Coulomb Barrier Energies Research Articles

Reaction mechanism study of 7Li(7Li,6He) reaction at above Coulomb barrier energies

The elastic scattering and the 6He angular distributions were measured in 7Li + 7Li reaction at two energies, Elab = 20 and 25 MeV. FRDWBA calculations have been performed to explain the measured 6He data. The calculations were very sensitive to the choice of the optical model potentials in entrance and exit channels. The one-step proton transfer was found to be the dominant reaction mechanism in 6He production.

Energy dependence of the nucleus-nucleus potential close to the Coulomb barrier

The nucleus-nucleus interaction potentials in heavy-ion fusion reactions are extracted from the microscopic time-dependent Hartree-Fock theory for the mass symmetric reactions ${}^{16}\mathrm{O} + {}^{16}\mathrm{O}$, ${}^{40}\mathrm{Ca} + {}^{40}\mathrm{Ca}$, and ${}^{48}\mathrm{Ca} + {}^{48}\mathrm{Ca}$ and the mass asymmetric reactions ${}^{16}\mathrm{O} + {}^{40, 48}\mathrm{Ca}$, ${}^{40}\mathrm{Ca} + {}^{48}\mathrm{Ca}$, ${}^{16}\mathrm{O} + {}^{208}\mathrm{Pb}$, and ${}^{40}\mathrm{Ca} + {}^{90}\mathrm{Zr}$. When the c.m. energy is much higher than the Coulomb barrier energy, potentials deduced with the microscopic theory identify with the frozen density approximation. As the c.m. energy decreases and approaches the Coulomb barrier, potentials become energy dependent. This dependence indicates dynamical reorganization of internal degrees of freedom and leads to a reduction of the ``apparent'' barrier felt by the two nuclei during fusion of the order of 2--3% compared to the frozen density case. Several examples illustrate that the potential landscape changes rapidly when the c.m. energy is in the vicinity of the Coulomb barrier energy. The energy dependence is expected to have a significant role on fusion around the Coulomb barrier.

Full coupled-channel description of three-body and four-body breakup reactions

Recent development of the continuum-discretized coupled-channels method (CDCC) is reported with introducing the eikonal CDCC method (E-CDCC) and four-body CDCC. The former is applied to analysis of 8B breakup by 208Pb at 52 A MeV and the astrophysical factor S 17 ( 0 ) for the 7Be( p , γ ) 8B reaction is extracted with the asymptotic normalization coefficient method. We obtain S 17 ( 0 ) = 20.9 −1.9 +2.0 eV b , which is significantly larger than the previous one, S 17 ( 0 ) = 18.9 ± 1.8 eV b , determined from an analysis with the virtual photon theory. The difference between the two values is found to be due to the contributions from nuclear breakup and higher-order processes. The latter, four-body CDCC, is applied to 6He+ 209Bi scattering near the Coulomb barrier energy and found to reproduce the elastic cross sections well, in contrast to the standard three-body CDCC. Accurate treatment of the three-body continuum states of 6He turns out to be essential to correctly describe the 6He+ 209Bi scattering.

Elastic scattering for the system 11Be + 209Bi at Coulomb barrier energies

The elastic scattering process for 11Be ions impinging on a 209Bi target was studied in the energy range around the Coulomb barrier. The angular distributions of the 11Be scattered particles were analyzed within the optical model framework in order to evaluate the reaction cross section, which turned out to be much larger than the fusion one, especially in the sub-barrier region. The comparison with the system 9Be + 209Bi showed that the reaction cross section is larger for the reaction 11Be + 209Bi in the energy range around the Coulomb barrier, while they are rather similar at higher bombarding energies. This result suggests that direct processes related to the 11Be halo structure and lower binding energy are more relevant at near-barrier energies.

Coulomb breakup effects on the elastic cross section ofHe6+Bi209scattering near Coulomb barrier energies

We accurately analyze the $^6$He+$^{209}$Bi scattering at 19 and 22.5 MeV near the Coulomb barrier energy, using the continuum-discretized coupled-channels method (CDCC) based on the $n$+$n$+$^4$He+$^{209}$Bi four-body model. The three-body breakup continuum of $^6$He is discretized by diagonalizing the internal Hamiltonian of $^6$He in a space spanned by the Gaussian basis functions. The calculated elastic and total reaction cross sections are in good agreement with the experimental data, while the CDCC calculation based on the di-neutron model of $^6$He, i.e., the $^2n$+$^{4}$He+$^{209}$Bi three-body model, does not reproduce the data.

