Positron and electron-impact multiple-ionization

A method is suggested for quickly and easily estimating multiple ionization (MI) cross sections of heavy atoms colliding with highly charged ions, using the independent-particle model (IPM). One-electron ionization probabilities p(b) are calculated using the geometrical model for values at zero impact parameter b and the relativistic Born approximation for one-electron ionization cross sections. Numerical results of MI cross sections are presented for Ne and Ar atoms colliding with Ar8+, Fe20+, Au24+, Bi67+ and U90+ ions at energies 1 MeV u–10 GeV uand compared with available experimental data and CTMC (classical trajectory Monte Carlo) calculations. The present method of calculation describes experimental dependencies of MI cross sections on the number of ejected electrons m within a factor of two to three. Numerical calculations show that at intermediate ion energies E = 1 − 10 MeV u, the contribution of MI cross sections to the total, i.e. summed over all m values, is quite large ∼35% and decreases with increasing energy.

New evaluations of the neutron cross sections for 14N and 16O have been made for ENDF/B-VI. The evaluations are based at low energies on R-matrix analyses of reactions in the 15N and 17O systems, and at higher energies on GNASH calculations and experimental data evaluations, including covariance analyses. The 15N system R-matrix analysis includes data from reactions among the channels n+14N, p+14C, and α+11B at energies corresponding to excitations in 15N below Ex=13 MeV. The resonance structure of all cross sections in this energy range is fairly well reproduced. New data indicate a different J-value for the first resonance, however. Sub-threshold S-wave levels required to explain the large n+14N total and elastic cross sections near zero energy give scattering lengths that differ significantly from the previous values. The R-matrix analysis of the 17O system includes many new measurements of the n+16O total cross section, done primarily at Oak Ridge and at Karlsruhe. The resonance structure of all the cross sections [total, (n,n), (n,α), and (α,α)] is well represented by the fit in the region below En = 6.5 MeV. The new total cross section information gives different positions for some of the resonances and implies a different normalization for the (n,α) cross sections than that obtained in the ENDF/B-IV analysis. The evaluations at energies above the ranges of the R-matrix analyses incorporate results from a number of experiments performed since the previous ENDF/B evaluations. Especially important are new measurements of the total cross sections and differential elastic, inelastic, and gamma-ray production cross sections.

Electron-impact cross sections for excitation of the 7 $^{2}$${\mathit{S}}_{1/2}$, 8 $^{2}$${\mathit{S}}_{1/2}$, 5 $^{2}$${\mathit{D}}_{3/2}$, and 6 $^{2}$${\mathit{D}}_{3/2}$ states of rubidium have been measured from threshold to 1000 eV. The optical-excitation-function method has been employed in a crossed atom- and electron-beam apparatus. Relative, total (cascade including) experimental cross sections are made absolute by comparison with the known total cross section of the Rb D1 line. Total excitation cross sections are compared with theoretical calculations employing first Born approximation and theoretical branching ratios. Born cross sections for the 7 $^{2}$${\mathit{S}}_{1/2}$ and 8 $^{2}$${\mathit{S}}_{1/2}$ states are obtained from the literature, while Born cross sections for the 5 $^{2}$${\mathit{D}}_{3/2}$, 6 $^{2}$${\mathit{D}}_{3/2}$, and all cascading states are calculated in this paper. At high energies, the measured total $^{2}$${\mathit{D}}_{3/2}$ state cross sections show 1/E behavior and converge to first Born theory; for Eg100 eV, experiment and theory agree within 6.7% for 5 $^{2}$${\mathit{D}}_{3/2}$ and within 3.7% for 6 $^{2}$${\mathit{D}}_{3/2}$. The total cross sections for the $^{2}$${\mathit{S}}_{1/2}$ states do not converge to Born theory even at 1000 eV, and it is shown that this cannot be attributed to cascading. At low energies, $^{2}$${\mathit{S}}_{1/2}$ and $^{2}$${\mathit{D}}_{3/2}$ state total excitation cross sections have similar shapes with sharply peaked maxima at about 0.9 eV above threshold. After cascading is corrected using first Born theory, estimated experimental cross sections for direct excitation of higher fine-structure states of rubidium are given.

We show that in relativistic heavy-ion collisions the independent-electron model can be used to predict cross sections for multiple inner-shell ionization in a single collision. Charge distributions of 430- and 955-MeV/amu ${\mathrm{U}}^{90+}$, ${\mathrm{U}}^{89+}$, ${\mathrm{U}}^{83+}$, and ${\mathrm{U}}^{68+}$ beams emerging from thin solid targets were used to obtain single- and multiple-electron stripping cross sections. The probabilities of stripping electrons from the K, L, or M shells were calculated using the semiclassical approximation and Dirac hydrogenic wave functions. The data generally agree with theory. An influence of the Auger effect is seen in ${\mathrm{U}}^{68+}$ collisions.

