This paper gives a short review of the past and recent activities of the Atomic Collisions in Solids Lyon-group, in collaboration with other groups, in the field of high energy channelling. The ion-channelling programme was performed at GANIL-Caen and at GSI-Darmstadt. The electron-channelling programme started at ALS-Saclay for relativistic incident energies and was then extended to SPS-CERN for ultra-relativistic energies. The last part of this paper presents the electron-channelling experiments performed originally at ALS-Saclay, then at BTF-Frascati and more recently at LS-Saga, in order to observe the electron “internal clock” predicted in 1924 by L. de Broglie.

The blocking technique in single crystals is a direct method to investigate the presence of long fission time components. With a lead beam impinging on a germanium single crystal, we tried to produce compound nuclei (CN) with atomic number Z=114 at high excitation energy. Blocking patterns for reaction products are reconstructed with position sensitive detectors at 20 degrees relative to the beam direction. The Z and the energies of all products are measured with Delta E-E telescopes of the 4 pi INDRA array, so that all reaction channels are unambiguously identified. With this setup, we can reach long fission times (>10(-8)s) that can be associated with CN fissions. However, in contrast to previous experiments in which such long fission times could be measured for Z = 120 and 124, no hint of long lifetimes within our sensitivity limit for Z=114 was observed, which may be due to the neutron deficiency of the formed isotopes.

Reaction mechanism analyses performed with a 4pi detector for the systems 208Pb + Ge, 238U + Ni and 238U + Ge, combined with analyses of the associated reaction time distributions, provide us with evidence for nuclei with Z=120 and 124 living longer than 10(-18) s and arising from highly excited compound nuclei. By contrast, the neutron deficient nuclei with Z=114 possibly formed in 208Pb + Ge reactions have shorter lifetimes, close to or below the sensitivity limit of the experiment.

. The blocking technique in single crystals has been applied to reaction time measurements for the 238 U + Ge system at 6.1 MeV/nucleon. Backed up with a reaction mechanism analysis using the INDRA 4π detector, it provides a direct experimental evidence for Z = 124 compound nuclei living longer than 10 -18 s, indicating thus very high fission barriers for this element.

The impact-parameter dependence of electron capture by 20-MeV/u U91+ ions has been studied by means of channeling in a 11-mu m-thick silicon crystal. Such ions are far from their equilibrium charge state in matter, and channeling offers a unique opportunity to study electron capture in conditions going from the extreme case of a single capture event (for the best channeled ions) to the case of multiple charge exchange events leading to charge state equilibrium (for unchanneled ions). For each incident ion, the charge states at emergence, energy loss, electron emission-, and x-ray yields are measured. The correlations between these quantities are studied. The data are reproduced by simulations based on the ion flux distribution. We show that the mechanical electron capture dominates at impact parameters smaller than 0.5 angstrom, whereas radiative electron capture is the only process occurring beyond. Specific features associated with highly charged heavy ions at intermediate velocities are discussed, in particular the ionization following capture into highly excited states and the local electron density enhancement due to the electron gas polarization. The measured impact-parameter dependence of capture probabilities is compared to continuum-distorted-wave eikonal-initial-state calculations, extrapolated to n > 5 final states.

Fission time measurements have been undertaken in the super-heavy elements domain around Z=120, using the crystal blocking technique. Complete fusion followed by fission mechanisms have been evidenced from time measurements and detailed kinematics analysis.

We report on the observation of a strong perturbation of the electron gas induced by 20 MeV/u U$^{91+}$ ions and 13 MeV/u Pb$^{81+}$ ions channeled in silicon crystals. This collective response (wake effect) in-duces a shift of the continuum energy level by more than 100 eV, which is observed by means of Radiative Electron Capture into the K and L-shells of the projectiles. We also observe an increase of the REC probability by 20-50% relative to the probability in a non-perturbed electron gas. The energy shift is in agreement with calculations using the linear response theory, whereas the local electron density enhancement is much smaller than predicted by the same model. This shows that, for the small values of the adiabaticity parameter achieved in our experiments, the density fluctuations are not strongly localized at the vicinity of the heavy ions.

In two experiments performed with 20–30 MeV/u highly charged heavy ions (Pb56+, U91+) channeled through thin silicon crystals, we observed the original features of superdensity, associated to the glancing collisions with atomic rows undergone by part of the incident projectiles. In particular, the very high collision rate yields a quite specific charge exchange regime, that leads to a higher ionization probability than in random conditions. X-ray measurements show that electrons captured in outer shells are prevented from being stabilized, which enhances the lifetime of the projectile inner shell vacancies. The charge state distributions and the energy loss spectra are compared to Monte-Carlo simulations. These simulations confirm, extend and illustrate the qualitative analysis of the experimental results.

