Heavy Ion-atom Collisions Research Articles

Multielectron Processes in Heavy Ion–Atom Collisions at Intermediate Velocity

Using high resolution x-ray spectroscopy, we have measured projectile electron single and multiple cross sections when a two-electron Ar{sup 16+} ion collides with neutral target atoms. For a fixed impact velocity (v{sub p}=23 a.u. ) , but using various targets from He to Xe, a range from the perturbative regime to the strong interaction regime has been investigated. Double excitation cross sections are found to be well reproduced by an independent electron model. First measurements of capture-ionization cross sections are also reported and show the importance of this often-neglected process. {copyright} {ital 1997} {ital The American Physical Society}

Importance of initial-final state correlations for the formation of fragments in heavy ion collisions

Using quantum molecular dynamics simulations, we investigate the formation of fragments in symmetric reactions between beam energies of $E=30A\mathrm{MeV}$ and $600A\mathrm{MeV}.$ After a comparison with existing data we investigate some observables relevant to tackle equilibration: $d\ensuremath{\sigma}{/dE}_{\mathrm{rat}},$ the double differential cross section ${d}^{2}\ensuremath{\sigma}{/p}_{t}{\mathrm{dp}}_{z}{\mathrm{dp}}_{t},\dots{}.$ Apart maybe from very energetic $(Eg~400A\mathrm{MeV})$ and very central reactions, none of our simulations gives evidence that the system passes through a state of equilibrium. Later, we address the production mechanisms and find that, whatever the energy, nucleons finally entrained in a fragment exhibit strong initial-final state correlations, in coordinate as well as in momentum space. At high energy those correlations resemble the ones obtained in the participant-spectator model. At low energy the correlations are equally strong, but more complicated; they are a consequence of the Pauli blocking of the nucleon-nucleon collisions, the geometry, and the excitation energy. Studying a second set of time-dependent variables (radii, densities, etc.), we investigate in detail how those correlations survive the reaction, especially in central reactions where the nucleons have to pass through the whole system. It appears that some fragments are made of nucleons which were initially correlated, whereas others are formed by nucleons scattered during the reaction into the vicinity of a group of previously correlated nucleons.

Two-body correlation transport theory for heavy ion collision

Abstract A two-body correlation transport theory (TBCTT) for describing the dynamical process in heavy ion collisions has been proposed by means of coupling the equation of motion for coherent single particle (representation basis) states with two-body correlation dynamics. This model contains time-dependent mean field effects, two-body correlations to all orders of interactions as well as fermionic (antisymmetric) effects. It is capable of describing the time-evolution of nonuniform nuclear matter including dynamical fluctuations and dynamical formation of fragments in heavy ion collisions. The preliminary numerical results show that this model is promising for the description of heavy ion collision dynamics.

CONTINUUM X RAYS IN HIGHLY CHARGED ION-ATOM COLLISIONS

Continuous X rays produced by highly charged heavy ion-atom collisions have been studied experimentally. 2–5.5 MeV/u F, Si, S and Cl ions with zero or one electron were bombarded with a thin gas target of H 2 and He. Emitted X-ray spectra were measured by using a Si(Li) X-ray detector at 90°. The characteristic X rays and radiative electron capture X rays were observed clearly, which were superimposed on the continuum X rays. The continuum X rays can be well explained by two types of radiative processes: mainly quasi-free electron bremsstrahlung (QFEB), and partly atomic bremsstrahlung (AB). It should be noticed that QFEB is predominant at low X-ray energy region and AB at high X-ray energy region in highly charged heavy ion-atom collision process.

Vapor-liquid phase transition and multifragmentation of nuclei.

Nucleation theory based on the steady-state solution of Fokker-Plank kinetic equation is applied to study of the liquid-gas phase transition in nuclear matter. Theoretical predictions are compared with the parameters extracted from the analysis of the charge distributions of fragments in heavy ion collisions at intermediate energies. The phenomenological droplet model is shown to be able to explain both the characteristic power-law falloff and the long tail of the yields of light and heavy nuclear fragments correspondingly. It is shown that the traditional equilibrium approach to the analysis of the multifragmentation data leads to significant overestimation of the parameters of nuclear equation of state.

Radiative double electron capture in heavy-ion atom collisions

Total cross sections for radiative double electron capture (RDEC) in collisions of fast highly charged ions with light target atoms were estimated. The applied method is based on the principle of detailed balance using the cross sections for the double photoionization process. Preliminary results of a dedicated RDEC experiment aiming at the observation of photons with twice the single REC photon energy, associated with double charge exchange, are discussed. Additionally, results of complementary measurements of double capture (without photon registration) by fully stripped U ions in collisions with C atoms at 295 MeV/u are presented. Here, a surprisingly enhanced cross section for double electron capture is found in accordance with observations reported in the literature.

