Relativistic Ion-atom Collisions Research Articles

Coincident mapping of e+ and e− from free-free pair production in a magnetic toroidal lepton spectrometer

SynopsisWe discuss the electron-optical properties of a toroidal magnetic sector spectrometer and its suitablilty for electron-positron pair spectroscopy in relativistic ion-atom collisions in the future HESR storage ring at FAIR. With the simultaneous mapping of electrons and positrons and geometric invariants in the lepton trajectorties this instrument offers a very high efficiency for studies of vector momentum correlation in free-free pair production.

Laser-assisted binary-encounter emission in relativistic ion-atom collisions

We consider binary-encounter electron emission in relativistic ion-atom collisions in the presence of a monochromatic low-frequency plane-wave electromagnetic field of circular polarization. The parameters of the field---its intensity and frequency---are chosen in such a way that the field per se has a negligible effect on the initial (ground) state of the atom. The results show, however, that this field may have a profound effect on the electron emission spectra, which is mainly caused by an enormous energy exchange between the emitted electron and the field.

Angular distribution of photons for the simultaneous excitation and ionization of He-like uranium ions in relativistic ion-atom collisions

Simultaneous ionization and excitation processes are studied for initially He-like uranium ions in collisions with xenon gaseous targets at relativistic energy, $220\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}/u$. The virtue of investigating the process of simultaneous excitation and ionization is that one electron ends up in the continuum, while the other electron ends up in a hydrogen-like final state. Experimentally, this process can be identified by observing the radiative decay of the excited levels in coincidence with ions that lost one electron (U${}^{91+}$). In particular, owing to the large fine-structure splitting of H-like U, the angular distribution of photons for the simultaneous ionization and excitation into the different total angular momentum $j=1/2$ and $j=3/2$ states of the L shell is determined directly from the obser-ved yields of Ly${}_{\ensuremath{\alpha}1}$ and Ly${}_{\ensuremath{\alpha}2}$ radiation at various observation angles. The experimental data show a progress for the dependence of the alignment on the collision impact parameter. It is shown that the current results confirm the theoretical predictions based on the independent-particle approximation and first-order perturbation, for which the simultaneous ionization and excitation processes occur at small impact parameter.

Polarization and anisotropic emission of K-shell radiation from heavy few electron ions1This article is part of a Special Issue on the 10th International Colloquium on Atomic Spectra and Oscillator Strengths for Astrophysical and Laboratory Plasmas.

The population of magnetic sublevels in hydrogen-like uranium ions has been investigated in relativistic ion–atom collisions by observing the subsequent X-ray emission. Using the gas target at the experimental storage ring facility we observed the angular emission of Lyman-α radiation from hydrogen-like uranium ions. The alignment parameter for three different interaction energies was measured and found to agree well with theory. In addition, the use of different gas targets allowed for the electron-impact excitation process to be observed.

Shell- and subshell-resolved projectile excitation of hydrogenlike Au78+ions in relativistic ion-atom collisions

The projectile excitation of high-$Z$ ions has been investigated in relativistic ion-atoms collisions by observing the subsequent x-ray emission. The x-ray spectra from the projectile excitation have been separated from the x-ray emission following electron capture into the excited states using a novel anticoincidence technique. For the particular case of hydrogenlike Au${}^{78+}$ ions colliding with Ar atoms, Coulomb excitation from the ground state into the fine-structure-resolved $n=2$ levels as well as into levels with principal quantum number $n\ensuremath{\geqslant}3$ has been measured with excellent statistics. The observed spectra agree well with simulated spectra that are based on Dirac's relativistic equation and the proper inclusion of the magnetic interaction into the amplitudes for projectile excitation. It is shown that a coherent inclusion of the magnetic part of the Lienard-Wiechert potential leads to the lowering of the excitation cross section by up to 35%. This effect is more pronounced for excitation into states with high angular momentum and is confirmed by our experimental data.

Symmetric eikonal model for projectile-electron excitation and loss in relativistic ion-atom collisions

At impact energies $ \stackrel{>}{\sim}1$ GeV/u the projectile-electron excitation and loss occurring in collisions between highly charged ions and neutral atoms is already strongly influenced by the presence of atomic electrons. In order to treat these processes in collisions with heavy atoms we generalize the symmetric eikonal model, used earlier for considerations of electron transitions in ion-atom collisions within the scope of a three-body Coulomb problem. We show that at asymptotically high collision energies this model leads to an exact transition amplitude and is very well suited to describe the projectile-electron excitation and loss at energies above a few GeV/u. In particular, by considering a number of examples we demonstrate advantages of this model over the first Born approximation at impact energies $\sim 1$--30 GeV/u, which are of special interest for atomic physics experiments at the future GSI facilities.

