arXiv subjects
Publications and source records attributed to C. A. Bertulani.
The transitions between complex configurations, to which the giant dipole resonance (GDR) and the double giant dipole resonance (DGDR) doorway states are coupled, are taken into account in second order perturbation theory for the reaction amplitude. It is proved that only transitions between GDR and DGDR doorway states play an essential role in the Coulomb excitation of the DGDR.
A new class of giant resonances in nuclei is discussed, i.e., giant resonances built on other giant resonances. These resonances are observed with very large cross sections in relativistic heavy ion collisions. A great experimental and theoretical effort is underway to understand the reaction mechanism which leads to the excitation of these states in nuclei, as well as the better microscopic understanding of their properties, e.g., strength, energy centroids, widths, and anharmonicities.
The production of antihydrogen in flight in antiproton-nucleus collisions is calculated theoretically in the Plane Wave Born Approximation (which is equivalent to the straight line semiclassical approximation). Antihydrogen has been produced in this way at LEAR/CERN and is presently studied at FERMILAB at various antiproton energies. Dirac wave functions for the leptons are used, taking first order (Z.alpha) corrections into account. Analytical results are obtained for differential cross-sections. Total cross sections are obtained by numerical integration. The dependence on the transverse momentum transfer is studied and the accuracy of the equivalent photon approximation and a recent variant by Munger, Brodsky,and Schmidt is discussed as a function of beam energy.
We study the postacceleration of charged fragments in reactions with unstable nuclear beams. For elastic breakup processes we show that the postaceleration effect can be well understood in terms of closed analytical forms derived in a quantum mechanical formulation. This gives theoretical support to the effect experimentally observed in the breakup of Li-11 projectiles at intermediate energies.
We study the energy dependence of the cross sections for nucleon removal of 8B projectiles. It is shown that the Glauber model calculations with nucleon-nucleon t-matrix reproduce well the energy dependence of the breakup cross sections of 8B. A DWBA model for the breakup cross section is also proposed and results are compared with those of the Glauber model. We show that to obtain an agreement between the DWBA calculations, the Glauber formalism, and the experimental data, it is necessary to modify the energy behavior of the effective interaction. In particular, the breakup potential has a quite different energy dependence than the strong absorption potential.
We study the combined effects of vacuum polarization, relativity, Bremsstrahlung, and atomic polarization in nuclear reactions of astrophysical interest. It is shown that these effects do not solve the longstanding differences between the experimental data of astrophysical nuclear reactions at very low energies and the theoretical calculations which aim to include electron screening.
The direct excitation of a "sea" of two-phonon configurations above 15 MeV from the ground state is considered. It is concluded that these configurations form a flat physical background in photoexcitation cross sections and Coulomb excitation in heavy ion reactions at relativistic energies. The contribution of the background to the total cross section in Coulomb excitation of Pb-208 in the DGDR energy region is about 15%. This shows that It should be considered while drawing out conclusions on anharmonicities effects for the DGDR.
We study the effect of vacuum polarization in nuclear reactions of astrophysical interest. This effect has the opposite sign compared to the screening by the atomic electrons. It is shown that vacuum polarization further increases the longstanding differences between the experimental data of astrophysical nuclear reactions at very low energies and the theoretical calculations which aim to include electron screening, albeit by a small amount.
At forward angles, and bombarding energies E > 200 MeV, the (p,n) and (n,p) reactions are thought to be directly proportional to the Gamow-Teller transition strengths in the nuclei. Assuming that this relationship also holds for charge exchange induced by high-energy heavy ions, it would be very useful in studies with radioactive beams. Contrary to this expectation, we show that the determination of Gamow-Teller and Fermi matrix elements from heavy-ion charge-exchange at forward angles is very inaccurate.
Nuclear reaction cross sections are usually very small in typical astrophysical environments. It has been one of the major challenges of experimental nuclear astrophysics to assess the magnitude of these cross sections in the laboratory. For a successful experiment high luminosity beams are needed. Increasing the target width, one also increases the reaction yields. But, this is of limited use due to multiple scattering in the target. Storage rings are a very good way to overcome these difficulties. In principle, they can be tuned to large luminosities, and have the advantage of crossing the interaction region many times per second (typically one million/s), compensating low density internal gas targets, or low reaction rates in beam-beam collisions. Storage rings are also ideal tools for precise measurements of masses and beta-decay lifetimes of nuclei of relevance for astrophysics.
We critically discuss the recently developed quantum-mechanical equivalent photon spectrum by Benesh et al. We point out that the key point, the strong absorption in heavy ion collisions, is not treated adequately. Conclusions drawn from such a spectrum are invalid. Equivalent photon spectra appropriate for heavy ions, have been given before in quantal as well as semiclassical versions and were found to be very satisfactory.
The electromagnetic excitation of the two-phonon isovector giant dipole resonance in relativistic projectiles incident on heavy targets can be proceed via several intermediate $1^-$ one-phonon giant resonance states. In two step electric dipole transitions the population of $0^+$, $1^+$, and $2^+ $ two-phonon states are possible. We calculate the amplitude distribution of $1^-$ excitations with an RPA formalism, and use it to calculate the electromagnetic excitation of two-phonon states in second order perturbation theory and coupled-channels. We show that a conspiracy between angular momentum coupling and the strength of the electromagnetic fields suppresses contributions of $1^+$ states to the total cross sections.
We study the effects of channel coupling in the excitation dynamics of giant resonances in relativistic heavy ions collisions. For this purpose, we use a semiclassical approximation to the Coupled-Channels problem and separate the Coulomb and the nuclear parts of the coupling into their main multipole components. In order to assess the importance of multi-step processes, we neglect the resonance widths and solve the set of coupled equations exactly. Finite widths are then considered. In this case, we handle the coupling of the ground state with the dominant Giant Dipole Resonance exactly and study the excitation of the remaining resonances within the Coupled-Channels Born Approximation. A comparison with recent experimental data is made.
We study the effects of channel coupling in the excitation of $^{11}Be$ projectiles incident on heavy targets. The contribution to the excitation from the Coulomb and the nuclear fields in peripheral collisions are considered. Our results are compared with recent data on the excitation of the $\halfm$ state in $^{11}Be$ projectiles. We show that the experimental results cannot be explained, unless very unusual parameters are used.
The extraction of the photodissociation cross sections of $^8B$ from Coulomb dissociation experiments is investigated. A careful study is done on the contributions of the E1, E2 and M1 multipolarities to the breakup. A comparison with the data of a recent experiment is performed. It is shown that the extraction of the radiative capture cross sections $^7Be(p,\ γ)^8B$ which are relevant for the solar neutrino problem is not affected appreciably by Coulomb reacceleration. A non-perturbative model is used for the purpose. Emphasis is put on the perspectives for future experiments which are planned at the University of Notre Dame, RIKEN (Japan), and GSI (Germany). An analysis of the total yields of ``photon-point" processes in inelastic electron scattering is also done.
We investigate the use of elastic and inelastic scatterings with secondary beams of radioactive nuclei as a mean to obtain information on ground state properties and transition matrix elements to continuum states. An eikonal model is developed for this purpose by using the folding potential. In particular we discuss possible signatures of halo wavefunctions in elastic and inelastic scattering experiments.