"Impact parameter dependent electron capture by dec elerated U 91+ ions at 20 MeV/u using crystal channeling conditions". D. Dauvergne, A. Bräuning-Demian, F. Bosch, H. Bräuning, M. Chevallier, C. Cohen, A. Gumberidze, R. Kirsch, C. Kozhuharov, A. l'Hoir, D. Liesen, P. H. Mokler, J.-C. Poizat, C. Ray, Th. Stöhlker, M. Tarisien, E. Testa, S. Toleikis et M. Toulemonde, Nuclear Instruments and Methods B 205 (2003) 773

We report observations of backward and forward electron emission by a thin silicon crystal target traversed by 29 MeV/u Pb{sup 56+} incident ions. For each incident ion we have performed measurements of backward and forward electron emission, of the energy loss and of the charge state of the transmitted ion. The crystal target was traversed by incident ions either in random incidence or in axial alignment conditions. In both cases these correlated measurements bring original information on electron emission. In random conditions, using an incident ion species with a charge quite far from equilibrium, we observe correlations between backward and forward electron emission, that we understand when analyzing the associated charge exchange and energy-loss data. In channeling conditions, we added electron emission measurements to simultaneous energy-loss and charge state measurements (that are known to characterize quite precisely the type of trajectory of a projectile transmitted through a thin crystal). This allowed us to observe the reduced electron emission due to hyperchanneled ions, that interact mainly with target valence electrons, and also the enhanced electron emission due to projectiles entering the crystal very close to atomic strings.

We present results of an experiment using decelerated U91+ ions, extracted from the GSI-ESR storage ring, and transmitted through a thin silicon crystal in channeling conditions. Charge state at emergence, secondary electron multiplicity and X-rays are measured simultaneously. These conditions allow to study the competition between mechanical electron capture (MEC) and radiative electron capture (REC) as a function of impact parameter. We observe that REC is the dominant charge exchange process for the best channeled ions, i.e. those travelling always far from the silicon target core electrons. For ions with high transverse energy, MEC into highly excited states is counterbalanced by enhanced ionization arising from a succession of close collisions along atomic rows.

We report a measurement of photon impact ionization of K and L shell of Au and K shell of Ag targets in the 1-GeV energy range. We show that the cross section is dominated by a contribution from a new channel called vacuum-assisted photoionization. In this process the energy-momentum balance associated with the removal of the innershell electron is obtained by conversion of a high-energy photon into an electron-positron pair. This measurement is consistent with the theoretical prediction that vacuum-assisted photoionization is the most probable ionization mechanism at very high energies.

Under impact of 150-keV/atom Au-n(+) projectiles (1less than or equal tonless than or equal to9) on gold targets the emission yield of anionic clusters increases much faster with n than expected from simple proportionality. Moreover the anion size distribution is much wider for bombardment by clusters. The cluster yield enhancements reflect the size dependence of the cluster stability according to the electronic shell structure. Accordingly, the most intense emission is observed for Au-7(-): under Au-9(+) impact the Au-7(-) yield per incident atom is enhanced by a factor of similar to6.

Axially oriented crystals, penetrated by high-energy electrons, are powerful photon sources and, hence, intense positron sources. Such kinds of positron sources have been studied experimentally at CERN, with the tertiary electron beam of the SPS having an energy of 6 and 10 GeV. Four and eight millimeters thick tungsten crystals and a compound target made of a 4 mm crystal followed by a 4 mm amorphous disk were used with an orientation along the 〈111〉 axis. The positrons were detected by a drift chamber, partially immersed in a magnetic field. The reconstructed trajectories allowed the determination of their energy and angular spectra. Significant enhancements were observed for the crystal source when compared to the amorphous one. The gain was about 3 for the 4 mm target and about 2 for the 8 mm and the compound targets. These preliminary results are described after short presentations of the experimental set-up and of the method of track reconstruction.

Absolute yields for multiple electron capture as well as projectile ionization have been measured for 46 MeV/u U91+ ions traversing thin carbon foils with different surface topologies. Multiple electron capture yields vary strongly, depending on the target manufacturer. The samples including the one with the smoothest surface show a deviation of the yield from the expected scaling based on subsequent single capture for decreasing target thickness. This indicates a strong contribution of the surface, whose nature cannot be identified by the current data.