Radiative electron capture in relativistic heavy-ion atom collisions

Based on systematic experimental studies and on exact relativistic calculations of the REC process in relativistic heavy-ion atom collisions, the limits of applicability of the nonrelativistic dipole approximation are discussed. In particular, direct comparison between a general REC scaling law and all available total electron-capture cross sections related to REC into bare and H-like ions is given. Similar to K-REC, a fortuitous agreement between the nonrelativistic approach and the experimental data is found. Moreover, the results of a dedicated L-REC experiment are presented, where this process was investigated for energy-resolved subshells. The measured angular distributions for REC into the j = 1 2 and j = 3 2 levels are in excellent agreement with exact relativistic calculations and manifest a complete breakdown of the nonrelativistic theory.

Relativistic ab initio description of the K-vacancy production in heavy-ion-atom collision systems with solid targets.

We investigate the problem of ion--solid-target collisions by means of a time-dependent coupled-channel description using ab initio relativistic four-component linear-combination-of-atomic-orbital molecular-orbital (LCAO-MO) wave functions as a basis. Large-impact-parameter calculations allow us to determine how the L and M shells of initially low ionized projectiles are fully stripped within a few atomic layers of the solid target. Once the inner-L-shell vacancies are created, our narrow-impact-parameter calculations give very good agreement with experimental results on K-shell vacancy production. As an example, we compare our results with measurements of 40.6-MeV ${\mathrm{Ar}}^{4+}$ and ${\mathrm{Ar}}^{12+}$ ions on Ca solid targets.

Unified description for the nuclear equation of state and fragmentation in heavy-ion collisions.

We propose a model that provides a unified description of the nuclear equation of state and fragmentations. The equation of state is evaluated in the Bragg-Williams as well as in the Bethe-Peierls approximations and compared with that in the mean field theory with Skyrme interactions. The model shows a liquid-gas type phase transition. The nuclear fragment distributions are studied for different densities at finite temperatures. Power law behavior for fragments is observed at the critical point. The study of the fragment distribution and the second moment ${\mathit{S}}_{2}$ shows that the thermal critical point coincides with the percolation point at the critical density. High temperature behavior of the model shows characteristics of chemical equilibrium.

Radiative electron capture studied in relativistic heavy-ion-atom collisions.

The process of radiative electron capture (REC) in relativistic collisions of high-Z ions with low-Z gaseous and solid targets is studied experimentally and theoretically. The observed x-ray spectra are analyzed with respect to photon angular distributions as well as to total K-REC cross sections. The experimental results for angle-differential cross sections are well reproduced by exact relativistic calculations which yield significant deviations from standard ${\mathrm{sin}}^{2}$\ensuremath{\theta} distributions. Total cross sections for K-REC are shown to follow a simple scaling rule obtained from exact relativistic calculations as well as from a nonrelativistic dipole approximation. The agreement between these different theoretical approaches must be regarded as fortuitous, but it lends support to the use of the nonrelativistic approach for practical purposes.

A schematic model for fragmentation and phase transition in nuclear collisions

We develop here a simple yet versatile model for nuclear fragmentation in heavy ion collisions. The model allows us to calculate thermodynamic properties such as phase transitions as well as the distribution of fragments at disassembly. In spite of its simplicity the model gives very good fit to recent data taken at the National Superconducting Cyclotron Laboratory. The model is an extension of a lattice gas model which itself has strong overlaps with percolation models which have been used in the past to compare with nuclear fragmentation data.

Ion induced L X-ray spectra of 3d transition elements

The L X-ray emission spectra from Cr, Mn, Fe and Co of the first-series transition metals excited by 1.8 MeV H +, 1.8 MeV He +, 1.1 MeV N + and 0.85 MeV Ne + ions have been studied using a high resolution Bragg crystal spectrometer. We found the following prominent effects induced by the heavy ion impacts. 1) Broad satellite bands are observed, arising from the multiple vacancy production in the M and N shells during heavy ion-atom collisions. This can be supported by the theoretical estimation using the Hartree-Fock-Slater calculations. 2) The peak shifts to the lower energy in the Lα 1,2 lines are also observed. This could be explained by the suggestion that the originally empty levels in the M and N shells of the target atoms may be occupied by electrons promoted from the inner shells of the projectiles through the quasimolecular orbitals or the purturbed Lα 1,2 X rays by the interatomic transition because of the complicated band structure of the 3d transition metals. The calculated values in the energy shift are a few electron volts and in agreement with the observed ones.

