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CAS; International audience; 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 outershells are prevented from being stabilized, which enhances the lifetime of the projectile innershell 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.

International audience; We report on the observation of a strong perturbation of the electron gas induced by 20 MeV/u U91+ ions and 13 MeV/u Pb81+ ions channeled in silicon crystals. This collective response (wake effect) induces 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 in the vicinity of the heavy ions.

Gold cluster beams delivered by the 2.5 MV Van de Graaff accelerator of the Institut de Physique Nucleaire de Lyon, equipped with a liquid metal ion source (LMIS), were used for secondary electron emission measurements. We investigated backward emission from thin carbon and gold targets bombarded by Au+n projectiles (1⩽n⩽9) of energies between 150 and 500 keV/atom in a standard vacuum of 10−7 mbar. For Au+ ions incident on carbon and gold targets it is observed that the electron yield follows the velocity dependence of the energy deposited at the surface of the target in ionization processes induced both by the incident projectile and by the recoiling target atoms. For Au+n clusters of a given velocity the electron yield per incident gold atom is observed to decrease when the cluster size increases. Given the large contribution of the recoil atoms to the electron emission, we studied the dependence of the cluster effect on the target thickness, for thicknesses of the order of the size of the atomic displacement cascades. The comparison of the electron yields of thin carbon targets of various thicknesses at the impact of Au+ and Au+n projectiles of the same velocity has effectively evidenced a thickness effect for cluster induced electron emission.

A liquid metal ion source (LMIS) was installed on the high-voltage terminal of the 2.5 MV single-stage Van de Graaff accelerator of the Institut de Physique Nucleaire de Lyon and was used to deliver intense MeV energy beams of cluster ions. After acceleration, the ions produced from an eutectic Au—Si alloy were mass-selected by means of a magnetic analysis. The beam was mainly composed of the monoatomic ions Au+ and Si+ and of the clustersAu n + , Au n Si+, and Au n Si 2 + (n = 2 − 13). The intensities decreased for increasing n values, but remained remarkably high. For pure gold ions, the maximutn intensity measured at the target site varied from 250 nA, for Au+, down to 20 pA, for Au 9 + . The research program on the study of the specific effects of the impact of energetic clusters on solid surfaces concerns the fundamental interaction processes and the solid modifications induced by high-density energy deposits. Our first experiments with Au n + , clusters led to the following results: Nonlinear dependence on the cluster size of kinetic secondary electron emission from thin carbon foil enhances the damage rate of irradiated silicon crystals as compared to monoatomic Au+ ion effects.

We have performed projectile-by-projectile measurements of backward and forward secondary-electron emission of thin carbon foils under impact of MeV H{sup 0} projectiles. The emitted electrons were detected in coincidence with the protons or the neutrals emerging from the target. We have used a very thin target for which we know that, at energies above 2 MeV, the emergent neutrals are essentially transmitted, i.e., they have kept their electron throughout the target. In these conditions the emission yields measured in coincidence with emergent neutrals are found lower than for protons of the same velocity, but the reduction factor is not the same for backward and for forward emission. We show that this can be explained by the screening of the proton charge by the electron during the H{sup 0}-target interaction. We have observed other effects related to forward electron emission: if the H{sup 0} projectile emerging from the foils results from an electron-capture event taking place close to the exit surface, the forward emission is enhanced (at energies above 500 keV) by the contribution of Auger electrons resulting from the rearrangement of the carbon atoms ionized in the capture events. For H{sup 0} projectiles ionized in the target we have usedmore » the statistics of the number of forward-emitted electrons to deduce the probability for an incident electron to be transmitted through a very thin target and to produce cascade electrons. {copyright} {ital 1997} {ital The American Physical Society}« less

We have performed a channelling experiment in which a ion beam, extracted by radiative recombination from the storage ring ESR at GSI after deceleration and cooling, was directed onto a thin silicon crystal. The directional properties of channelling have been used to test the quality of the extracted beam. An upper limit of the beam emittance has been measured to be via x-ray measurements. Coincidence measurements between photons and ions transmitted after capture of one electron in the crystal have allowed us to study L-REC and the associated emission.

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.

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.

We used channeling through a thin crystal to get an estimate of the cross section of the resonant trielectronic capture by ${\mathrm{Kr}}^{34+}$ ions. K x-ray\char21{}$K$ x-ray coincidence measurements were performed with a selection on the charge state and energy loss of transmitted ions. An upper limit of $1.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}27} {\mathrm{cm}}^{2}$ at the resonance is obtained, and this represents an improvement by two orders of magnitude with respect to previous ion-gas experiments. The possibility to reach the theoretical predictions experimentally is discussed.

