Projectile Breakup Research Articles

Fusion, reaction and break-up cross sections of weakly bound projectiles on 64Zn

We present new measurements and a general discussion of the behavior of the fusion, break-up and reaction cross sections of different projectiles on the same target 64Zn, at near and above barrier energies. The projectiles are the tightly bound 16O, the stable weakly bound 6Li, 7Li and 9Be and the radioactive very weakly bound 6He nuclei. We also compare the results with the ones for heavier targets.

Further Study and Analysis of the 7 Li on 56 Fe Reaction at 50 MeV Incident Energy

We present deuteron and triton spectra measured at 12 laboratory angles from the reaction of 50 MeV 7Li on 56Fe together with evaporative component’s as modeled by CASCADE and fitted to the backward angle data. The deuteron and triton “break-up” spectra obtained by subtracting the evaporative components from the measured spectra are also presented. The break-up of the 7Li projectile near the surface occurs with high probability and the major fraction of the break-up cross-section is taken by (7Li, α) transfer process. A crude estimate of the fraction of the total cross-section is found to be of the same order of magnitude as the overall spectroscopic factor determined by the diffraction model. The value of the estimated fraction of total cross section at 50 MeV incident energy is compared to that at 68 MeV incident energy for the same reaction. However, these values of estimated fraction of total cross section are found very much consistent with the measured yields at both incident energies. The importance of the level density parameter in locating the maximum of excitation energy is indicated in the diffraction model.

209Bi(6He, α) reaction mechanisms studied near the Coulomb barrier using n–α coincidence measurements

The 6He+209Bi reaction displays a remarkably large cross section for α-particle emission at energies near the Coulomb barrier. The possible reactions that may produce the observed α particles include two-neutron transfer, one-neutron transfer, and direct projectile breakup. Each of these mechanisms results in a distinctive angular correlation between the α particle and outgoing neutron(s). A neutron-α-particle coincidence experiment was performed to separate these different modes. The neutron data show significant angular correlations. Monte Carlo simulations of one-neutron transfer are compared with the experimental data. It is shown that approximately 20% of the observed α-particle yield is due to this process.

Extended optical model analyses of elastic scattering and fusion cross sections for heavy-ion collisions with loosely bound projectiles at near-Coulomb-barrier energies

Within the framework of an extended optical model, simultaneous ${\ensuremath{\chi}}^{2}$ analyses are performed for elastic scattering and fusion cross-section data for $^{9}\mathrm{Be}+^{209}\mathrm{Bi}$ and $^{6}\mathrm{Li}+^{208}\mathrm{Pb}$ systems, both involving loosely bound projectiles, at near-Coulomb-barrier energies to determine the polarization potential as decomposed into direct reaction $(\text{DR})$ and fusion parts. We show that both $\text{DR}$ and fusion potentials extracted from ${\ensuremath{\chi}}^{2}$ analyses separately satisfy the dispersion relation, and that the expected threshold anomaly appears in the fusion part. The $\text{DR}$ potential turns out to be a rather smooth function of the incident energy, and has a magnitude at the strong absorption radius much larger than the fusion potential, explaining why a threshold anomaly has not been seen in optical potentials deduced from fits to the elastic-scattering data without such a decomposition. Using the extracted $\text{DR}$ potential, we examine the effects of projectile breakup on fusion cross sections ${\ensuremath{\sigma}}_{F}$. The observed suppression of ${\ensuremath{\sigma}}_{F}$ in the above-barrier region can be explained in terms of the flux loss due to breakup. However, the observed enhancement of ${\ensuremath{\sigma}}_{F}$ in the subbarrier region cannot be understood in terms of the breakup effect. Rather, the enhancement can be related to the $Q$ value of the neutron transfer within the systems, supporting the ideas of Stelson et al. [Phys. Lett. B 205, 190 (1988); Stelson et al.Phys. Rev. C 41, 1584 (1990)] that subbarrier fusion starts to occur when the colliding ions are at a distance where the barrier against the flow of the valence neutrons disappears and thus neutron exchange can take place freely.