We have studied the stabilities of multiply charged van der Waals dimers in slow Xe30+ + [C60]2([C60C70], [C70]2) → Xe(30-s)+ + [C60]2r+([C60C70]r+, [C70]2r+) + (r-s)e-electron-transfer collisions (v = 0.4 a.u. and r ≤ 7). The relative ionization cross sections display even-odd variations as functions of r for [C60]2r+, [C60C70]r+, and [C70]2r+. This is in clear contrast to the typical smooth decreasing behavior of the cross sections for multiple ionization of fullerene monomers, which can be explained within the framework of the classical over-the-barrier model. In addition, we report the branching ratios and the kinetic energy releases for the dominant fragmentation processes that yield intact fullerenes. The experimental results are discussed in view of a simple electrostatic model for dimer ionization and recent results from high level density functional theory calculations.

The cross sections for single and multiple ionization of uranium by electron impact for energies 7.5 to 500 eV have been measured using a modulated crossed-beam experiment. The absolute total ionization cross section was found by comparing the cross section for ionization to the cross section for associative ionization in the reaction U + O/sub 2/ ..-->.. UO/sub 2//sup ts+/+e. The total ion number cross section has a maximum value of (5.8 +- 0.9) x 10/sup -16/ cm/sup 2/ occurring at an energy of about 50 eV. The single ionization cross section has a maximum of (4.80 +- 1.24) x 10/sup -16/ cm/sup 2/ at an energy of 27 +- 5 eV. The cross sections for higher-order ionization have values at 500 eV that are 49%, 33%, and 17% that of single ionization for n = 2, 3, and 4, respectively. These ratios are significantly higher than those for other elements for which data are available.

The change in the $K\ensuremath{\alpha}$ x-ray spectra of neon is studied as a function of projectiles of ${\mathrm{C}}^{+3,+4,+5,+6}$, ${\mathrm{N}}^{+4,+6,+7}$, ${\mathrm{O}}^{+4,+5,+6,+7,+8}$, and ${\mathrm{F}}^{+9}$ having an energy between 1 and 2 MeV/amu. The spectra are observed using a curved crystal spectrometer. Combined with previous measurements data are obtained for projectiles from carbon to argon. X rays are observed from different ionization states of Ne designated by $K{L}^{n}$ for $n=1\ensuremath{-}7$. An 81% correction between the $K{L}^{5}$ peak and $K{L}^{6}$ peak is derived. Semiempirical fluorescence yields are applied to the relative x-ray intensities to obtain relative multiple ionization cross sections. These cross sections are fitted by a binomial distribution as predicted by Coulomb ionization. All cases give good fits except for the ${\mathrm{F}}^{+9}$ induced spectra. A possible explanation is that a different excitation mechanism may exist for this near-symmetric collision case.

We calculate total hadron-hadron cross sections at high energies using the Low-Nussinov two-gluon model of the Pomeron. The gluon exchange is represented by a phenomenological potential including screened color-Coulomb, screened confining, and spin-spin interactions. We use bound-state wave functions obtained from a potential model for mesons, and a Gaussian wave function for a proton. We evaluate total cross sections for collisions involving pion, kaon, rho, phi, D, J/psi, psi', Upsilon, Upsilon', and the proton. We find that the total cross sections increase with the square of the sum of the root-mean square radii of the colliding hadrons, but there are variations arising from the spin-spin interaction. We also calculate the total cross sections of a mixed-color charmonium state on a pion and on a proton. The dependence of the total cross section on the size and the color state of the charmonium is investigated.

Measurement of fusion cross sections for the $^{6,7}\mathrm{Li}$ $+$ $^{24}\mathrm{Mg}$ reactions by the characteristic $\ensuremath{\gamma}$-ray method has been done at energies from below to well above the respective Coulomb barriers. The fusion cross sections obtained from these $\ensuremath{\gamma}$-ray cross sections for the two systems are found to agree well with the total reaction cross sections at low energies. The relatively large difference between total cross sections and measured fusion cross sections at higher energies is consistent with the fact that other channels, in particular breakup, open up with an increase of bombarding energy. The breakup channel, however, appears not to have any influence on fusion cross sections. The critical angular momenta (${l}_{\mathrm{cr}}$) deduced from the fusion cross sections are found to have an energy dependence similar to other Li-induced reactions.