The emission statistics of secondary electrons emitted from solid targets under ion impact has been recently the object of much interest from a theoretical as well as experimental point of view since it provides very useful information about the electron excitation process in the solid. In the case of very thin targets, it is possible to measure simultaneously the number of electrons emitted from both sides of the target and the exit charge state of the projectile. The resulting two variable (number of electrons emitted in the backward and in the forward directions) statistical distribution shows in some cases a clear correlation between the numbers of electrons emitted from both sides of the target. This correlation between backward and forward emission has been studied from Monte Carlo simulations and from experiments for H0 and H+ projectiles incident on thin carbon foils in the MeV energy range. It is shown that the charge exchange processes of the incident projectiles in the target play an important role in the interpretation of the backward–forward correlation of the secondary electron emission.

The secondary electron emission has been measured for 2 MeV H+ and for H0 projectiles passing through very thin carbon foils without charge changing. We have studied the statistics of the emission from both entrance and emergence surfaces. Simultaneously, Monte Carlo calculations have been performed for similar experimental conditions (500 keV

Recently, measurements on the statistics of the number of electrons emitted due to MeV light ion impact on solid targets have been fitted adequately by negative binomial distributions. We present here Monte Carlo simulations for backward and forward emissions induced by protons incident on thin carbon foils in the energy range 250 keV –2 MeV, and compare them with experimental results. A general discussion on the statistical distribution of the number of emitted electrons is given. A transport model giving the general form of the probability generating function (pgf) of the distribution of the number of emitted electrons is also proposed.

Owing to the results of Beuhler et al. [Phys. Rev. Lett. 63 (1989) 1292] on cluster impact fusion we undertook experiments with accelerated (D2)n+, (N2)n+ and (CD4)n+ clusters to investigate a possible enhancement of (d,d) nuclear fusion rates in cluster impact on deuterated titanium and polyethylene targets. With (n ≥ 100) clusters containing deuterium atoms of energies below 1 keV/D, no fusion event was detected. Under these conditions, the experimental background for the detection of the 3 MeV protons emitted in the fusion reaction sets an upper limit to the fusion yield of 10−13 per cluster impact. This is four orders of magnitude below the proton yields previously reported by Beuhler et al. for (D2O)n+ clusters in the same energy range. Nevertheless, the main difficulty of this kind of experiments is to avoid experimental artefacts that may result from the presence of light and fast deuterated components in the beam at the target site. Experiments with energy analysed (CD4)n+ and (N2)n+ clusters have evidenced that a magnetic deviation of light projectiles in front of the target is not an absolutely safe procedure. It is shown that a D+ acceleration process might explain why the time-of-flight experiments by Beuhler et al. [Phys. Rev. Lett. 67 (1991) 473] were not able to reveal the presence of accelerated light ions at the target site.

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.

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.

We have measured the radiation emitted by 150-GeV ${e}^{\ensuremath{-}}$ incident along the $〈110〉$ axis of Ge crystals. The on-axis total radiated energy is 25 times larger than for nonaligned directions for 0.4-mm-thick Ge. The distribution of the radiated energy versus the angle of the electron beam yields a half-width much larger than the channeling critical angle. The on-axis results confirm the predictions of the crystal-assisted radiation theory. The Born approximation to the coherent bremsstrahlung fits the data at large angles.

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.

Radiation spectra measured for 150 GeV positrons directed at or close to the (10) planar direction of a 0.185 mm thick Ge crystal show clear evidence for increasing lower cut-off frequency with decreasing pathlength in the planar electric field. The observed features are well reproduced by the theory of "synchrotronlike" radiation off a continuum planar potential.

Measured energy spectra of photons emitted at 0° by a 54 MeV electron beam channeled along the most open planar directions of a 20 μm thick diamond single crystal show unusually sharp lines which are identified as transitions between discrete states of transverse energy of the channeled electrons. Comparison with the spectrum obtained for random incidence reveals each channeling spectrum to exhibit a broad hump feature which is attributed to the radiative transitions of dechanneled electrons ending in bound and unbound states.

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.

We have investigated the energy loss of the single-charged fraction of beam transmitted through extremely thin carbon foils bombarded with He+ projectiles having energies from 1.5 to 2.4 MeV. A reduced energy loss, leading to an effective charge of 1.1 ± 0.3, was observed for the He+ component that is transmitted without charge exchange in the target, strongly supporting the idea that this component keeps its bound electron when passing through the target.