DIRAC: A new version of computer algebra tools for studying the properties and behavior of hydrogen-like ions

During recent years, the Dirac package has proved to be an efficient tool for studying the structural properties and dynamic behavior of hydrogen-like ions. Originally designed as a set of Maple procedures, this package provides interactive access to the wave and Green's functions in the non-relativistic and relativistic frameworks and supports analytical evaluation of a large number of radial integrals that are required for the construction of transition amplitudes and interaction cross sections. We provide here a new version of the Dirac program which is developed within the framework of Mathematica (version 6.0). This new version aims to cater to a wider community of researchers that use the Mathematica platform and to take advantage of the generally faster processing times therein. Moreover, the addition of new procedures, a more convenient and detailed help system, as well as source code revisions to overcome identified shortcomings should ensure expanded use of the new Dirac program over its predecessor.

Simultaneous electron capture and bound-free pair production in relativistic collisions of heavy nuclei with atoms

We consider a two-step process that may occur in relativistic ion-atom collisions. In this process, within a single ion-atom collision, an atomic electron is captured by the ion via the radiative and/or nonradiative capture channels, while the colliding nuclei of the atom and ion simultaneously produce an electron-positron pair with the electron created in a bound state of the ion. As a result, the ion picks up two electrons and a positron is emitted. We calculate the total cross section for this process in collisions of incident $1--4\text{ }\text{GeV}/\text{u}$ ${\text{U}}^{92+}$ projectiles with various atomic targets and discuss its dependence on the collision energy and target atomic number. We also analyze whether this process can compete with electron recombination proceeding via the excitation of the negative-continuum states, in which an atomic electron incident on a bare nucleus is captured into a bound state of the ion by transferring the energy excess to a negative-continuum electron with the latter undergoing a transition into a bound state. According to our estimates, the electron recombination dominates the two-step process in collisions with light and intermediately heavy atoms $({Z}_{a}\ensuremath{\lesssim}50)$, while in collisions with heavy atoms the latter leads to much larger cross sections.

Polarization and angular correlation studies of X-rays emitted in relativistic ion-atom collisions

Particle and photon polarization phenomena occurring in collisions of relativistic ions with matter have recently attracted particular interest. Investigations of the emitted characteristic x-ray and radiative electron capture radiation has been found to be a versatile tool for probing our present understanding of the dynamics of particles in extreme electromagnetic fields. Owing to the progress in x-ray detector technology, in addition, accurate measurements of the linear polarization for hard x-ray photons as well as the determination of the polarization plane became possible. This new diagnostic tool enables one today to derive information about the polarization of the ion beams from the photon polarization features of the radiative electron capture process.

ATOMIC PHYSICS RESEARCHES AT COOLER STORAGE RING IN LANZHOU

The commissioning of the cooler storage rings (CSR) was successful, and the facility provides new possibilities for atomic physics with highly charged ions. Bare carbon, argon ions, were successfully stored in the main ring CSRm, cooled by cold electron beam, and accelerated up to 1 GeV/u. Heavier ions as Xe 44+ and Kr 28+ were also successfully stored in the CSRs. Both of the rings are equipped with new generation of electron coolers which can provide different electron beam density distributions. Electron-ion interactions, high precision X-ray spectroscopy, complete kinematical measurements for relativistic ion-atom collisions will be performed at CSRs. Laser cooling of heavy ions are planned as well. The physics programs and the present status will be summarized.

Breit interaction effects on the Kα angular distribution following the dielectronic recombination of high-Z ions

The angular distribution of the Kα hypersatellite radiation has been investigated for high-Z, helium-like ions following the K−LL dielectronic recombination of initially hydrogen-like projectiles in relativistic ion–atom collisions. A particular strong effect is found for the alignment of the 2s2p1/2J=1 (spin-forbidden) resonance that changes sign if, in addition to the static Coulomb repulsion, the Breit interaction is taken into account for the resonant electron capture. This change in the alignment of the recombined ion also leads to a remarkable shift in the angular distribution of the subsequent 2s2p1/2J=1→1s2s J=0 Kα2 photons from a dominantly perpendicular emission to one that occurs in parallel with the ion beam, and vice versa for the 2s2p1/2J=1→1s2s J=1 line.

Relativistic ion–atom collisions and very strong fields

After characterizing the properties and experimental possibilities of very strong fields, a brief outline is given of the basic processes that can occur if highly charged projectiles up to bare uranium nuclei collide with heavy atoms at relativistic velocities. Besides direct reactions and transfer, the attention is focused on single and multiple electron–positron pair production. Open problems and difficulties are pointed out. The resulting opportunities and challenges are addressed.

Angular correlations in the two-photon decay of aligned hydrogenlike ions

The two-photon decay of highly charged hydrogenlike ions is studied within the framework of second-order perturbation theory, based on Dirac's equation. Attention has been paid to the angle-angle correlations between the two emitted photons. In particular, the density matrix theory is applied in order to explore how these angle-angle correlations are affected by the alignment of the excited states of the ions. Detailed computations have been carried out for the 3d{sub 5/2}{yields}1s{sub 1/2} transition in hydrogenlike uranium U{sup 91+} ions for different (degrees of) initial alignment. It is shown that the angular correlation function is very sensitive to the alignment of the 3d{sub 5/2} state, an effect which has to be accounted for in the analysis of all (future) two-photon decay experiments, for which the excited state of the ions is produced by relativistic ion-atom collisions.