Secondary electron emission in swift heavy-ion atom collisions

(1993). Secondary electron emission in swift heavy-ion atom collisions. Radiation Effects and Defects in Solids: Vol. 126, No. 1-4, pp. 35-40.

Application of inclusive probability theory to heavy ion-atom collisions

Using the independent particle model as our basis we present a scheme to reduce the complexity and computational effort to calculate inclusive probabilities in many-electron collision system. As an example we present an application toK−K charge transfer in collisions of 2.6 MeV Ne9+ on Ne. We are able to give impact parameter-dependent probabilities for many-particle states which could lead toKLL-Auger electrons after collision and we compare with experimental values.

Delta-electron emission in fast heavy ion atom collisions.

Biological damages such as mutations, chromosomal aberrations etc. are a consequence of biochemical changes mostly in the DNA. With ionizing radiation, these chemical changes are due to primary ionization events and secondary ionization effects caused by the primarily produced electrons. Differences in the biological response of densely ionizing radiation, like heavy charged particles, in comparison to sparsely ionizing radiation, such as X- or gamma-rays, are mainly due to the differences in the production of the so called delta-electrons. Therefore, the emission process of electrons i.e. the cross section for the primary ionization event as well as the energy and angular distribution of the emitted electrons should be understood in detail. The delta-electron emission processes occuring in fast heavy ion atom collisions are explained qualitatively. The different spectral structures of electron emission arising from either the target or the projectile are explained in terms of simple models of the kinetics of momentum transfer induced by the COULOMB forces. In collisions of very heavy ions with matter, high nuclear COULOMB forces are created. These forces lead to a strong polarization of the electronic states of the participated electrons. The effects of this polarization are discussed.

Electron excitations in heavy ion collisions

Coherence effects in electron excitations in heavy ion-atom collisions are discussed, and new methods to treat the role of residual interactions are suggested. Among experimental consequences may emerge new predictions of energy variations in e+ e− pair production probabilities.

A study of scattered ion charge states for Kr n+ -Kr collisions ( n = 1, 2, 3, 4 and 5)

The final ionization states have been measured for Kr ions scattered to 1, 2, and 4° from Kr targets for selected incident ion energies from 0.4 to 1.4 MeV. For projectile states ranging from 1 to 5 the average final scattered charge state is observed to have only a weak dependence on the initial charge state. For example, for 1.4 MeV, 4° scattering the average final charge state for Kr +-Kr collisions is 10.2 and for Kr 5+-Kr it is 10.3. For 0.7 MeV, 1° scattering the final charge states ranged from 4.8 to 5.3 for incident charge states from 1 to 4. It is not clear for these very heavy ion-atom collisions that existing models provide an appropriate description of the ionization processes that give rise to the observed final charge states.

?-Electron emission in fast, highly-charged heavy ion-atom collisions

Systematic studies have been started to investigate the double differential cross sections for the emission of delta-electrons, differential in emission energy Ee and -angle ϑein U - rare gases collisions. These studies have been performed in a projectile energy range between 0.4 MeV/u and 1.4 MeV/u. The energy spectra for different emission angles show unexpected structures in the region of the binary encounter peak [1], which vary strongly with the emission angle ϑe and projectile energy EP. These structures appear e.g. at ϑe≈40° for an impact energy EP/A of 1.4 MeV/u, at ϑe≈32.5° for Ep/A = 1 MeV/u and at ϑe≈22.5° for Ep/A = 0.5 MeV/u.

The Boltzmann-Langevin equation and its application to intermediate mass fragment production

We present the first simulations of the Boltzmann-Langevin equation recently introduced for taking into account high order correlations not contained in extended mean field theories. This framework is very promising for phenomena involving large fluctuations such as presumably the formation of Intermediate Mass Fragments in heavy ion collisions at some tens of MeV/A. We apply the simulation to this energy regime and try to exhibit some characteristics of multifragmentation patterns.

The physics of asymmetric atomic collisions studied by the SCA model

The semiclassical trajectory method for atomic processes induced by charged projectiles is reviewed. The perturbative SCA formalism is sketched with emphasis on the first-order time-dependent approach. Selected results from computations on asymmetric collision systems are presented. The kinematic scaling law for ionization and pair-production phenomena is treated and commented upon. Applications of this scaling law in high-energy atomic collision physics are discussed. Stress is put on its possible consequences for the interpretation of the anomalous positron production in very heavy ion-atom collisions.