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For the collision system ${\text{U}}^{88+}+{\text{N}}_{2}$ at a collision energy of 90 MeV/u, the energy distribution of electrons being nonradiatively captured from the target into the projectile continuum has been measured under an angle of ${0}^{\ensuremath{\circ}}$ with respect to the projectile beam axis. This measurement of the electron-capture-to-continuum cusp with the highest effective projectile charge ${Z}_{p}^{\mathrm{eff}}=88$ at a near-relativistic collision velocity of $\ensuremath{\beta}\ensuremath{\approx}0.41$ is shown to be characterized by a strong asymmetry in the cusp shape. By comparing the data to measurements of the radiative-electron-capture-to-continuum cusp for the same collision system, the opposite asymmetry of the cusp is traced back to the varying underlying mechanisms. The experimental results are compared with the two theoretical calculations available for this process, one of them in the semirelativistic impulse approximation and the other in the nonrelativistic continuum-distorted-wave approach. A corresponding fully relativistic treatment may be motivated by the presented experimental data.

The radiative electron capture of a target electron into the projectile continuum has been studied for the collision system ${\text{U}}^{88+}+{\text{N}}_{2}\ensuremath{\rightarrow}{\text{U}}^{88+}+{[{\text{N}}_{2}^{+}]}^{*}+{e}^{\ensuremath{-}}+\ensuremath{\gamma}$ at 90 MeV/$\mathrm{u}$. Using a magnetic electron spectrometer, the energy distribution of cusp electrons emitted under an angle of ${0}^{\ensuremath{\circ}}$ with respect to the projectile beam and with a velocity close to the projectile velocity has been measured in coincidence with the emitted photons under various observation angles. The experimental results provide a stringent test for the corresponding process in inverse kinematics, namely, the theory of electron-nucleus bremsstrahlung at the high-energy endpoint. For comparison this process is calculated using fully relativistic Dirac wave functions and using semirelativistic Sommerfeld-Maue wave functions.

The momentum distribution of fast binary-encounter electrons emitted from solid-state targets of various thicknesses is investigated both theoretically and experimentally. The production of electrons in binary-type collisions with fast, highly stripped ions is calculated in the electron-impact approximation. A transport theory is formulated which allows for energy loss as well as angular deflection of the electrons (within the separation of energy loss and angular scattering approximation) during penetration of the target. The broadening of the binary encounter peak with increasing target thickness observed experimentally in $13.6\ensuremath{-}\mathrm{M}\mathrm{e}\mathrm{V}/\mathrm{u}{\mathrm{Ar}}^{17+}$ and ${\mathrm{Ar}}^{18+}$ colliding with carbon foils is well described by theory.

Fast electron velocity spectra have been measured by means of the multidetector ARGOS (actually located at Laboratorio Nazionale del Sud, Catania) in a large angular range for atomic collisions induced by a $77A\ensuremath{-}\mathrm{MeV}$ ${}^{40}\mathrm{Ar}$ beam on an Al target. The results are in a fair agreement with a recently proposed relativistic billiard ball model and quasielastic scattering approximation calculations. However, the experimental binary-encounter peak velocities are found to be significantly lower than theoretically predicted. The data also show evidence for an excess of fast electrons in the backward direction.

We have performed K-shell radiative electron capture (K-REC) measurements with bare 60.1-MeV/u incident krypton ions, both in channeling conditions and for random orientation of a 37-\ensuremath{\mu}m silicon crystal. The sampled electron densities are quite different in each case, which has an influence both on the shape and on the amplitude of the K-REC photon peak. We have developed simulations of the K-REC photon lines: for this we have determined the impact parameter distribution at statistical equilibrium for various beam incidence conditions (direction and angular spread) using the continuum potential model for channeled ions. Multiple scattering effects were included. The K-REC photon peak was calculated within the nonrelativistic dipole approximation, K-REC being assumed to be a purely local process. Solid state electron densities were used, and impact parameter dependent electron momentum distributions (Compton profiles) were calculated for 2s and 2p silicon electrons. A remarkable agreement is found between the spectra measured with very high statistics, and the calculated ones, which leads to the following results: (i) The dependence of the K-REC yield on the beam incidence angle is obtained separately for silicon core and valence electrons, which was never observed before. We find that the core electron contribution to REC is still significant for axial alignment, whereas it is generally neglected in the literature. (ii) Electron Compton profiles are found to vary significantly with impact parameter. (iii) The free electron gas model represents a fair approximation for the description of valence electron Compton profiles. (iv) The K-REC cross section is measured with an absolute accuracy better than 20%, and found to be close to the value calculated within the nonrelativistic dipole approximation. \textcopyright{} 1996 The American Physical Society.

We have measured electron yields from the beam entrance and exit surfaces of thin carbon foils (d\ensuremath{\approxeq}4--700 \ensuremath{\mu}g/${\mathrm{cm}}^{2}$) bombarded with swift (13.6 MeV/u) highly charged (q=16--18) argon ions. The dependence of the electron yields on target thickness and charge state of the ions is analyzed within the framework of an extended semiempirical model. Due to the high velocity of the ions, it is possible to distinguish electron production in primary ionization (related to the stopping power and the effective charge of the ions) from secondary electron production due to the transport of so-called \ensuremath{\delta} electrons (cascade multiplication). By combining the experimental results with numerical simulations of electron transport in matter by a Monte Carlo method, we have obtained electron transport lengths of high energy (E\ensuremath{\gg}100 eV) \ensuremath{\delta} electrons parallel and perpendicular to the ion trajectory, as well as diffusion lengths of slow electrons (E\ensuremath{\ll}100 eV). In order to study the velocity dependence of these transport lengths, we have not only investigated 13.6 MeV/u Ar ions, but also 1 MeV/u C and 3.9 MeV/u S, for which experimental results are available [Koschar et al., Phys. Rev. A 40, 3632 (1989)]. We discuss the origin of electron yield reductions (compared to a simple scaling with the square of the nuclear charge) with heavy ions and present measurements of double differential energy and angular electron distributions of 13.6 MeV/u ${\mathrm{Ar}}^{17+}$ ions.