Distributions in angle and in energy have been measured for ions transmitted through thin solid targets bombarded by various molecular-ion beams. The results, which differ markedly from those expected on the basis only of a simple “Coulomb explosion”, indicate the existence of an additional force tending to align the internuclear vectors of each cluster with the beam direction. An explanation is suggested in terms of a potential attributable to the plasma oscillations generated by the particles as they traverse the solid. Calculations are presented to substantiate this idea.

The velocity dependences of the population of 2p and 4p states in fast (0.4–2 MeV) He + ion beams transmitted through a thin carbon foil have been measured. They are found to be quite different to each other. These results are analyzed in the framework of a simple model based on the possibility for a given excited state to be formed inside the solid. The conclusion is that in the studied velocity range the 2p state can exist in carbon whereas the 4p state cannot.

Distributions in angle and in energy have been measured for ions transmitted through thin solid targets bombarded by various molecular-ion beams. The results, which differ markedly from those expected from simple Coulomb explosions, indicate that for the ion clusters created inside the target, the internuclear vectors tend to align with the beam direction. An explanation is suggested in terms of a wake potential generated behind each particle as it traverses the solid.

Three different measurements on the structure of the H/sub 3//sup +/ molecular ion are reported. The measurements all make use of a new technique: the foil-induced dissociation of a fast molecular-ion beam. It is shown that the structure is equilaterally triangular in shape. The most probable length of side of the triangle is determined by the three measurements to be 0.97 +- 0.03 A, 0.95 +- 0.06 A, and 1.2 +- 0.2 A, respectively.

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.

The channelling effects on a (110) plane of vanadium single crystal with 2 MeV protons are investigated by the study of the cross-section of the 51V(p,n)51Cr nuclear reaction. Neutrons are detected by means of a time-of-flight method, and so energy measurements are possible. The channeling effect is correlated to the observation of a minimum in the cross section of the reaction. The attenuation rate is 36% for the reactions occuring near the surface of the crystal. The effect decreases exponentially with the depth where the reaction occurs in the crystal. An attenuation is also observed as a function of the integrated proton flux. This effect is reduced by a factor two when the integrated flux reaches 4 × 1016 protons/mm2.

The shape of the spectra of fast channeled particles transmitted through a single crystal is computed. A simplified channel-model is described. Results give, in particular, an explanation of the “high energy loss component” of the observed spectra.

An indirect method of determination of the energy loss of channeled particles in a single-crystal is described in the particular case where the incoming protons can induce a (p,γ) reaction in the target. In this way dechanneling effect as a function of depth in the crystal was studied by means of both (p,γ) reaction and Rutherford scattering. This method was applied to 1.4 MeV protons channeled through an aluminum single crystal along a [110] direction. Stopping power of aluminum crystal for protons incoming along a [110] direction, normalized to the “normal” stopping power was computed from our experimental results, and it was found to be 0.35±0.02.

We have measured angular and charge state distributions of atomic fragments resulting from the foil dissociation of 30–120 keV/p Hn+ clusters. The fragment neutralization probability has been investigated for beam velocities above and around the Bohr velocity. At a given velocity the width of the angular distribution of neutral atoms and their yield are observed to increase with n up to n > 5 and n > 7, respectively. Moreover we have used the simpler H2+ case to propose new ideas to explain the vicinity effects observed on the charge exchange processes.

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.

Deux mesures directes de vies moyennes d'états nucléaires excités ont été effectuées à l'aide d'un accélérateur pulsé. Elles concernent le premier niveau de 19F et de 25Mg. Les valeurs trouvées sont les suivantes : niveau à 110 keV du 19F : τm = 0,84 ± 0,04 × 10-9 s, niveau à 580 keV du 25Mg : τ m = 4,97 ± 0,15 × 10-9 s.

We have observed the channeling effects on the transmission of H2 + molecular ions through thin gold crystals. Energy and thickness dependences have been tested in planar channeling conditions. Similarly, the production of H2 + ions from an incident H2 + beam has been studied. All these observations are consistent with a molecular recombination taking place at the back surface of the crystal.