Target proximity effect and dynamical projectile breakup at intermediate energies

Projectile binary breakup has been investigated in 58Ni+ 12C, 24Mg, 197Au at 34.5 MeV/A and 58Ni+ 70Zn at 40 MeV/A. The fragment angular distributions exhibit an anisotropic pattern showing that breakup is aligned with the direction of scattered quasi-projectile (QP). The correlation functions of the two heaviest fragments have been studied as a function of charge asymmetry. They suggest that the QP decays while still in close proximity of the target. The correlation between the charge and velocity of the two heavy fragments shows that the binary breakup of the QP might originate from an important deformation of the projectile by the target, and that the lighter of the colliding partners also contributes to the aligned emission pattern.

Spallation of 32S ions at ultra-relativistic energies in nuclear emulsion

Partial production cross sections of electromagnetic breakup of 32S projectiles at two widely differing energies were measured using an emulsion target. The electromagnetic dissociation represents about 6% and 17% of the total number of nuclear events at Elab/A = 3.7 and 200 GeV, respectively. The experimental electromagnetic cross sections σexpEMD show an energy dependence that is well reproduced by the Weizsacker–Williams approximation. The value of σexpEMD at 200A GeV (with γ >> 1) shows a good agreement with that predicted by the combined model of Pshenichnov et al. Examination of the reactions (γ, p) and (γ, He) reveals that the ratio of 1 p/1 He cross sections is close to unity at Dubna energy (3.7A GeV), while it rapidly increases to 3.0 ± 0.6 at CERN energy (200A GeV). In most of the observed dissociation modes, the total kinetic energy of the fragments in the projectile rest frame is lower than 50 MeV. Nevertheless, it is possible that the contribution of excitation modes of the different multipolarities is determining in this region. PACS No.: 25.70

Modelling Effects of Halo Breakup on Fusion

Theories of breakup and fusion of two-body projectiles are examined, to see how complete and incomplete fusion may be predicted separately. A proposal is made for an 'optical decoherence model' which uses semigroup evolution equations to describe the decoherence effects of the imaginary parts of optical potentials without also losing flux.

Optical Model Analyses of Elastic Scattering, Fusion, and Breakup Reaction Induced by Loosely Bound Nuclei

Within the framework of an extended optical model, simultaneous Χ 2 analyses are performed for elastic scattering and fusion cross section data for 6 He+ 2 0 9 Bi, 6 Li+ 2 0 8 Pb, and 9 Be+ 2 0 9 Bi systems, involving loosely bound projectiles, at near-Coulomb-barrier energies to determine the polarization potential decomposed into direct reaction (DR) and fusion parts. We show that both DR and fusion potentials extracted from Χ 2 analyses separately satisfy the dispersion relation and that the expected threshold anomaly appears in the fusion part. The DR potential turns out to be rather a smooth function of the incident energy, and has a magnitude at the strong absorption radius much larger than the fusion potential, explaining why a threshold anomaly has not been seen in optical potentials deduced from the fits to the elastic scattering data without such a decomposition. Using the extracted DR potential, we examine the effects of projectile breakup on fusion cross sections, σ F . The observed suppression of σ F in the above barrier region can be explained in terms of the flux loss due to breakup.

Role of virtual break-up of projectile in astrophysical fusion reactions

We study the effect of virtual Coulomb break-up, commonly known as the dipole polarizability, of the deuteron projectile on the astrophysical fusion reaction 3He(d,p)4He. We use the adiabatic approximation to estimate the potential shift due to the E1 transition to the continuum states in the deuteron, and compute the barrier penetrability in the WKB approximation. We find that the enhancement of the penetrability due to the deuteron break-up is too small to resolve the longstanding puzzle observed in laboratory measurements that the electron screening effect is surprisingly larger than theoretical prediction based on an atomic physics model. The effect of the 3He break-up in the 3He(d,p)4He reaction, as well as the 7Li break-up in the 7Li(p,alpha)4He reaction is also discussed.

Fusion of light weakly bound nuclei

The effect of breakup in the fusion cross section in terms of suppression versus enhancement, discussed in a conflicting way in the literature is addressed. Data and theoretical predictions available in the literature are compared. Excitation functions of the sub- and near-barrier fusion cross-sections for a wide variety of light and heavy systems are presented and interpreted. We have measured fusion excitation functions and breakup correlation functions for the medium weight systems 6Li + 59Co and 7Li + 59Co. These measurements help to establish the influence of the projectile breakup on the fusion process at near-barrier energies and contribute to the determination of how the mass of the target affects the breakup role. The results indicate a light fusion enhancement at sub-barrier energies and a geometry dominated cross section at barrier energies.