The slowing down of fast electrons emitted from insulators [Mylar, polypropylene (PP)] irradiated with swift ion beams (C, O, Kr, Ag, Xe; 20--64 MeV/u) was measured by the time-of-flight method at LNS, Catania and GANIL, Caen. The charge buildup, deduced from both convoy- and binary-encounter electron peak shifts, leads to target material-dependent potentials (6.0 kV for Mylar, 2.8 kV for PP). The number of projectiles needed for charging up (charging-up time constant) is inversely proportional to the electronic energy loss. After a certain time, a sudden decharging occurs. For low beam currents, charging-up time, energy shift corresponding to maximum charge buildup, and time of decharging are regular. For high beam currents, the time intervals become irregular (chaotic).

We have measured the continuum electron yield in coincidence with the charge state of the recoiling target ion for the collision systems He{sup +}{r arrow}Ne,Ar (170{le}{ital E}{sub {ital p}}{le}330 keV/u). The target ions were detected with a standard time-of-flight spectrometer employing a pulsed electric field to avoid distorting the trajectory of the continuum electron. Comparing our results with cross sections for single- and multiple-target ionization given by DuBois (Phys. Rev. A 39, 4440 (1989)) we find a similarity in the relative proportions of the recoiling charge states for argon at lower energies, but not for neon. Additionally, the yield of continuum electrons associated with each particular charge state of the recoil target ions falls off more rapidly with increasing projectile energy than might be expected.

We investigate both experimentally and theoretically the energy and angular distribution of fast monoenergetic electrons penetrating very thin foils with a thickness of the order of a few {lambda}{sub {ital f}}, the mean free path for free-electron scattering. The measured distribution functions display three distinctive components of zero scattering, single scattering, and multiple scattering, in both angle and energy. The description in terms of a kinetic equation (or equivalently, phase-space master equation) is discussed. Its numerical application is facilitated by a multiple-scattering expansion. Good agreement between experiment and theory is found.

Doubly differential electron velocity spectra induced by 95-MeV/u {sup 36}Ar{sup 18+} from thin carbon foils were measured at GANIL (Caen, France) by means of the ARGOS multidetector and the time-of-flight technique. The spectra allow us to determine absolute singly differential cross sections as a function of the emission angle. Absolute doubly differential cross sections for binary encounter electron ejection from C targets are compared to a transport theory, which is based on the relativistic electron impact approximation for electron production and which accounts for angular deflection, energy loss, and also energy straggling of the transmitted electrons. For the thinnest targets, the measured peak width is in good agreement with experimental data obtained with a different detection technique. The theory underestimates the peak width but provides (within a factor of 2) the correct peak intensity. For the thickest target, even the peak shape is well reproduced by theory.

We report absolute cross sections for binary encounter and convoy electron emission. Fast-electron velocity spectra were measured for atomic collisions induced by a 45 MeV/u ${}^{58}\mathrm{Ni}$ beam impinging on solid targets by means of the multidetector ARGOS in a large angular range (from 1.5\ifmmode^\circ\else\textdegree\fi{} to 165\ifmmode^\circ\else\textdegree\fi{}). Different conducting elemental targets ${(}^{12}\mathrm{C},$ ${}^{27}\mathrm{Al},$ ${}^{58}\mathrm{Ni}$ and ${}^{64}\mathrm{Ni},$ ${}^{\mathrm{nat}}\mathrm{Ag},$ ${}^{197}\mathrm{Au})$ and polystyrene were used. Characteristics of electrons with a velocity close to the beam velocity (convoy electrons) were found to be very sensitive to the incoming charge state of the projectile. Their yield increases with the target atomic number. The yield of binary encounter electrons with a velocity of almost twice the beam velocity at small ejection angles is roughly proportional to the area density of encountered target electrons. The velocity spectra centroids of these electrons are shifted towards lower velocities than predicted by simple two-body relativistic kinematics. Surprisingly, this effect seems to be mostly independent of target material and thickness. With increasing target atomic number, the high-energy tails of the spectra exhibit an unexpectedly large number of very fast electrons. A multiple-scattering mechanism involving multiple collision sequences of electrons between projectile and target atoms is invoked to explain this effect. The data also show evidence for an excess of fast electrons in the backward direction.

The energy-gain spectra and angular distributions for single-electron capture in collisions between Ar/sup 4+/ and Ar have been measured with a new technique for six laboratory energies in the range 33--200 eV. The spectra show that the cross section is dominated by capture to the 4p state of Ar/sup 3+/, while there is no evidence of capture to the 4s state. For energies below 68 eV, forward scattering is suppressed and strong angular oscillations within the acceptance of +- 7/sup 0/ indicate interferences between different paths on the 4p potential curves.