Effect of neutron transfer channels in fusion reactions with weakly bound nuclei at subbarrier energies

The role of neutron transfer in fusion reactions of weakly-bound nuclei at subbarrier energies is studied within the empirical model of channel coupling. The results from calculating the fusion cross sections for the ⁷Li + ²⁰⁹Bi, ^{9, 11}Li + ^{208, 206}Pb, ^{6, 7, 9, 11}Li + ¹⁵²Sm reactions are presented. Good agreement with the available experimental data is shown. Several combinations of colliding nuclei for which the strong enhancement of subbarrier fusion due to the effect of neutron transfer processes are predicted.     

Alignment effects in subbarrier fusion reactions

Subbarrier fusion reactions in which one constitutent is deformed and aligned are considered in the framework of a coupled-channel model with incoming-wave boundary conditions. The effect of alignment on the fusion cross section is estimated to be 40% for aligned ¹⁶⁵Ho. The semiclassical analog to the coupled-channel equations is developed, yielding an expression for the cross section in terms of a weighted average over all orientations of the deformed nucleus.     

Reactions with 9Be at subbarrier energies

The ⁹Be+⁹Be, ⁹Be+¹⁰B, ⁹Be+¹¹B and ⁹Be+¹³C reactions have been studied at subbarrier energies measuring the cross sections for the characteristic γ-rays emitted by the residual nuclei and comparing them with the statistical evaporation prediction. A substantial enhancement in the αn channel, which contains the the α-transfer reaction from ⁹Be, has been observed in all the systems studied. Evidence suggesting that the α-transfer reaction is not exclusively a direct transfer process is being presented.     

Cluster transfer reactions at low energies

Acta physica Academiae Scientiarum Hungaricae - The cluster transfer processes below the Coulomb barrier are investigated for arbitrary charges and angular momentum transfer, taking into account...     

Proton and neutron radii of208Pb from transfer reactions and single particle energies

Neutron and proton transfer cross sections and single-particle energies are converted into rms radii of density distributions via independent particle wave functions. The radii of the neutron excess and the analogous “proton excess” are found to be the same. Comparison is made with the results of other experiments.     

Limiting excitation energies in fusion evaporation reactions

The mass dependence of the critical excitation energy above which a hot nucleus does not decay any more by particle evaporation, has been studied in the mass range 75≦A≦130. The experimental values of the critical excitation energy have been obtained by means of a new integral method based on the analysis of the widths of the evaporation residue mass distributions as a function of the residue velocity. The obtained mass dependence appears to be stronger than expected by various models and a predicted dependence on theZ/A ratio of the decaying nucleus seems to be confirmed.     

Experiments on screening effect in deuteron fusion reactions at extremely low energies

The enhanced electron screening effect in nuclear reactions taking place in dense astrophysical plasmas is extremely important for determination of stellar reaction rates in terrestrial laboratories as well as in prediction of cross sections enhancement in interiors of stars such as White and Brown Dwarfs or Giant Planets. This effect resulting in reduction of the nuclear Coulomb potential by the atomic electrons has been confirmed in many laboratory experiments. Unfortunately, experimental screening energies are much higher than the theoretical predictions and the reason for that remains unknown. Here, we present absorbing results of the experiment studying d + d nuclear reactions in different deuterized metallic targets under ultra high vacuum conditions. The total cross sections and angular distributions of the ²H(d,p)³H and ²H(d,n)³He reactions have been measured using a deuteron beam of energies between 8 and 30 keV provided by the electron cyclotron ion source. The atomic cleanness of the target surface has been secured by combining Ar sputtering of the target and Auger electrons spectroscopy. Due to application of an on-line analysis method, the homogeneity of the implanted deuteron densities could be continuously monitored. We will discuss probable causes of the large discrepancy between theoretical and experimental data.     

Study of fusion cross-section in heavy-ion fusion-fission reactions at around fusion barrier energies using the Langevin dynamical approach

The neutron detector with ³He -filled counters placed in the focal plane of the VASSILISSA separator is used for measuring the average number and determining the multiplicity distribution of prompt neutrons from the spontaneous fission of heavy short-lived isotopes. The test reaction $\ensuremath \mathrm{{}^{48}Ca}+\mathrm{{}^{206}Pb}=2{\rm n}+\mathrm{{}^{252}No}$ is used for tuning the separator settings and calibrating the detector system with the spontaneous fission of the ²⁵²No . The average neutron number per ²⁵²No spontaneous fission event is as large as $\ensuremath \bar{\nu}=4.06 \pm 0.12$ . The short-lived heavy isotope ²⁴⁴Fm , produced in the complete fusion reaction ⁴⁰Ar + ²⁰⁶Pb , is investigated. The average number of neutrons per spontaneous fission of ²⁴⁴Fm from the experimental data ( $\ensuremath \bar{\nu}=3.3 \pm 0.3$ is determined for the first time.     

Cross sections of fusion and transfer reactions in interactions between 3He and Pt and 197Au nuclei in the subbarrier energy region

The reactions of complete and incomplete fusion, along with nucleon transfer reactions in the range of ³He energies from 10 to 24.5 MeV, are investigated by irradiating gold and platinum targets with an accelerated ³He ion beam on the U-120M cyclotron at the Institute of Nuclear Physics, Czech Academy of Sciences, ?e?. Activation is used to determine the yield of nuclides resulting from nuclear reactions. The γ-activity induced in the targets is measured using a with high-resolution HPGe detector. Despite the low binding energy of ³He and the reactions with positive Q values, the measured cross sections of fusion reactions exhibit no specific features, compared to reactions conducted using beams of other light stable particles. Transfer neutron reactions in the subbarrier energy region have relatively high cross sections. These cross sections continue to grow as the energy of ³He rises. When a neutron is captured by a nucleus of ³He, the cross sections of these reactions reach their maximum values in the Coulomb barrier region.     