H. Bräuning, A. Bräuning-Demian, G. Bednarz, F. Bosch, X. Cai, C. Cohen, D. Dauvergne, A. Gumberidze, H. Harutyunyan, R. Kirsch, C. Kozhuharov, D. Liesen, P.H. Mokler, J.-P. Rozet, Z. Stachura, Th. Stöhlker, M. Terasawa, S. Toleikis, A. Warczak, ESR-Team JLU Giessen (Germany), GSI (Germany), University of Cracow (Poland) IMP Lanzhou (China), GPS Paris (France), IPNL Villeurbanne (France), Yerevan SU (Armenia) INP Carcow (Poland), Himeji Inst. of Technology (Japan),

Fission times of lead and uranium nuclei have been measured at GANIL by the crystal blocking method. The inverse kinematics was used. Fragment atomic numbers and total excitation energies were determined. For data analysis, full Monte-Carlo trajectory calculations were used to simulate the blocking patterns. The effect of post-scission emissions, included in our simulations, is discussed. At high excitation energies, the scissions occur dominantly at times shorter than 10(-19) s, whereas at low excitation energies (E* < 250-300 MeV), scissions occurring at much longer times with sizeable probabilities are observed both for uranium and for lead nuclei, leading to average scission times much longer than those inferred from pre-scission emission. (C) 2002 Elsevier Science B.V. All rights reserved.

A new domain in physics is opened, with the possibility to study the recombination of highly-charged decelerated ions, far from their equilibrium charge state in matter. In particular, in solids, a strong polarization of the electron gas will be induced in the vicinity of the ion, thus influencing both the charge exchange probabilities, and the energy deposition along the ion track. Crystal channeling presents the unique feature to provide a dense quasi-free electron gas target for ions travelling far from the target atomic rows or planes. It also allows, by selecting impact parameters, to study in details the competition between Radiative Electron Capture (REC) and Mechanical Electron Capture (MEC). Indeed, REC probabilities depend mainly on the electron density sampled by the projectile, and thus REC (dominated by the capture into Kand L-shells) can occur in regions where only valence or conduction electrons are present. On the other hand MEC probabilities are much more peaked at small impact parameters, because, first, the target atom recoil is needed for energy and momentum conservation during the charge exchange process, and, second, the overlap between initial and final wave functions of the transferred electron is required both in spatial and momentum spaces. After the first success of the deceleration–extraction technique for Au ions at 53 MeV/u in 1995 [1], experiments at lower energies had been performed only inside the ESR storage ring, using the internal gas target. The present experiment, performed in December 2001, succeeded in decelerating, cooling, extracting and transporting a U ion beam at 20 MeV/u into cave A. Fig.1 presents a spectrum of transmitted ions at the focal point after a magnetic analysis by the cave A spectrometer. This spectrum was obtained in axial channeling conditions along the <110> direction of a 11.7 μm thick silicon crystal. A broad charge state distribution is observed, that is governed by the large REC probabilities for channeled ions. For a random crystal orientation none of these high charge states are transmitted. The significant fraction of the ions frozen in their initial 91+ state is an encouraging indication that thicker crystals could be used to decelerate highly-charged ions down to very low velocities. More information will be extracted from the X-ray spectra, as shown in figs. 2-a and b for random and axial crystal orientations, respectively. These spectra were recorded at 90° from the beam direction by a collimated Ge detector. For the random orientation, the MEC probabilities are more than two orders of magnitude higher than the REC ones, and incident U ions captured, in average, more than 15 electrons in the target; Thus the only visible lines are projectile Kand L-lines, resulting from the decay after capture into high n,l states. When transmitting the ions along the <110> crystal axis, Kand LREC peaks appear nicely, because most of the ions, travelling far from the atomic strings in the crystal, can no longer undergo MEC. The comparison of the two spectra shows that, for the random orientation, the projectile K and L shells are filled already within the very first thousands Angströms by MEC, thus inhibiting K-and L-REC during the whole path inside the target. On the other hand, the K-and L-REC line shape reflect the electron momentum distribution in the initial state (Compton profile). Indeed, these lines, observable in channeling conditions, are very narrow. This clearly shows that only valence electrons can be captured by REC, because MEC is already dominant at impact parameters of the order of the spatial extension of the silicon core electrons (i.e. about 0.5 Å). A detailed analysis of the experiment is currently carried out. Support by IN2P3-GSI collaboration agreement is highly acknowledged.

Tungsten crystals oriented on their <111> axis, were submitted to 6 and 10 GeV electron beams on the SPS-CERN transfer lines. The crystals, 4 and 8 mm thick, used alone or associated to 4 mm thick amorphous disk, were studied as positron sources. The emerging positrons were detected by a Drift Chamber partially immersed in a magnetic field, where their trajectories were reconstructed providing the energy spectrum and the angular distribution. Significant enhancements were observed for the crystal source when compared to the amorphous one of the same thickness. The gain was larger than 3 and 2 for the 4 mm and 8 mm targets, respectively. The presented results look very promising for e(+)e(-) linear colliders. (C) 2002 Elsevier Science B.V. All rights reserved.