Is the inhibition/enhancement of fusion due to breakup still a puzzle?

The effect of breakup in the fusion cross section in terms of suppression versus enhancement, discussed in a conflicting way in the literature, is addressed. Data and theoretical predictions available in the literature are compared. Excitation functions of the sub- and near-barrier fusion cross-sections for a wide variety of light and heavy systems are presented and interpreted. We have measured fusion excitation functions and breakup correlation functions for the medium weight systems 6Li + 59Co and 7Li + 59Co. These measurements help to establish the influence of the projectile breakup on the fusion process at near-barrier energies and contribute to the determination of how the mass of the target affects the breakup role. The results indicate a light fusion enhancement at sub-barrier energies and a geometry dominated cross section at barrier energies.

Exclusive breakup of6Liby208Pbat Coulomb barrier energies

Differential and total cross sections for the exclusive breakup of a 6Li projectile into α+d and α+p fragments from a 208Pb target were measured at several energies around the Coulomb barrier. The α+d exclusive cross sections are well reproduced by full continuum discretized coupled channel calculations with excitation up to 11.5 MeV of the α+d (6Li*) system. The total exclusive cross sections are much smaller than the inclusive ones for the production of α particles. This large difference gives very clear experimental evidence for the stripping breakup mechanism. Such a process, relevant in the reaction dynamics even at the Coulomb barrier with loosely bound projectiles such as 6Li, needs to be carefully considered and described theoretically.Received 20 December 2002DOI:https://doi.org/10.1103/PhysRevC.67.044607©2003 American Physical Society

Coupling effects in the elastic scattering of6Heon12C

To study the effect of the weak binding energy on the interaction potential between a light exotic nucleus and a target, elastic scattering of ${}^{6}\mathrm{He}$ at 38.3 MeV/nucleon on a ${}^{12}\mathrm{C}$ target was measured at Grand Acc\'el\'erateur National d'Ions Lourds (GANIL). The ${}^{6}\mathrm{He}$ beam was produced by fragmentation. The detection of the scattered particles was performed by the GANIL spectrometer. The energy resolution was good enough to separate elastic from inelastic scattering contributions. The measured elastic data have been analyzed within the optical model, with the real part of the optical potential calculated in the double-folding model using a realistic density-dependent nucleon-nucleon interaction and the imaginary part taken in the conventional Woods-Saxon (WS) form. A failure of the ``bare'' real folded potential to reproduce the measured angular distribution over the whole angular range suggests quite a strong coupling of the higher-order breakup channels to the elastic channel. To estimate the strength of the breakup effects, a complex surface potential with a repulsive real part (designed to simulate the polarization effects caused by the projectile breakup) was added to the real folded and imaginary WS potentials. A realistic estimate of the polarization potential caused by the breakup of the weakly bound ${}^{6}\mathrm{He}$ was made based on a parallel study of ${}^{6}\mathrm{He}{+}^{12}\mathrm{C}$ and ${}^{6}\mathrm{Li}{+}^{12}\mathrm{C}$ optical potentials at about the same energies.

Breakup of 9Be on 209Bi above and near the Coulomb barrier as a molecular single-particle effect: Its influence on complete fusion and scattering

The breakup of the 9Be projectile on the 209Bi target at bombarding energies above and near the Coulomb barrier is studied in the adiabatic two-center shell model approach. The effect of 9 Be→ n+2α breakup channel on complete fusion, elastic and inelastic cross sections is investigated. Results show that the breakup of the projectile 9Be could be due to a molecular single-particle effect shortly before the colliding nuclei reach the Coulomb barrier.

H2+(20–100-keV) collisions with H: Dissociative and nondissociative capture and ionization and pure-H-target ionization

Collision of the molecular hydrogen ion, ${\mathrm{H}}_{2}^{+}$ in the energy range 20--100 keV with a H target has been investigated. Production of one or two fast protons or hydrogen atoms formed from the projectile breakup was distinguished by use of a Si-barrier detector. Cross sections were determined by coincidence-counting techniques between the target ions (separated by time-of-flight analysis), electrons, and one or two fast product particles. Capture processes are shown to dominate at the lower energies whereas pure target ionization is demonstrated to be the most likely process at 90 keV and higher energies. Such data are of importance in the understanding of astrophysical and high-temperature laboratory plasmas.