This paper reports the measurement of the differential photon yield per electron of planar channeling radiation and coherent bremsstrahlung emitted by 54-, 80-, and 110-MeV electrons incident on diamond and silicon crystals of various thicknesses (1--100 \ensuremath{\mu}m) and at several tilt angles relative to the incident beam. Information on the evolution of the electron transverse energy with the penetration depth, and thus on the relative population of the transverse energy states either bound or in the continuum, is obtained. All of the coherent-radiation processes are found to be well described by theories, except for the observed initial level populations, which differ markedly from predictions using the channeling wave functions. A discrepancy with theoretical models is also found for the incoherent radiation, but the influence of incoherent processes on level lifetimes is correctly predicted. The observed forbidden \ensuremath{\Delta}n=2 dipolar transitions may be due to the parity-breaking fluctuations in the potential.

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.

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.

Fast ionic clusters Hn+ interact with matter in a specific way which is observed to deviate strongly from the interaction of atomic ions at the same velocity. We present some results obtained at Lyon about foil and gas interactions of hydrogen clusters (5 ≤ n ≤ 23) at projectile velocities close to the Bohr velocity, i.e. dynamics of the cluster fragmentation, charge state of atomic fragments and absolute dissociation cross sections in gas. We also discuss future experiments specially at higher velocities.

A new mechanism by which the resonant capture of a target electron by a swiftly moving heavy ion is translated in one step into nuclear excitation is identified. It is in fact the inverse of internal conversion. Predicted resonances have energies ≥ 10 MeV/u, widths specified by the target electrons' momentum distribution, and cross-sections approaching tens of millibarns for some, and appear to be observable under channelling condition.

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 orientation dependence of the radiative emission of 150 GeV electrons and positrons incident at small angles with respect to the 〈110〉 axial direction of a thin (0.185 mm) Ge crystal has been observed. The processes are well understood, except for channeled electrons, which radiate unexpected high energy photons.

We have observed variations with incident energy of the scattering yield of H+ and He+ ions measured in a particular configuration of planar double alignment. The structure of these variations is related to the oscillating nature of the trajectories of the ions in planar channeling. Experimental oscillation lengths are deduced for H+ and He+ ions in the (111) direction, and for H+ ions in the (110) direction.

Interaction of moving ions with single crystals is known to be very sensitive to the orientation of the incident beam with respect to the crystalline directions of the target. We have shown that channeling conditions strongly modify the slowing down and the charge exchange processes of high energy heavy ions. The reason is that channeled particles are prevented from approaching the target atoms, and then can interact only with loosely bound target electrons. This results not only in drastically reducing the electron density experienced by them, which lowers energy loss and electron Ioss, but also in inhibiting the radiationless capture of bound electrons, since most of the available electrons are quasi-free valence electrons. In an experiment where high energy Xeq+ ions, with q = 52, 53, 54, are directed onto a thin Si single crystal we observe that, whereas unchanneled ions reach charge equilibrium very rapidly, most channeled ions keep their incident charge state unchanged all along their passage through the crystal. On one hand the loss of their electron(s) (q = 52, 53) is impossible. On the other hand the only way that is left to them to capture quasi-free electrons is the radiative electron capture (REC). This process does occurs also for unchanneled ions but is quite difficult to observe, particularly because of the overwhelming non radiative Mechanical Electron Capture(MEC). REC photons have been observed in channeling conditions and correspond to electron capture into the K, L and M shells of the projectiles. The shape of the REC photon lines reflects the momentum distribution of the electron encountered by channeled ions. Channeling offers a unique opportunity to study the radiative electron capture since it allows ions of well defined charge state to travel through a dense quasi-free electron target. This opportunity could be extended to other processes involving high energy heavy ions.

In recent years a great deal of theoretical work has been devoted to the description of the interaction of ultrarelativistic electrons and positrons and high energy photons moving along the major directions of crystals.1–9 It was predicted that quantum electrodynamic (QED) effects must be taken into account. In particular, the perfect alignment of photons with a crystal axis was predicted to increase the pair production rate above the Bethe-Heitler (BH) value.

We have studied the break-up of accelerated hydrogen cluster ions passing through an argon gas target. The absolute dissociation cross section has been measured for a wide variety of H n + (odd masses only) cluster ions, withn between 5 and 23 and with projectile velocities ranging from 1.5 to 5 × 108 cm/s. We discuss the dissociation processes and the dependence of their cross-sections upon the cluster mass and velocity.

The scattering yield of protons measured in a particular configuration of planar double alignment from a thin gold crystal exhibits strong oscillations when the incident energy varies, providing a new method of observation of particle trajectories in planar channeling.

The light emission of a 600 keV He + beam excited by 300–600 A thick gold crystals has been measured. For the three studied spectral lines in He II we have seen no influence of channeling.