuclei. Large enhancements up to EF ≃ 10³ are observed. This is the first observation of neutron pair transfer enhancement for a heavy nuclear binary system with super-fluid properties with experimentally separated levels. The calculations with microscopic 2-neutron wave functions, with configuration mixing over six shell model configurations and using the coupled reaction channels approach, reproduce well the observed probabilities and the enhancement. Received: 27 August 2002 / Accepted: 9 December 2002 / Published online: 25 March 2003 RID="a" ID="a"e-mail: oertzen@hmi.de Communicated by D. Schwalm     

Transfer reactions and sub-barrier fusion in 40Ca + 90, 96Zr

The two systems ⁴⁰Ca + ^90,96Zr have been studied by measuring nucleon transfer reactions at two energies near the Coulomb barrier, thus complementing the available sub-barrier fusion cross-sections. Angular distributions for various transfer channels have been determined. Significantly larger neutron transfer cross-sections are found for the target ⁹⁶Zr that exhibits the larger enhancement in the sub-barrier fusion cross-sections. All data have been analyzed with a new model for heavy-ion collisions that calculates simultaneously transfer cross-sections, fusion excitation functions and barrier distributions. The model gives a good account of both transfer and fusion data. Received: 2 May 2002 / Accepted: 4 June 2002 / Published online: 26 November 2002 RID="a" ID="a"e-mail: montagnoli@pd.infn.it, Fax +39049 8277102, Tel. +39049 8277117. RID="b" ID="b"On leave from the China Institute for Atomic Energy, 102413 Beijing, China. Communicated by C. Signorini     

Elastic and inelastic scattering of 58Ni +90,94Zr

Pure elastic and inelastic scattering cross sections have been measured for the systems ⁵⁸Ni +^90,94Zr at energies near the Coulomb barrier where not only quasi-elastic and fusion but also deep-inelastic process come into play. Coupled channels calculations including both projectile and target inelastic excitations can successfully explain the elastic and inelastic scattering angular distributions with an energy-independent semi-empirical bare potential. The calculation reproduces also the sum of the total quasi-elastic, fusion and deep-inelastic cross sections. Received: 14 September 1998 / Revised version: 21 October 1998     

O-与CH3CN反应机理的密度泛函理论研究

采用G3MP2B3方法研究了氧负离子与乙腈反应的势能剖面.在(U)B3LYP/6-31+G(d,p)理论水平下分别优化了该反应势能面上反应物、产物、中间体和过渡态的分子结构,采用G3MP2B3方法校正了这些关键点的能量. 势能面上的各个反应路径均通过针对过渡态的內禀反应坐标理论计算加以确定. 分别考察了四个可能的热力学产物通道,即质子转移、氢原子转移、H₂⁺转移和双分子亲核取代反应途径. 其中,经H₂⁺转移生成H₂O的反应通道为该反应的主要产物通道.     

Mössbauer Study of the Ni/Ca0.8Sr0.2Ti1−x Fe x O3−α Catalyst System for Partial Oxidation of Methane to Synthesis Gas

The Ni/Ca_0.8Sr_0.2TiO₃ catalyst system prepared by the citrate method shows high activity in partial oxidation of methane to synthesis gas. It is assumed that the interaction of Ni with the perovskite lattice may be responsible for the increased catalytic activity. 1% ⁵⁷Fe dopant substituted for Ti was used in order to see if the presence of Ni has any perturbation effect on the structure of the perovskite. One may expect systematic changes in the Mössbauer parameters of the substitutional Fe impurity as a function of the NiO content if the bulk properties of the perovskite are affected. Samples with different Ni/Ca_0.8Sr_0.2Ti_0.99 ⁵⁷Fe_0.01O_3– ratios from 0:1 to 1:1, and others having Fe substitutions for Ti up to 30%, all prepared by the citrate method, have been investigated. The Mössbauer spectra contained doublets of paramagnetic Fe³⁺ and Fe⁴⁺ species as well as paramagnetically relaxed Fe³⁺. These species were assigned to the bulk perovskite, the perovskite surface and the NiO/perovskite interface. The perturbation of the perovskite structure by Ni could not be verified.     

Transfer of1H and13C dynamic nuclear polarization from immobilized nitroxide radicals to flowing liquids

In this study,¹H and¹³C dynamic nuclear polarization (DNP) was generated at a magnetic field strength of 0.33 T utilizing silica phase immobilized nitroxide (SPIN) samples. The polarization was subsequently transferred to flowing liquids and monitored at a magnetic field strength of 4.7 T. These solid/liquid intermolecular transfer (SLIT) experiments provide efficient polarization transfer without the necessity of the free radical system present in the monitoring fluid. Specifically, ultimate¹H SLIT DNP Overhauser enhancements of ?56 and ?110 have been observed for benzene and chloroform in the presence of SPIN system 2, respectively. The¹³C SLIT DNP enhancement for benzene is dominated by three-spin effects and poor leakage factors (f _c). However, a particularly favorable case is the chloroform/SPIN 2 system which exhibits a scalar dominated enhancement. For this case, positive enhancements 40–60 times the¹³C thermal Boltzmann magnetization at 4.7 T have been observed. The large scalar dominated¹³C DNP enhancement for this system represents one of the largest experimental enhancements reported to date. The¹³C DNP spectra for other samples which exhibit favorable scalar¹³C dominated enhancements (e.g., Freon 113) are also presented. Three different SPIN systems were also prepared and characterized in the present study.     

Scattering and reactions of 14C + 14C and 14C + 14C

We have studied elastic scattering, inelastic scattering and several transfer channels of the systems ¹⁴C + ¹⁴C and ¹⁴C + ¹²C over a wide range of energies up to E_c.m. = 35 eMeV and 32 MeV, respectively. The reaction channels were identified by means of kinematic coincidences between solid-state detectors, γγ coincidences were measured to determine cross sections for mutual inelastic scattering of ¹⁴C + ¹⁴C.Pronounced regular gross structures, similar to those found for ¹⁶O + ¹⁶O, are observed in the elastic excitation function of ¹⁴C + ¹⁴C at θ_c.m. = 90°, The angular distributions measured at the energies of the maxima and an optical-model analysis suggest that one or a few surface partial waves dominate the scattering behaviour. Correlated structure of narrower width is found in the inelastic channels and, to a lesser degree, in the transfer channels which appear with rather small cross sections.In ¹⁴C + ¹²C elastic scattering the gross structures are strongly fragmented, in contrast to ¹⁴C + ¹⁴C but similar to ¹²C + ¹²C. While the ¹²C(2⁺) excitation is very weak, the observed strengths of the ¹⁴C(3^?) excitation and of neutron transfer point to a substantial role of these channels as coupling partners to the elastic configuration and to their influence on the elastic scattering behaviour. A correlated intermediate structure is observed near 23.5 MeV, where a dominance of l = 18 is suggested by the elastic scattering angular distribution. This unexpectedly high l-value exceeds l_graz at this energy by at least two units of ?.