In this paper, we extend our previous work of COBI model to study the multiple ionization and mean charge state of noble gases collided by heavy ions at energies not far from Bragg peaks of several to hundreds keV/amu. Being in good agreement with various experimental data and taking extremely small computational time make the model helpful to study the charge state distribution and multiple ionization of gases transforming to plasmas.

The ionization and dissociation of ammonia by impact of protons is investigated experimentally in a wide range of energies from 125 to 2700 keV. Coincidence measurements are used to separate single from multiple ionization. The role of the charge sign of the projectile was analyzed for the ionization by comparing the proton with previous electron data. The energy dependence of the single and multiple-ionization cross sections allowed us to evaluate the role of second-order mechanisms. For single ionization, the role of a two-step mechanism was inferred. The double-ionization cross section showed that postcollisional deexcitation takes over the two-step sequential ionization above 800 keV. Triple ionization is prompted mostly by postcollisional Auger decay.

The goal of this thesis is to study the interaction of the jet particle with the strongly interacting QCD medium within the framework of the effective dynamical quasiparticle model (DQPM), which describes the nonperturbative QGP medium in terms of strongly interacting off-shell massive quarks and gluons, whose properties are fitted to reproduce the lattice QCD thermodynamics at zero baryon chemical potential. In addition to the temperature $T$ and chemical potential $\mu_B$ dependencies of quasiparticle masses, the DQPM includes information about their interactions in the sQGP through dynamically generated widths, which also depend on $T$ and $\mu_B$. In chapter 1 of the thesis we discuss the basic DQPM ingredients and provide the details of the calculation of the DQPM propagators, spectral functions, strong coupling, thermal masses and widths, showing that the thermodynamic quantities evaluated within the DQPM are consistent with the lattice QCD results. The chapter 2 is dedicated to the partonic interactions in the QGP. Using the DQPM propagators for massive quarks and gluons, we construct partonic scattering amplitudes for elastic and inelastic reactions. By performing the integration over the phase space, we calculate the differential and total elastic and inelastic cross sections and investigate various observables, demonstrating the following key findings. First, we show that the DQPM amplitudes for both $2 \to 2$ and $2 \to 3$ processes align with the pQCD calculation when quasiparticle masses and widths are zero, thus highlighting the consistency between pQCD and the DQPM. Also, we show that high-energy elastic and radiative processes are dominated by the $t$ channel and their cross sections scale with QCD color factors, which is also consistent with pQCD. For the $qq \to qqg$ process, significant contributions arise from gluon emission from exchange lines, which emphasizes the need for precise evaluations without approximations. Next, we show that $T$ and $\mu_B$ dependencies of the cross sections are mainly driven by the DQPM strong coupling, showing a strong $T$ dependence, but a weaker $\mu_B$ dependence. Whereas elastic cross sections dominate at low energies and high temperatures, radiative cross sections increase at low temperatures and high energies, becoming comparable or even dominant. Furthermore, we find that the partonic cross sections are only slightly affected by the leading parton mass but are strongly sensitive to the mass of the emitted gluon, which indicates its importance for the radiative processes. Finally, we show that the interaction rate and relaxation time in the sQGP are primarily governed by elastic scattering. In chapter 3 we focus on the attenuation of jet partons in the QGP by applying together the DQPM and kinetic theory to calculate the jet transport coefficients, specifically $\hat{q}$ (transverse momentum broadening per unit length) and $dE/dx$ (energy loss per unit length). The calculations are performed for both elastic and inelastic reactions, including all possible contributions and interference terms. Our results demonstrate several key findings. First, we find that both elastic and inelastic $\hat{q}$ and $dE/dx$ rise rapidly as the medium temperature approaches the critical temperature $T_c$. This rise is more pronounced for inelastic interactions and is driven by the behavior of the DQPM strong coupling. We also find that the off-shellness of medium partons generally reduces the value of $\hat{q}$, especially at low temperatures. Furthermore, both elastic and inelastic $\hat{q}$ and $dE/dx$ exhibit a strong dependence on jet momentum. For low temperatures and high jet momenta, the contribution from the inelastic reactions dominates over the elastic one, indicating the importance of radiative processes for jet partons. A comparison of DQPM results with pQCD models shows that elastic energy loss in the QGP medium is stronger than that with massless pQCD partons. Nevertheless, at high temperatures the DQPM results for $\hat{q}$ align qualitatively with pQCD calculation as well as with lattice QCD results and phenomenological estimates. Then, investigating different scenarios for strong couplings in thermal, jet, and emitted gluon vertices, we find that $\hat{q}$ and $dE/dx$ are also highly sensitive to the dependencies of these couplings on energy, momentum transfer, or temperature. In addition, we examine the scaling $\eta/s \approx 1.25\; T^3/\hat{q}$ and show that this scaling is valid in the weak coupling regime at high temperatures but is violated in the strong coupling regime at low temperatures. Lastly, we apply the DQPM together with the soft gluon approximation to study the transport coefficients of a charm quark and show that the inclusion of radiative processes gives a small contribution at low charm momenta and a large contribution at high charm momenta, suggesting the consideration of the non-thermal strong coupling in the charm vertex.

