Barrier Penetration Models Research Articles

Fusion and proton transfer in the B10+Al27 system at sub-barrier energies

Background: Significant enhancement in sub-barrier fusion cross sections by several orders of magnitude, in the frame of one-dimensional barrier penetration models, is observed for a variety of nuclear systems. To understand this, one needs to find the fundamental degrees of freedom relevant in the sub-barrier fusion region by incorporating nonelastic channels such as coupling of the excited target and/or projectile nuclei, particle transfer channels, threshold variation of nuclear potential, etc.Purpose: We probe what degrees of freedoms are associated with the $^{10}\mathrm{B}+^{27}\mathrm{Al}$ reaction at energies $\ensuremath{\approx}18%$ below to $\ensuremath{\approx}12%$ above the Coulomb barrier.Method: An online $\ensuremath{\gamma}$-ray spectroscopy technique is used to determine the populated excitation functions in the $^{10}\mathrm{B}+^{27}\mathrm{Al}$ reaction. The reaction cross sections obtained from these $\ensuremath{\gamma}$ rays are compared with the statistical model to explore the mechanism of fusion reactions.Results: At energies near and below the Coulomb barrier, two isotopes of sulfur ($^{34}\mathrm{S}$ and $^{32}\mathrm{S}$) were found to reproduce cross section within the frame of the pace2 model based on Hauser-Feshbach calculations, whereas isotopes of chlorine ($^{35}\mathrm{Cl}$), sulfur ($^{35}\mathrm{S}$), and phosphorous ($^{32}\mathrm{P}$) agree well but at above-barrier energies. Isotopes of Ar, Cl, S, P, and Si show significant enhancement over an entire band of energies. This enhancement (suppression) in the experimental cross section at energies above (below) the barrier points towards the involvement of some nuclear reaction mechanism other than fusion-evaporation. Distorted-wave Born approximation calculations strongly support the population of $^{28}\mathrm{Si}$ residue via the one-proton transfer process. The total fusion cross section is found to follow the theory after the inclusion of coupling of low lying excited states of projectile and target. Calculations concerning the astrophysical $S$ factor and the logarithmic derivative factor support the idea of no fusion hindrance at the studied energies. The universal fusion function benchmark shows consistency with previous data for lithium (or Li isotopes) induced systems.Conclusions: The inclusion of inelastic couplings associated with both target and projectile is important to understand the behavior of total fusion cross section. The formation of $^{28}\mathrm{Si}$ via the one-proton transfer reaction supports the present experimental technique to make inclusive transfer measurements. The calculations with the universal fusion function present a slight suppression in the higher energy region, indicating possible breakup effects in this region. The present calculations show that till $18%$ below the barrier there is no fusion hindrance. On extrapolating the theoretical values we further observed that the present system has no fusion hindrance. No connection of $^{10}\mathrm{B}+^{27}\mathrm{Al}$ fusion with astrophysics has been made, which raises the question to think of cosmic ray reactions and perhaps reactions in the early solar system, but how the fusion reaction would fit into the picture is not clear at all.

Effect of increasing neutron-excess on the fusion cross-section in 12−15C + 12C at above-barrier energies

Fusion excitation functions at energies above the fusion barrier for 12−15C + 12C are examined. From these excitation functions the average fusion cross-section, <σF>, is calculated. For this isotopic chain, the measured dependence of <σF> on neutron excess is compared with static barrier penetration models. The more rapid increase observed for the experimental cross-sections above the geometric increase predicted by the static models suggests that the stronger dependence on neutron-excess measured may be attributable to dynamics. Calculations with a time-dependent Hartree-Fock model also fail to describe the observed trend suggesting that for neutron-rich nuclei, neutron dynamics may play a larger role than is presently accounted for.