Competing processes for electron capture to continuum in relativistic ion-atom collisions

The relative importance of the two mechanisms for the capture of a target electron by a fast, heavy projectile, radiative ionization (RI) and Coulomb capture to continuum (ECC), is studied in the vicinity of the forward peak. For both processes a consistent relativistic description, based on the impulse approximation, is provided. It is found that the differential cross-sections scale with the projectile charge and exhibit a common velocity dependence. As a result, RI starts to dominate over ECC near the same impact energy (∼11 MeV/amu) for arbitrary bare projectiles colliding with hydrogen. For electrons from the inner shells of heavier targets this energy increases, however, which is confirmed by a coincidence experiment on 90 MeV/amu U 88+ +N 2.

FEW-BODY QUANTUM DYNAMICS IN RELATIVISTIC ION-ATOM COLLISIONS

Until recently the theory of relativistic ion-atom collisions was basically restricted to the description of the motion of a single electron in the external field generated by the atomic nucleus and that of the ion-projectile. In the present article we review some of the very recent developments in the theory of relativistic ion-atom collisions which go beyond the single-active-electron picture. Here we discuss processes in which two electrons actively participate and focus our attention in the following two kinds of such processes: (i) double ionization of helium by relativistically moving highly charged nuclei and (ii) two-center dielectronic transitions in relativistic collisions between an atom and an ion.

Electron–positron pair production in relativistic ion–atom collisions

The creation of electron–positron pairs constitutes an example for the conversion of energy into mass. We here give a brief outline of the various processes and theoretical approaches in a simple fashion. We point out some recent results and difficulties that have yet to be overcome.

Resonance effects in projectile-electron loss in relativistic collisions with excited atoms

The theory of electron loss from projectile-ions in relativistic ion–atom collisions is extended to the case of collisions with excited atoms. The main feature of such collisions is a resonance which can emerge between electron transitions in the ion and atom. The resonance becomes possible due to the Doppler effect and has a well-defined impact energy threshold. In the resonance case, the ion–atom interaction is transmitted by the radiation field and the range of this interaction becomes extremely long. Because of this the presence of other atoms in the target medium and the size of the space occupied by the medium have to be taken into account and it turns out that microscopic loss cross sections may be strongly dependent on such macroscopic parameters as the target density, temperature and size. We consider both the total and differential loss cross sections and show that the resonance can have a strong impact on the angular and energy distributions of electrons emitted from the projectiles and the total number of electron loss events.

Radiation Field and Resonant Two-Center Dielectronic Transitions in Relativistic Ion-Atom Collisions

We consider projectile-electron excitation and loss in relativistic collisions of ionic projectiles with excited atoms. We show that under certain conditions electron transitions in the ion and atom can be resonantly coupled in the collision via the radiation field. The resonance becomes possible due to the Doppler effect, has a well-defined impact energy threshold, and clearly manifests itself in the cross sections. Since the range of the ion-atom interaction in the resonance case is very long, the presence of other atoms in the target medium as well as the size of the space occupied by the medium have to be taken into account. As a result, the cross section may become dependent on the density of the target atoms and/or the target size.

Radiative electron capture to continuum in relativistic ion–atom collisions

The capture of a target electron to the projectile continuum with simultaneousphoton emission, known as radiative ionization (RI), is investigated theoreticallyfor fast, highly charged projectiles colliding with light target atoms. Based on theimpulse approximation in its post form, the features of radiative ionization arestudied and contrasted with the related processes of radiative electron capture andnonradiative electron capture to continuum (ECC). A large RI cusp asymmetry isfound, increasing strongly with projectile charge but decreasing with projectilevelocity. However, in contrast to ECC, the cusp is skewed to the high-energyside. Results are shown for 20 MeV amu−1 Kr36+ + H and for 100 and 400 MeV amu−1 U92+on H and N.

Asymptotic energy dependence of projectile excitation in relativistic ion-atom collisions

The Coulomb excitation of inner-shell electrons around heavy and highly charged projectile ions impinging on light targets is investigated in the highly relativistic energy regime for projectiles as heavy as gold $(Z=79).$ First-order time-dependent perturbation theory is employed for treating the interaction of the active electron with the light target, and exact relativistic Coulomb-Dirac wave functions are used for the description of electronic states in the strong field of the projectile nucleus. The energy dependence of transition probabilities and partial cross sections is studied up to extreme relativistic energies of hundreds of GeV/u and beyond. An analytic representation of the asymptotic energy dependence of the projectile excitation cross section complements the numerically calculated impact-parameter-dependent probabilities and cross sections, providing additional insights.