Quantitative predictions for the energy and $A$ dependence of the cross sections for nuclear photoabsorption and inelastic electron-nucleus scattering are given. In general, the nucleons do not contribute equally to the total photon-nucleus cross section when coherent contributions of photoproduced hadrons are taken into account. At low energies (${E}_{\ensuremath{\gamma}}\ensuremath{\sim}1$ BeV), the cross sections are proportional to nuclear number $A$, but at high energies, they become proportional to the number of surface nucleons---provided that the photon interactions are mediated by hadrons of sufficiently low mass. The condition on the masses is that the momentum transfers in forward photoproduction of these states should be small compared to the reciprocals of their mean free paths in nuclear matter. In the case of $\ensuremath{\rho}$ dominance, the real-photon photoabsorption cross section has the same $A$ dependence as hadron-nucleus total cross sections for photon energies above $\ensuremath{\approx}10$ BeV, whereas the cross section for virtual photon absorption at that energy, obtained from inelastic electron scattering is nearly proportional to $A$ for spacelike momentum transfer $|{Q}^{2}|\ensuremath{\gtrsim}5$ Be${\mathrm{V}}^{2}$. We then generalize to an arbitrary spectrum of intermediate particles, and discuss the sensitivity of feasible experiments to various models in which the spectrum contains important structure beyond the $\ensuremath{\rho}$. Measurements of the photon-nucleus cross sections will provide a fundamental test of "hadron dominance" in general, and of $\ensuremath{\rho}\ensuremath{-}\ensuremath{\omega}\ensuremath{-}\ensuremath{\varphi}$ dominance in particular, as well as help to determine the basic parameters of photon-nucleon and $\ensuremath{\rho}$-nucleon interactions. We also calculate the photon-deuteron cross section and discuss the multiple scattering approach to photon-nucleus interactions. This discussion provides insight into the many-body processes which underlie the eikonal, optical-model calculations; it is also relevant to the determination of ${\ensuremath{\sigma}}_{\ensuremath{\gamma}n}$ at high energies.

TILDA, a new Monte Carlo track structure code for ions in gaseous water that is valid for both high-LET (approximately 10(4) keV/microm) and low-LET ions, is presented. It is specially designed for a comparison of the patterns of energy deposited by a large range of ions. Low-LET ions are described in a perturbative frame, whereas heavy ions with a very high stopping power are treated using the Lindhard local density approximation and the Russek and Meli statistical method. Ionization cross sections singly differential with energy compare well with the experiment. As an illustration of the non-perturbative interaction of high-LET ions, a comparison between the ion tracks of light and heavy ions with the same specific energy is presented (1.4 MeV/nucleon helium and uranium ions). The mean energy for ejected electrons was found to be approximately four times larger for uranium than for helium, leading to a much larger track radius in the first case. For electrons, except for the excitation cross sections that are deduced from experimental fits, cross sections are derived analytically. For any orientation of the target molecule, the code calculates multiple differential cross sections as a function of the ejection and scattering angles and of the energy transfer. The corresponding singly differential and total ionization cross sections are in good agreement with experimental data. The angular distribution of secondary electrons is shown to depend strongly on the orientation of the water molecule.

Doubly differential cross sections for single and multiple ionization of Ar have been measured for 500, 750, and 1000 eV electron impact. The cross sections were measured as a function of projectile energy loss and scattering angle. The energy loss range was 0--85 % of the initial projectile energy and scattering angles were between $\ifmmode\pm\else\textpm\fi{}22\ifmmode^\circ\else\textdegree\fi{}.$ The data were put on an absolute scale by normalizing to total ionization cross sections available in the literature and found to be in good agreement with the absolute electron impact cross sections from DuBois and Rudd. For 750 eV impact, a comparison was made between the present electron impact data and positron impact data obtained using the same experimental conditions. The same energy dependence and yields for single ionization were found for both electron and positron impact. On the other hand, the double- and triple-ionization yields are smaller for positron impact as compared to electron impact. Comparisons with photoionization data showed that for outer shell ionization the fractions of double and triple ionization of argon by photon impact are in quite good agreement with the present electron impact data.