Barrier distributions and signatures of transfer channels in theCa40+Ni58,64fusion reactions at energies around and below the Coulomb barrier

Background: The nuclear structure of colliding nuclei is known to influence the fusion process. Couplings of the relative motion to nuclear shape deformations and vibrations lead to an enhancement of the sub-barrier fusion cross section in comparison with the predictions of one-dimensional barrier penetration models. This enhancement is explained by coupled-channels calculations including these couplings. The sub-barrier fusion cross section is also affected by nucleon transfer channels between the colliding nuclei.Purpose: The aim of the present experiment is to investigate the influence of the projectile and target nuclear structures on the fusion cross sections in the $^{40}\mathrm{Ca}+^{58}\mathrm{Ni}$ and $^{40}\mathrm{Ca}+^{64}\mathrm{Ni}$ systems.Methods: The experimental and theoretical fusion excitation functions as well as the barrier distributions were compared for these two systems. Coupled-channels calculations were performed using the ccfull code.Results: Good agreement was found between the measured and calculated fusion cross sections for the $^{40}\mathrm{Ca}+^{58}\mathrm{Ni}$ system. The situation is different for the $^{40}\mathrm{Ca}+^{64}\mathrm{Ni}$ system where the coupled-channels calculations with no nucleon transfer clearly underestimate the fusion cross sections below the Coulomb barrier. The fusion excitation function was, however, well reproduced at low and high energies by including the coupling to the neutron pair-transfer channel in the calculations.Conclusions: The nuclear structure of the colliding nuclei influences the fusion cross sections below the Coulomb barrier for both $^{40}\mathrm{Ca}+^{58,64}\mathrm{Ni}$ systems. Moreover, we highlighted the effect of the neutron pair-transfer channel on the fusion cross sections in $^{40}\mathrm{Ca}+^{64}\mathrm{Ni}$.

Interaction spatial barrier distributions for the proton-halo system 8B+58Ni

Radial barrier distributions for the proton-halo system 8B+58Ni are obtained by the use of Woods-Saxon polarization potentials calculated from a fit to elastic scattering angular distribution data. The strength, diffuseness and reduced radii of the polarization potentials, besides fitting the data, are such that reproduce the barrier height and position as extracted from a Wong's fit to the universal behavior of reduced interaction cross sections for halo systems. It is found that, for the proton-halo system 8B+58Ni, the interaction barrier height VI and the interaction barrier radius RI result correspondingly lower and larger than the Coulomb nominal barrier height VB and barrier position RB as expected from Barrier Penetration Models and double folding density dependent interactions such as the São Paulo Potential. It is also shown that, the determined energy dependent polarization potentials, besides satisfying the dispersion relation, present a behavior consistent with the Breakup Threshold Anomaly. Keywords. Nuclear Reactions, weakly bound nuclei, threshold anomaly.

Comparison of brain capillary endothelial cell-based and epithelial (MDCK-MDR1, Caco-2, and VB-Caco-2) cell-based surrogate blood–brain barrier penetration models

An accurate means of predicting blood–brain barrier (BBB) penetration and blood–brain partitioning of NCEs (new chemical entities) would fulfill a major need in pharmaceutical research. Currently, an industry-standard BBB drug penetration model is not available. Primary brain capillary endothelial cells, optionally co-cultured with astrocytes and/or pericytes, are the most valued models of BBB. For routine use, establishing and maintaining a co-culture system is too costly and labor intensive. Alternatively, non-cerebral cell lines such as MDCK-MDR1 are used, and most recently, the suitability of native and modified Caco-2 for predicting brain penetration has also come under investigation. This study provides comparative data on the morphology and functionality of the high integrity brain capillary endothelial BBB model (EPA: triple culture of brain capillary endothelial cells with pericytes and astrocytes) and the epithelial cell-based (native Caco-2, high P-glycoprotein expressing vinblastine-treated VB-Caco-2 and MDCK-MDR1) surrogate BBB models. Using a panel of 10 compounds VB-Caco-2 and MDCK-MDR1 cell lines show restrictive paracellular pathway and BBB-like selective passive permeability that makes them comparable to the rat brain BBB model, which gave correlation with the highest r2 value with in vivo permeability data. In bidirectional assay, the VB-Caco-2 and the MDCK-MDR1 models identified more P-glycoprotein drug substrates than the rat brain BBB model. While the complexity and predictive value of the BBB model is the highest, for the screening of NCEs to determine whether they are efflux substrates or not, the VB-Caco-2 and the MDCK-MDR1 models may provide a simple and inexpensive tool.