Measurement of 42 MeV 7Li projectile breakup on 208Pb target near grazing incidence

Breakup of 42 MeV 7Li projectiles in the field of a 208Pb target are studied near the grazing angle. In inclusive breakup measurements, angular distribution of the α and t fragments are found to be rather flat in nature. The α–t coincidence measurements are carried out with wide angular separations between the detected fragments. The choice of detection angles enables measurement in the same range of fragment relative energies but very different 7Li∗ scattering angles. The direct breakup contribution is found to dominate over the sequential breakup for the θα>θt set, while the reverse is true when θα<θt. Calculations performed in a full quantum mechanical framework reproduce the overall shape of the direct breakup part of the present data and several existing data on 197Au and 208Pb targets at nearby energies. The sequential breakup is observed only from the decay of the 4.63 MeV (7/2−) resonant excited state of 7Li and its angular distribution does not fully agree with the existing theoretical predictions. Unlike the direct part, the sequential breakup cross section does not decrease with increasing scattering angles. Similar measurements with halo nuclei would be helpful to study their unknown resonant states.

αbreakup of6Liand7Linear the Coulomb barrier

Angular distributions of the $\ensuremath{\alpha}$-particle production differential cross section from the breakup of ${}^{6}\mathrm{Li}$ and ${}^{7}\mathrm{Li}$ projectiles incident on a ${}^{208}\mathrm{Pb}$ target have been measured at seven projectile energies between 29 and 52 MeV. The $\ensuremath{\alpha}$-breakup cross section of ${}^{6}\mathrm{Li}$ was found to be systematically greater than that of ${}^{7}\mathrm{Li}$ across the entire energy range. These data have been compared with previously reported results and with the predictions of continuum-discretized coupled channels (CDCC) calculations including resonant and nonresonant projectile breakup. The present data compare well with previous measurements, while the CDCC calculations provide a reasonable prediction of the relative $\ensuremath{\alpha}$-breakup cross sections but underpredict their absolute values. The calculations confirm that a major factor in the enhancement of the ${}^{6}\mathrm{Li}$ to ${}^{7}\mathrm{Li}$ $\ensuremath{\alpha}$-breakup cross section is the difference between the $\ensuremath{\alpha}$-breakup thresholds of the two isotopes. These results have implications for structural studies of light exotic nuclei based on elastic scattering.

Evidence for a dissipative friction mechanism based on 8Be fragments from the interaction of 12C with 59Co

It is suggested that the spectra of the 8Be ground-state (gs) nuclei produced in the interaction of 12C with 59Co at incident energies varying from 8.3 to 33.3MeV/amu can be explained by introducing a dissipative friction interaction mechanism preceding projectile break-up.

Dynamical effects in nuclear collisions in the Fermi energy range: aligned breakup of heavy projectiles

Recent experimental results concerning heavy systems (Pb+Au, Pb+Ag, Pb+Al, Gd+C, Gd+U, Xe+Sn, …) obtained at GANIL with the INDRA and NAUTILUS 4π arrays will be presented. The study of reaction mechanisms has shown the dominant binary and highly dissipative character of the process. The two heavy and excited fragments produced after the first stage of the interaction can decay into various decay modes from evaporation to multifragmentation including fission. However, deviations from this simple picture have been found by analyzing angular and velocity distributions of light charged particles, and fragments. Indeed, there is a certain amount of matter in excess emitted between the two primary sources suggesting either the existence of a mid-rapidity source similar to the one observed in the relativistic regime (participants) or a strong deformation induced by the dynamics of the collision (neck instability). This last possibility has been suggested by analyzing in detail the angular distributions of the fragments. More precisely, we observe an isotropic component which is compatible with the prediction of statistical models and a second one corresponding to breakup aligned with the recoil direction of the projectile like source which should be compared with the predictions of dynamical calculations based on microscopic transport models.

Inclusive and exclusive measurements in the projectile breakup of 7Li

The inclusive and exclusive measurements were carried out for 7Li projectile breakup on 27Al target at 48 MeV. In the inclusive data we have observed a broad peak around the beam velocity for alphas and tritons. The exclusive data for alpha-triton coincidences show good agreement with the post-form DWBA theory of breakup reactions.