Cross section measurements for the 9Be+9Be System at subbarrier energies

The total reaction cross section and the characteristic y-ray cross sections have been measured for the ⁹Be+ ⁹Be reaction in the energy range E_cm = 1.4–3.4 MeV, detecting the prompt γ-rays emitted by the various residual nuclei with two Nal detectors in nearly 4π geometry and with a germanium detector, respectively. The differential elastic cross sections for the same system have also been measured at e_c.m.= 2.2, 2.7 and 3.2 MeV. The cross sections calculated with the “standard” and the proximity optical model potentials, which describe well the total reaction cross sections of the light nuclei, agree with the ⁹Be + ⁹Be elastic-scattering data, but underpredict the total reaction cross section by a factor of 2 to 3. The characteristic γ-ray measurements show that all two-particle emission channels, nα ¹³C, nn¹⁶O, np¹⁶N and αα¹⁰Be are enhanced by about that factor, while the single-particle emission channel, p¹⁷N, is not enhanced.     

Angular and energy dependences of cross sections for deuteron electrodisintegration at subbarrier energies

A method is proposed for analyzing a large set of spectra calculated for the Coulomb breakup of deuterons. The regularities in the behavior of the respective cross section as a function of the emission angles and energies of particles constituting the deuteron are studied on the basis of this method. The deuteron-breakup reaction on lead nuclei at an energy of E _d = 8 MeV is considered by way of example. It is shown that the approximate Landau-Lifshitz formulas for this process are unable to predict some of its features. An experiment is proposed for studying the physical nature of the Nemets effect.     

Heavy-ion fusion at low energies

Through precision measurements of fusion cross sections at energies close to the Coulomb barrier and through the application of the method of “experimental barrier distributions” which these permit, many recent advances have been made in our understanding of the dynamical processes occurring during a heavy-ion collision. It is now clear that the target and projectile reach one another in superpositions of states which correspond to different orientations for rotational nuclei or to different induced deformations for vibrational nuclei. The creation of a neck of neutron matter has also long been postulated and by studying the isotopic dependence of the fusion reaction, some recent results in the ¹⁰Ca+^90,96Zr systems appear to confirm this result. For large Z ₁ Z ₂ a type of extra-push effect can arise from the same inelastic entrance-channel effects which enhance the fusion of lighter systems, though this will be absent in cases where the enhancement arises from neutron transfers. The existence of different barriers will of course influence all other reaction channels. Fusion simply allows one to visualise the barriers most easily, since for this process, the total cross section is an incoherent sum of the contributions from all relevant eigenchannels. Some effects in other channels have already been observed. Other possible effects will be discussed. These include; the exploitation of the lowest-energy barrier to produce exotic evaporation residues and strongly deformed high-spin states at low excitation energy.     

Measurement of evaporation residue cross-sections of the reaction 30Si + 238U at subbarrier energies

The reaction ³⁰Si + ²³⁸U → ²⁶⁸Sg^* was studied at beam energies close to the Coulomb barrier. At a center-of-mass energy of E _c.m. = 144.0MeV for reactions at half thickness of the target we measured three decay chains of ²⁶³Sg produced by evaporation of five neutrons. The cross-section was ( 67⁺⁶⁷ _-37) pb. At E _c.m. = 133.0MeV we measured three spontaneously fissioning nuclei which we assigned to the isotope ²⁶⁴Sg. The production cross-section was ( 10⁺¹⁰ _-6) pb and a half-life of ( 120⁺¹²⁶ _-44) ms was determined. This half-life is a factor of twenty shorter than theoretical predictions. At E _c.m. = 128.0MeV an upper cross-section limit of 15pb was measured. The cross-section data reveal a strong influence of the orientation of the deformed target nucleus on the production yield. Compared to excitation functions measured for the lighter system ¹⁶O + ²³⁸U → ²⁵⁴Fm^*, a reduction of the fusion probability was observed at low beam energies indicating increasing competition from quasifission processes.     

Analysis of the27Al(p, γ)28Si reaction at subbarrier energies in terms of the direct-semidirect model

Differential cross sections forγ-transitions to 12 states in²⁸Si following nonresonant proton capture in²⁷Al nuclei (E _p=1,625 keV) were measured and analysed in terms of the direct-semidirect model. The experimental data are reproduced only when the complex coupling constant with the GDR is enhanced for thef partial wave in the entrance channel.     

Ion-molecule reactions at thermal energies

Ion-molecule reactions is a generic word for reactions involving ions (both positive and negative), radicals and stable neutrals. In this presentation, use of the flowing afterglow technique to study ion-molecule reactions at thermal energies is demonstrated using the examples of positive ion-negative ion mutual neutralization of molecular nitrogen ion (N ₂ ⁺ ) with F⁻ and the reaction of atomic nitrogen with SF _n (n=1 to 5) to form NF.     

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     

Multistep direct reactions at low energies

The cross sections for neutron inelastic scattering by niobium at 20 MeV and for the charge-exchange (p,n) reaction on copper at 27 MeV have been calculated using the multistep direct reaction theory of Feshbach, Kerman and Koonin [1]. The theory was modified to include the non-DWBA matrix elements and the collective vibrations. The results show enhanced contributions from two-, three- and four-step direct reactions in agreement with experiment.