Near-barrier Fusion and Breakup/Transfer induced by Weakly Bound and Exotic Halo Nuclei

The influence on the fusion process of coupling to collective degrees of freedom has been explored. The significant enhancement of the fusion cross section at sub-barrier energies was compared to predictions of one-dimensional barrier penetration models. This was understood in terms of the dynamical processes arising from strong couplings to collective inelastic excitations of the target and projectile. However, in the case of reactions where at least one of the colliding nuclei has a sufficiently low binding energy, for breakup to become an important process, conflicting model predictions and experimental results have been reported in the literature. Excitation functions for sub- and near-barrier total (complete + incomplete) fusion cross sections have been measured for the 6,7Li+59Co reactions. Elastic scattering as well as breakup/transfer yields have also been measured at several incident energies. Results of Continuum-Discretized Coupled-Channel (Cdcc) calculations describe reasonably well the experimental data for both reactions at and above the barrier. A systematic study of 4,6He induced fusion reactions with a three-body Cdcc method is presented. The relative importance of breakup and bound-state structure effects on total fusion (excitation functions) is particularly investigated. The four-body Cdcc model is being currently developed.

Constructing Optimum Blood Brain Barrier QSAR Models Using a Combination of 4D-Molecular Similarity Measures and Cluster Analysis

A new method, using a combination of 4D-molecular similarity measures and cluster analysis to construct optimum QSAR models, is applied to a data set of 150 chemically diverse compounds to build optimum blood-brain barrier (BBB) penetration models. The complete data set is divided into subsets based on 4D-molecular similarity measures using cluster analysis. The compounds in each cluster subset are further divided into a training set and a test set. Predictive QASAR models are constructed for each cluster subset using the corresponding training sets. These QSAR models best predict test set compounds which are assigned to the same cluster subset, based on the 4D-molecular similarity measures, from which the models are derived. The results suggest that the specific properties governing blood-brain barrier permeability may vary across chemically diverse compounds. Partitioning compounds into chemically similar classes is essential to constructing predictive blood-brain barrier penetration models embedding the corresponding key physiochemical properties of a given chemical class.

Fusion and breakup in reactions involving weakly bound nuclei

Fusion and breakup in reactions involving weakly bound nuclei A. Szanto de Toledo”, F.A. Souza”, C. Beckb, S.J. Sandersc, M.G. Munhoza, J Takahashi”, N. Carlin”, A.A.P. Suaide”, M.M. de Moura” and E.M. Szanto” “Instituto de Fisica - Universidade de Slo Paulo, Deptartamento de Fisica Nuclear, Laboratorio Pelletron, CP 66318, 05315-970, Sao Paulo - SP, Brazil bCentre National de la Recherche Scientifique Universitd Louis Pasteur, 23 rue du Loess, B.P. 28, F-67037 Strasbourg Cedex 2, France ‘University of Kansas, Lawrence, KS 66045, USA The effect of collective degrees of freedom on the fusion process has been extensively investigated over the past few years [l]. A significant enhancement of the sub-barrier fu- sion cross section is often found as compared to the predictions of one-dimensional barrier penetration models. This enhancement is understood in terms of dynamical processes involving couplings to collective inelastic excitations of the target and/or projectile. A precise determination of the “barrier distributions” leading to the enhancement requires an understanding of the dominant channels that couple to the fusion channel[2]. However, in the case of reactions where at least one of the colliding nuclei has a sufficiently low binding energy so that breakup becomes an important process, conflicting experimental and theoretical results have been reported [3-111. The many questions regarding the influence of breakup become more relevant with the recent availability of radioactive beams and the renewed interest in super-heavy element formation. Radioactive ion beams are likely to produce intense breakup yields. In the fusion processes, and more specifically in the fusion of weakly bound nuclei, that can be used in super-heavy element studies, two different and independent processes can be distinguished both experimentally and theoretically. One, denoted as “Complete Fusion” (CF), is associated with the capture of all of the projectile constituents by the target. The other, denoted as “Incomplete Fusion” (ICF) or Partial Fusion, occurs when part of the projectile is captured by the target and the remaining part escapes. “Total Fusion” is understood as the sum of these two processes (CF + ICF). In order to avoid misinterpre- tations, a clear definition of the experimental and theoretical quantities being compared is essential. This is the main difficulty in comparing data and/or calculations from different authors. Depending on the theoretical approach, different results are achieved. If fusion occurs incoherently, i.e., with possible breakup of the incoming particle [4,5], the survival probability of this particle prior to fusion, is lower than unity, resulting in a decrease in the effective entrance-channel flux and a corresponding reduction of the fusion cross section. On the other hand, if the breakup channel is coupled coherently to the fusion channel, as expected in references [3,6], entrance barrier fluctuations will lead to an effective lowering

