Yves Quéré

Defect production rates in normal and in superconducting states

Abstract Electronic thermal conductivity of metals is different in normal (N) and superconducting (S) states. An experiment is described in which various metals (Nb, V, Pb) are irradiated with fast neutrons or with uranium fission fragments, alternatively in (N) and in (S) state at constant temperature. The damage rate is the same in both eases, showing that electronic thermal conductivity does not influence significantly the damage rate.

Impact parameter dependence of energy loss and target-electron-induced ionization for 27 MeV/u Xe35+ incident ions transmitted in [110] Si channels

Abstract 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.

RTE measurement with Xe52+ ions channeled in a Si crystal

Abstract KLL resonant transfer and excitation (RTE) of 33–43 MeV/nucleon He-like Xe ions channeled along the 〈110〉 axis of a thin Si crystal has been investigated in measurements of charge state distributions and of the resulting X-ray production. The resonance peaks obtained by the two methods are quite similar. The charge state measurements show that RTE appears, for well channeled ions, to be rather independent of the electron density they sample. Resonance energy, width and intensity are compared with theoretical estimates.

An experimental argument — in Nb3Ge — for the Labbé-Barisic-Friedel theory of A-15 superconductors

Abstract When Nb 3 Ge is irradiated with electrons of energy E , a range E (0.5–0.8 MeV) is observed where the critical temperature is unaffected by irradiation whereas the electrical resistivity increases. This fact is interpreted as being due to disordering of the germanium sublattice only, and gives strong support for the Labbe-Barisic-Friedel theory of one-dimensional superconductivity in A-15 compounds.

Neutron radiation damage in superconductor Nb3Ge

Abstract A sample of Nb 3 Ge of initial critical temperature T c = 19.82 K has been irradiated by fast neutrons at low temperature. Critical temperature decreases linearly with neutron flux even for the lowest observable changes of T c , the rate being ΔT c / Δϑt = −0.5 × 10 −18 K/ncm −2 . The damage rate, measured as the change of resistivity at ≅ 22 K, does not depend on the state (normal or superconducting) in which the sample is being irradiated.

Influence of intrashell excitation (n=2) on the population of mestastable states of H- and He-like krypton ions in channeling conditions

Abstract We have observed delayed K α 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.

Electronic capture and excitation of highly charged channeled ions

Abstract Two aspects of heavy ion channeling are presented. The first aspect is related to the fact that channeled ions interact only with the most loosely bound target electrons. One can take benefit of this feature to study processes such as radiative electron capture (REC) and resonant transfer and excitation (RTE) in a dense quasi-free electron gas. The experimental work, performed at GANIL, devoted to these two processes is described. A possible extension to Nuclear RTE or NEEC (nuclear excitation by electron capture) studies is also described. The second aspect discussed is related to the periodicity of the potential experienced by channeled ions. We show that in a well chosen case this could lead to a significant and detectable coherent excitation of the projectile nucleus.

Charge exchange processes of high energy heavy ions channeled in crystals

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).

Radiation effects in (old and new) superconductors

Abstract 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.

Point defects in superconductors

Abstract 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