Studies of neutron-rich isotopes with the CPT mass spectrometer and the CARIBU project

The heavy neutron-rich isotopes area is the least explored region of the nuclear landscape. Although sensitive techniques exist to gather the required information on these isotopes, they just have not been made available in sufficient quantity and with the right properties for many of the most basic studies. Recent measurements at the Canadian Penning trap (CPT) mass spectrometer, using isotopes produced from the fission of Cf 252 stopped in the CPT gas catcher system, have allowed the mass of a number of neutron-rich isotopes to be determined. This approach is being further pursued in the CARIBU project with the installation of a new dedicated heavily shielded source and gas catcher that will yield four orders of magnitude higher neutron-rich isotope yield at low energy for mass measurements and Coulomb barrier energy for nuclear structure studies.

Single nucleon transfer reactions near Coulomb barrier on 197Au

Single neutron transfer reactions on 197Au induced by 12C and 16O at near Coulomb barrier energies have been studied using radiochemical and off-line gamma-ray spectrometric techniques. High spin yield fraction (HSF = σm/(σm+σg)) for 196m,gAu and formation cross section ratio (σ198/σ196) of 198Au and 196Au have been determined for both 197Au+12C and 197Au+16O reactions at different excitation energies near the Coulomb barrier. The HSF of 196m,gAu and σ198/σ196 values in both reactions are seen to sharply increase with increasing excitation energy near the Coulomb barrier and then slowly increase reaching a constant value at high projectile energy. These observations have been analyzed in terms of nuclear structural and kinematic parameters. A comparison the effects of Coulomb excitation and nucleon transfer process during heavy-ion interactions indicates a predominant effect of the latter process in the background of the Coulomb excitation. Observed reaction systematics seem to follow the quasi-elastic transfer process.

Simultaneous DWBA-description of elastic, fusion and 1 n-transfer reactions on the system 9Be + 208Pb for energies around the barrier energy

A distorted wave born approximation is applied to give a simultaneous description of elastic scattering, total reaction, fusion and one neutron transfer cross sections for the system 9Be + 208Pb at energies below and above the Coulomb barrier energy. In the approach, Woods–Saxon optical potentials are applied. For the entrance channel, the imaginary part W of the optical potential U is split into two parts; a volume part W F which is responsible for fusion absorption and a surface part W DR which accounts for direct reactions. The optical potential parameters are determined from a simultaneous χ 2 -analysis of recent experimental data of (a) elastic scattering and complete fusion and (b) elastic scattering and total fusion (complete + incomplete). In this manner, an account of a single α-particle partial fusion can be given. The one-neutron transfer cross section is determined with the same elastic distorted waves obtained from U. A search of the threshold anomaly is made for the real and imaginary parts of U although a no conclusive determination of it can be given. In general the results obtained in the calculations are in close agreement with the data.

Breakup dynamic polarization potential forHe6+Pb208: Energy dependence and generic properties

We extend to a series of higher energies an earlier study, R. S. Mackintosh and N. Keeley, Phys. Rev. C 70, 024604 (2004), of the local dynamic polarization potential for the $^{6}\mathrm{He}+^{208}\mathrm{Pb}$ interaction. The dynamic polarization potential generated by the coupling to breakup channels is determined using the CDCC-plus-inversion method and does not involve semiclassical approximations. The long-ranged potential generated by the dipole breakup persists, with very little change in radial form and relative strength of real and imaginary terms, to energies far above the near Coulomb barrier energy. The dynamic polarization potentials presented here extend from 10 fm, well inside the strong absorption radius, out to 60 fm (i.e., including the transition region from where nuclear excitation dominates to the asymptotic region dominated by electric dipole excitation). Real and imaginary components of the dynamic polarization potential each contribute substantially to the scattering.

Elastic scattering of 17F on 208Pb and 17F breakup cross section at Coulomb barrier energies

We have studied the elastic scattering of the exotic 17 F nuclei from 208 Pb at Coulomb barrier energies. For the measurement we used a new detector array with pixel structure and a large solid angle. Optical potential analysis shows that 17 F has an imaginary potential much less deep than 19 F. Information on the exclusive 17 F breakup cross section has been deduced too.

