We have observed delayed Kα photons emitted by 60 A MeV H-like and He-like krypton ions leaving a 37 μm silicon crystal, both for random and 〈110〉 axial alignment of the target. We could extract intrashell (2s→ 2p) excitation probabilities, which are compared with values deduced from PWBA calculations.

We have performed {ital K}-shell radiative electron capture ({ital K}-REC) measurements with bare 60.1-MeV/u incident krypton ions, both in channeling conditions and for random orientation of a 37-{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 {ital K}-REC photon peak. We have developed simulations of the {ital 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 {ital K}-REC photon peak was calculated within the nonrelativistic dipole approximation, {ital 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 2{ital s} and 2{ital p} 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 {ital K}-REC yield on the beam incidence angle is obtained separately for silicon core and valence electrons, which was never observed before. Wemore » 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 {ital K}-REC cross section is measured with an absolute accuracy better than 20{percent}, and found to be close to the value calculated within the nonrelativistic dipole approximation. {copyright} {ital 1996 The American Physical Society.}« less

(1993). Channeling of swift heavy ions. Radiation Effects and Defects in Solids: Vol. 126, No. 1-4, pp. 313-318.

We present original experiments providing information on the impact parameter dependence of stopping power in axial channeling and on electron impact ionization. A beam of 27 MeV/u Xe35+ ions (far from the equilibrium charge state in matter) has been transmitted through a Si crystal, parallel to the [110] axis. The very broad and out-of-equilibrium emerging charge state distribution (35 ⩽ Qout ⩽ 53) arises nearly only from electron impact ionization. The corresponding cross sections σ(Q → Q + 1) obtained, when fitting the experimental distribution by Monte Carlo simulations. The energy loss spectra measured for each Qout have been analyzed, using the same simulation program. The choice of Qout corresponds to a selection in the transverse energy distribution of the ions and on their accessible transverse space. Detailed information is thus obtained on “local stopping power”, i.e. in a given point of the transverse [110] space, and on its relation with the corresponding local electronic density. The respective influence of core and valence electrons is studied. For very well channeled ions, the energy loss, which is only induced by the valence electron gas, depends little on the local electronic density sampled by the particles. It is very close to the .energy loss corresponding to a homogeneous electron gas with density equal to the mean density of valence electrons.

The interaction of moving ions with single crystals is very sensitive to the orientation of the incident beam with respect to the crystalline directions of the target. The experiments show that high energy heavy ion channeling deeply modifies the slowing down and charge exchange processes. In this review, we describe the opportunity offered by channeling conditions to study the charge exchange processes. Some aspects of the charge exchange processes with high energy channeled heavy ions are selected from the extensive literature published over the past few years on this subject. Special attention is given to the work performed at the GANIL facility on the study of Radiative Electron Capture (REC), Electron Impact Ionisation (EII), and convoy electron emission. Finally we emphasize the interest of studying resonant charge exchange processes such as Resonant Coherent Excitation (RCE), Resonant Transfer and Excitation (RTE) or Dielectronic Recombination (DR) and the recently proposed Nuclear Excitation by Electron Capture (NEEC).

We have measured the emerging charge-state distribution of 27-MeV/u ${\mathrm{Xe}}^{35+}$ beams channeled through a thin Si single crystal, and deduced electron impact ionization cross sections for ${\mathrm{Xe}}^{35+}$ to ${\mathrm{Xe}}^{45+}$ by 14.7-keV electrons. They are \ensuremath{\approxeq}2 to 4 times higher than predicted by usually accepted empirical estimations. We have also measured the energy loss versus emerging charge state. For hyperchanneled ${\mathrm{Xe}}^{\mathrm{Q}+}$ ions, the stopping power depends only on the mean (and not on the actually sampled) density of valence electrons and compares well with the prediction of the electron gas model.

The federating theme of superconductivity has given rise to a number of experimental studies of point defects in solids as different as transition metals (V, Nb, …), A-15 compounds (V3Si, Nb3Ge, …), or perovskite-like copper oxides. Some of these experiments are presented here

Irradiation disorder generally alters superconductivity either increasing, or decreasing, the critical temperature. Some experimental results are presented especially in the cases of “old” A-15 structures and of “new” perovskites irradiated by electrons or by fast heavy ions.

Transient response of a 2500 Å thick superconducting Nb film to the impact of a single 129Xe50+ ion of total energy 3.2 GeV, transferring about 2.4 keV/Å of electronic excitation energy, was observed by means of a superconductor-insulator-superconductor tunnel junction attached to the film some distance away from the impact point. The response had rise time of less than 150 ps (resolution limit of the electronic detection system) and decay time varying from 2 to 1.3 ns as function of temperature. Main experimental features, if not finer details, are in good agreement with Rothwarf and Taylor theory of nonequilibrium processes in superconductors.

Channeling phenomena have been extensively studied in the last twenty years, and have been shown to be very helpful for the more general study of ion-matter interactions. However, the experimental work has been mostly performed with light projectiles, protons or alpha particles, and also electrons and positrons, over the whole range of available energies, from a few keV to several hundreds of GeV. Channeling investigations with heavy ions have been mainly performed with moderately heavy ions of Tandem energies, that is, between 1 and 3 MeV/u, and particularly by the Oak Ridge group in a nice series of experiments. It can be expected that channeling studies of very fast heavy ions, then in high charge states, will reveal specific properties of their interactions with matter.

We present the first experimental channelling study with very energetic heavy ions. This work, performed with 60 MeV/u argon projectiles bombarding a 100 μm thick germanium crystal, aimed at determining the experimental conditions and techniques suitable to the observation of channelling effects with such projectiles. We show that the detection of forward emitted gamma-rays, neutrons and beam fragmentation products allows us to get the crystal aligned and to measure the channelling parameters, χmin and ψ1/2, which we find to be in reasonable agreement with theoretical estimates.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not.The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

A Nb3Ge sample, of transition temperature T c = 20.8 K, has been irradiated at low temperatures with electrons of various energies E. It is shown that: 1. The defect production rate at E = 2.5 MeV does not depend drastically on the state (normal or superconducting) of the sample.2. The minimum displacement energy occurs at E = 0.5 MeV, which corresponds to a threshold energy of 15 eV for Ge.

We have studied by transmission the planar channeling of protons through tantalum without and then with dislocations. The number of channeled particles decreases significantly when the sample contains dislocations, but no shift is observed in the emergent energy of these particles. Nous avons étudié par transmission la canalisation planaire de protons dans des échantillons de tantale, ne contenant pas puis contenant des dislocations. Le nombre de particules décanalisées diminue de façon significative lorsque l'echantillon contient des dislocations, mais on n'observe aucune variation de l'énergie des particles émergentes.