No evidence of break-up effects on the fusion of 9Be with medium-light nuclei

Fusion cross sections were measured for the 9Be+27Al and 19F+9Be, 12C systems, at energies above the Coulomb barrier, in order to investigate the possible effect of fusion hindrance due to the break-up of the weakly bound nuclei. Comparisons with one-dimensional barrier penetration models and with other similar systems, where no break-up is expected to occur, show no evidence of fusion hindrance.

Inelastic scattering and nucleon transfer in the system232Th+206Pb near the Coulomb barrier

Nucleon transfer accompanied by Coulomb excitation was studied in the system206Pb+232Th atELab=6.4 MeV/u. Particle-particle-gamma coincidence techniques were used to identify excited states of reaction products populated through inelastic scattering and nucleon transfer reactions. The mean excitation energy was measured by means of aγ-ray energy and multiplicity filter consisting of 6 NaI detectors. Large cross sections for one-neutron and two-neutron pick-up from232Th are observed. The impact-parameter dependence of the neutron transfer is analyzed in terms of semiclassical barrier penetration models. Using realistic neutron potentials with a diffuse surface, the experimental data are in accordance with the assumption of a “cold” transfer to states near the yrast line.

Angular momentum effects in subbarrier fusion of 64Ni+100Mo.

Angular momentum distributions ${\ensuremath{\sigma}}_{l}$ for the compound nucleus $^{164}\mathrm{Yb}$ were deduced from measurements of \ensuremath{\gamma}-ray multiplicity for all significant evaporation residues from fusion of $^{64}\mathrm{Ni}$ and $^{100}\mathrm{Mo}$. At the lowest bombarding energies the ${\ensuremath{\sigma}}_{l}$ extend to higher l values than do predictions of barrier-penetration models that include coupling of the principal inelastic channels, even if the coupling strengths are increased to match the experimental excitation function. Likewise, ${\ensuremath{\sigma}}_{l}$ from a real potential with an energy-dependent shape fitted to the excitation function fails to reproduce the experimental ${\ensuremath{\sigma}}_{l}$ distribution. Better success is achieved with models in which fusion occurs at large distances.

Hindrance of fusion in central collisions of heavy, symmetric nuclear systems

The energy dependence of the fusion probabilities for central collisions of the systems124Sn +92,94,96Zr and86Kr +123Sb was determined from neutron-evaporation-residue cross sections. Near the barrier as calculated from one-dimensional barrier-penetration models, the fusion probability was found to be reduced by several orders of magnitude.

Global Increase of Near- and Below-Barrier Fusion for Heavier Systems

Twenty excitation functions for $^{12}\mathrm{C}$-, $^{16}\mathrm{O}$-, $^{40}\mathrm{Ar}$-, and $^{86}\mathrm{Kr}$-induced fusion were deduced from measured neutron production cross sections. A global increase of the near-barrier fusion is observed as reactant masses increase, which by far cannot be accounted for by one-dimensional barrier penetration models. Static properties such as deformation introduce only small modifications to this tendency, whereas the mean curvature radius provides a good scaling to this phenomenon, thus indicating shape distortions or neck formation.