6Li breakup from 208Pb target at Coulomb barrier energies: doorway to reaction mechanism induced by loosely bound/halo nuclei

systemhave beendeduced. Comparisonoftheexperimentaldatawithtwodifferenttheoretical approaches strongly supports the direct breakup process description.1. INTRODUCTIONThe growing interest and effort of the nuclear physicists community for experimentsperformed with Radioactive Nuclear Beams (RIBs), has triggered several experimentsthat can be performed using stable beams and could give a first taste of what kind ofdifficulties and results we will face once RIBs will be available. In this perspective, it’snatural to choose stable beams that have characteristics very close to those of the RIBs.For example, if one considers the binding energy of the nuclei, it’s immediately clear thatsome stable light nuclei are weakly bound, like many light RIBs:

Which Degrees of Freedom Play a Role in Multinucleon Transfer Processes?

A presentation of some recent significant results obtained in the field of multinucleon transfer reactions at Coulomb barrier energies is given. The comparison of the experimental observables with the predictions of theoretical models treating quasi-elastic and deep-inelastic processes on the same ground reveals interesting features of the reaction mechanisms not clearly identified in the past.

Nucleon Coalescence in the Interaction of 16 O with 59 Co at an Incident Energy of 250 MeV

We have observed the emission of intermediate mass fragments with a maximum yield at the Coulomb barrier energy in the interaction of 250 MeV 16O ions with 59Co. These emissions, which become dominant at emission angles larger than 20o, are explained by assuming that they are produced by nucleon coalescence during the thermalization of the composite nuclei produced in the complete and incomplete fusion of 16O with 59Co.

Semiclassical character and optical model description of heavy ion scattering, direct reactions, and fusion at near-barrier energies

An approach is proposed to calculate the direct reaction (DR) and fusion probabilities for heavy ion collisions at near-Coulomb-barrier energies as functions of the distance of the closest approach D within the framework of the optical model that introduces two types of imaginary potentials, DR and fusion. The probabilities are calculated by using partial DR and fusion cross sections, together with the classical relations associated with the Coulomb trajectory. Such an approach makes it possible to analyze the data for angular distributions of the inclusive DR cross section, facilitating the determination of the radius parameters of the imaginary DR potential in a less ambiguous manner. Simultaneous ${\ensuremath{\chi}}^{2}$ analyses are performed of relevant data for the ${}^{16}\mathrm{O}{+}^{208}\mathrm{Pb}$ system near the Coulomb-barrier energy.

Effects of nuclear deformation on fusion probability in the reactions of82Se+natCeand76Ge+150Ndnear the Coulomb barrier

Evaporation residue (ER) cross sections for ${}^{82}\mathrm{Se}{+}^{\mathrm{nat}}\mathrm{Ce}{(Z}_{1}{Z}_{2}=1972)$ were measured in the vicinity of the Coulomb barrier and the fusion probability was obtained with the aid of calculated survival probability. This fusion probability was compared with that for ${}^{76}\mathrm{Ge}{+}^{150}\mathrm{Nd}{(Z}_{1}{Z}_{2}=1920),$ which was determined with the same manner by using the data published by us. The former system represents fusion of two spherical nuclei, the latter fusion involving the prolately deformed target ${}^{150}\mathrm{Nd}.$ The collision of ${}^{76}\mathrm{Ge}$ with the side of ${}^{150}\mathrm{Nd}$ is the most compact configuration at touching. The system ${}^{82}\mathrm{Se}{+}^{\mathrm{nat}}\mathrm{Ce}$ showed fusion hindrance in the form of extra-extra-push energy of $27\ifmmode\pm\else\textpm\fi{}5\mathrm{MeV},$ whereas the system ${}^{76}\mathrm{Ge}{+}^{150}\mathrm{Nd}$ does not show fusion hindrance at and above the Coulomb barrier energy, suggesting that the reaction starting from the compact touching point results in a higher fusion probability.

Coupled-channel analysis of scattering and fusion reactions on the system [formula omitted] for energies below and above the Coulomb barrier energy

Elastic, 2 1 +-inelastic and fusion cross sections of the nuclear system 16 O + 60 Ni are calculated for several energies below and above the Coulomb barrier energy. The calculations are done by using a unified coupled-channel approach, that is consistent with the direct reaction theory and can describe elastic, inelastic and fusion reactions simultaneously. All the cross sections are calculated with only one set of parameters of the optical potential where the imaginary part W has been divided into two parts, W F responsible for fusion reactions and W DR responsible for other absorption processes. The inelastic channels explicitly included in the CC-calculation are 2 1 + and 3 1 − of the target and 3 1 − of the projectile, it is found that fusion through the 2 1 +-excitation channel of the target gives the main contribution. It is also shown that, elastic, inelastic and fusion cross section below and above the Coulomb barrier energy can be simultaneously fitted only if the fusion radius parameter r F of W F takes values larger than those normally assumed in other theories.

