J.C. Poizat

Slowing down of hydrogen clusters in thin foils

Abstract We present experimental results for the energy loss in carbon of 10 to 120 keV/proton H n + clusters (up to n =25). Special attention is given to the ratio between the total energy loss of a H n + cluster and the total energy loss of n individual protons of the same velocity. At low cluster energies this ratio is less than unity, while it is larger than unity at high energies. Saturation effects with cluster size are observed at least for lower cluster energies. A theoretical model describing these effects is developed, and the theoretical results compare well with experimental data.

Electron loss and capture cross sections of 800 KeV amu−1 H and He species in carbon foils

Abstract Transmitted charge fractions φ ij of incident 800 keV amu −1 projectiles through 1−8 μg cm −2 carbon foils have been measured for one-electron H 0 , 3 He + and H 2 + as well as two-electron H − , 3 He 0 and H 3 + . For the thinnest targets (⪅3 μg cm −2 ), the transmitted fraction φ ij of the incident species can be characterized by an exponential decay with a coefficient of unity. This feature can be reasonably understood only if the incident projectile is transmitted with its original electron(s) intact. For the atomic projectiles, the charge changing cross sections σ ij ( i ≠ j ) deduced from the φ ij are simulated well by the predictions based on binary collisions between a free projectile and target atoms.

The dependence of H2+ molecular-ion yields on the proton neutral fraction

Abstract We have measured the fraction of H 2 + molecules emerging from 1.8−72 μg/cm 2 carbon targets bombarded with 0.75–2.25 MeV H 2 + and H 3 + projectiles. The H 2 + molecular yields decrease from ∼10 −3 for 1.8 μg/cm 2 to ∼10 −10 for 72 μg/cm 2 targets and are only weakly dependent upon the projectile velocity. The distance between the two protons upon emergence from the target depends upon the mutual Coulomb repulsion during the dwell-time in the target, upon multiple scattering, and upon the wake-potential. The velocity dependence of the molecular ion yield, for a given dwell-time, follows the proton neutral fraction. The dependence of the probability of H 2 + molecular-ion formation upon the proton neutral fraction, the projectile dwell-time, and the influence of multiple scattering are considered. Additional factors influencing the molecular ion yields were investigated in double-target experiments.

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.

Measurement of axial effects on the pair creation by 30–150 GeV photons and on the radiation emitted by 150 GeV electrons and positrons in Ge crystals

Abstract Crystal-assisted processes around the 〈110〉 axis of a Ge crystal cooled to 100 K have been investigated at CERN with 150 GeV e− and e+ incident beams and 30–150 GeV incident photons. A large enhancement of the radiation emitted by incident e+ and e− is observed. Electrons are found to radiate more than positrons. When the crystal is tilted around 〈110〉 clear channeling effects are still observed. However the radiation enhancement persists at angles much larger than the channeling critical angle. The total pair creation rate by photons aligned along 〈110〉 has also been measured: in contrast with what is observed for a random orientation, the rate increases sharply with the photon energy. A scan through 〈110〉 shows that the pair production does not reach a maximum for axial incidence but at a tilt angle which decreases when the photon energy increases. These measurements indicate that crystal assisted pair production is not governed by the channeling of the created electrons.

Another experimental evidence for coherent electronic excitation of channeled He+ ions

Abstract The coherent excitation of He + ions passing along the [110] axis of a thin gold crystal has been studied. Excitation occurs for several resonant energies, from 250 to 350 keV, which could be related to the Fourier frequency spectrum of the Coulomb perturbation.

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.

Simultaneous study of non-equilibrated charge and excitation states of MeV/amu light atomic and molecular ions emerging from thin solid targets

Abstract The evolution of the relative population of a given excited state of atoms (H0) or ions (H+) emerging from thin carbon foils bombarded with MeV/amu atomic (H0, He+) and molecular (H2+, H3+) beams has been observed in equilibrium and non-equilibrium situations which are known to occur in charge exchange processes. The comparison between charge state and photon yield (transition n = 4 → 3) variations for He+ gives new information about capture and excitation processes in the target. Strong enhancement of the relative population of the low excited state n = 2 in the H0 emerging beam is observed in the non-equilibrium regime for the H0 incident beam as for molecular hydrogen incident beams. In addition, specific molecular effects occur in the equilibrium regime.

Neutral fraction in proton beams after channeling through thin crystals

Abstract The effects of planar and axial channeling on the neutral fraction Φ0 in proton beams emerging from thin nickel and gold crystal were measured in the energy range 0.4 to 2 MeV, corresponding to velocities 4–9 atomic units. Channeling reduces Φ0 relative to the values observed for random directions, approximately by 15% at 0.4 MeV and by a factor ∼4 at 2 MeV in gold for incidence along the 〈110〉 axial direction. By tilting the crystal, the angular dependence of Φ0 was measured near planar directions. These results show that Φ0 depends on the distance of closet approach of protons to the atomic planes and, for a given distance of closet approach, on the details of the particle trajectories at emergence. It is concluded that, for proton velocities comparable with the velocities of the target core electrons, the contribution of atomic cores inside the solid to the electron capture by emerging protons dominates.

Foil dissociation of 40–120 keV/p hydrogen clusters

Abstract We report on measurements of angular and charge state distributions of hydrogen fragments resulting from the dissociation of fast H + n clusters ( n ≤ 13) in a carbon foil. The proximity effects on the fragment neutralization have been investigated for beam velocities above and around the Bohr velocity. At a given velocity the angular width and the yield of neutral atoms are observed to saturate at n ≥ 5 and n ≥ 7, respectively. The interpretation of this behaviour provides some insight into the collective aspects of the collisions and into the structure of hydrogen clusters.