Fusion cross sections for 6Li+ 12C and 6Li+ 13C reactions at low energies

The partial cross sections for the 6Li+ 12C and 6Li+ 13C reactions below and above the Coulomb barrier energy have been determined from the cross sections of the characteristic γ-rays measured with HPGe detectors and using the branching factor σ γ σ ch obtained from the statistical model calculations. The fusion cross sections obtained from the sum of these partial cross sections are found to be equal to the total reaction cross sections upto an energy well above the Coulomb barrier energy and there appears to be no evidence for limitation of fusion cross sections for these systems at such energies contrary to the evaporation residue measurements. The measured cross sections are also found to agree nicely with the IWBC (Incoming Wave Boundary Condition) and one-dimensional BPM (Barrier Penetration Model) calculation.

Structure and reactions at Coulomb-barrier energies

A variety of nuclear reactions at energies close to the Coulomb barrier are used to populate residual nuclei in order to study their structure through methods such as γ-ray spectroscopy. The most common of these is the fusion-evaporation reaction. Recent detailed studies of heavy-ion fusion have shown how this reaction proceeds not through a single Coulomb barrier but through a distribution of barriers whose properties are intimately related to the collective modes of the target and projectile. This paper will explore possible ways of exploiting structure effects in the entrance channel to populate systems of interest in the compound nucleus and its evaporation residues. This leads to a complementarity of charged-particle and γ-ray work. Some comments will also be made on the complementarity of experiments with stable and exotic beams in the same energy region.

A new 14 GHz electron-cyclotron-resonance ion source for the heavy ion accelerator facility ATLAS

A 14 GHz electron-cyclotron-resonance (ECR) ion source has been designed and built at Argonne National Laboratory. The source is a modification of the AECR [D. J. Clark, C. M. Lyneis, and Z. Q. Xie, 14th Particle Accelerator Conference (PAC), IEEE Conference 91 CH3038-7, 1991 (unpublished), p. 2796 and C. M. Lyneis, Z. Q. Zie, D. J. Clark, R. S. Lam, and S. A. Lundgren, 10th International Workshop on ECR Ion Sources, Oak Ridge, ORNL CONF-9011136, 1990 (unpublished), p. 47.] at Berkeley and incorporates the latest results from electron-cyclotron-resonance (ECR) developments to produce intense beams of highly charged ions, including an improved magnetic confinement of the plasma electrons with an axial mirror ratio of 3.5. The aluminum plasma chamber and extraction electrode as well as a biased disk on axis at the microwave injection side donate additional electrons to the plasma, making use of the large secondary electron yield from aluminum oxide. The source is capable of ECR plasma heating using two different frequencies simultaneously to increase the electron energy gain for the production of high charge states. The main design goal is to produce several eμA of at least U35+238 in order to accelerate the beam to coulomb-barrier energies without further stripping. First charge state distributions for gaseous elements have been measured and 210 eμA16O7+ has been achieved. A normalized, 90% emittance from 0.1 to 0.2 π mm mrad for krypton and oxygen beam has been found.

Quasielastic transfer in the136Xe+64Nireaction

Single and multinucleon transfer yields for the {sup 136}Xe+{sup 64}Ni reaction at a scattering energy {approx}5{percent} above the Coulomb barrier energy are studied using particle{endash}{gamma}-ray coincidence data. Q-value and scattering-angle distributions are extracted for the stronger channels. A fast transfer mechanism dominates the yields to these channels over an extended Q-value range, leading to a concentration of the cross section near the grazing angle. Analysis of the angular distributions based on a semiclassical barrier penetration model suggests that the single-nucleon and two-neutron exchange channels are dominated, respectively, by direct and two-step sequential transfer from the ground or low-lying excited states of the participating nuclei. The multiproton transfer channels have angular distributions that indicate a more complex mechanism, although direct cluster transfer from an excited configuration cannot be fully discounted. In a separate analysis, the relative population of different mass channels is found to be in general agreement with the expectations of a {open_quotes}random walk{close_quotes} model of particle exchange. {copyright} {ital 1997} {ital The American Physical Society}