Shidong Liu

Energy deposition by heavy ions: Additivity of kinetic and potential energy contributions in hillock formation on CaF2

Modification of surface and bulk properties of solids by irradiation with ion beams is a widely used technique with many applications in material science. In this study, we show that nano-hillocks on CaF2 crystal surfaces can be formed by individual impact of medium energy (3 and 5 MeV) highly charged ions (Xe22+ to Xe30+) as well as swift (kinetic energies between 12 and 58 MeV) heavy xenon ions. For very slow highly charged ions the appearance of hillocks is known to be linked to a threshold in potential energy (Ep) while for swift heavy ions a minimum electronic energy loss per unit length (Se) is necessary. With our results we bridge the gap between these two extreme cases and demonstrate, that with increasing energy deposition via Se the Ep-threshold for hillock production can be lowered substantially. Surprisingly, both mechanisms of energy deposition in the target surface seem to contribute in an additive way, which can be visualized in a phase diagram. We show that the inelastic thermal spike model, originally developed to describe such material modifications for swift heavy ions, can be extended to the case where both kinetic and potential energies are deposited into the surface.

A study of highly charged ions interacting with a plasma target at the Institute of Modern Physics

A new experimental area for the investigation of ion beam matter interaction and ion beam plasma interaction was completed at the Institute of Modern Physics (Lanzhou, China). We report the details of this low-energy setup and first results on ion beam matter interaction, where we measured the charge state of O5+ ions at 1 MeV passing through a hydrogen gas target. The data were compared with the Monte-Carlo simulation results for cold hydrogen gas and hydrogen plasma.

Energy dependence of H2+ ions guided through tapered capillaries in PC

Transmission of H+2 ions through tapered capillaries in insulating polycarbonate (PC) foil is reported. Guiding, focusing effects and multi-peaks phenomena on ions through capillaries have been studied.

Target Z dependence of Xe L x-ray emission in heavy ion–atom collision near the Bohr velocity: influence of level matching

X-ray yields for the projectile L-shell have been measured for collisions between Xe20+ and thick solid targets throughout the periodic table with incident energies near the Bohr velocity. The yields show a very pronounced cyclic dependence on the target atomic number. This result indicates that Xe L x-ray emission intensity is greatly enhanced either in near-symmetric collisions or if the binding energy of the Xe M-shell matches the L- or N-shell binding energy of the target.

Simulations of guiding of low-energy ions through a single nanocapillary in insulating materials

Simulations of guiding of low-energy ions through a single nanocapillary in insulating polymers are reported. The nanocapillary has a diameter of 100 nm and a length of 10 μm. Different from previous work, in our simulations a hyperbolic function is used to describe the decay of the charges deposited on the capillary surface. The present simulations reproduce the self-organized charge-up process occurring in the capillary. It is shown that lower-energy ions undergo more oscillations to get guiding equilibrium than those of higher-energy ions, resulting in a longer charging time, which is in good agreement with previous experimental results. Moreover, the experimentally observed mass independence of ion guiding is proved in our simulations. In particular, it is found that the maximum of the repulsive field within the capillary is independent of the ion energy as well as the tilt angle. To counterbalance the increasing of the transversal energy caused by increasing the tilt angle or incident energy, the effective length of the repulsive field is expanded in a self-organizing manner.

Analytical model for describing ion guiding through capillaries in insulating polymers

An analytical description for guiding of ions through nanocapillaries is given on the basis of previous work. The current entering into the capillary is assumed to be divided into a current fraction transmitted through the capillary, a current fraction flowing away via the capillary conductivity and a current fraction remaining within the capillary, which is responsible for its charge-up. The discharging current is assumed to be governed by the Frenkel–Poole process. At higher conductivities the analytical model shows a blocking of the ion transmission, which is in agreement with recent simulations. Also, it is shown that ion blocking observed in experiments is well reproduced by the analytical formula. Furthermore, the asymptotic fraction of transmitted ions is determined. Apart from the key controlling parameter (charge-to-energy ratio), the ratio of the capillary conductivity to the incident current is included in the model. Differences resulting from the nonlinear and linear limits of the Frenkel–Poole discharge are pointed out.

Transmission of 200 keV H+2 Ions through Tapered Capillaries in PC

The transmission of 200 keV H+2 ions through tapered capillaries with inlet/outlet diameters of 4 μm/2 μm and a length of 30 μm is reported. The results indicate that 200 keV H+2 ions can be guided through foils tilted up to ±12.5°. The areal density of the transmitted ion beam is estimated to be ~3.7 times larger than that of incident beam.

Secondary Electron Emission from Carbon Foils under O2+ Ion Impact

Secondary electron emission yields in forward and backward direction from carbon foils (thickness of 74 nm) induced by O2+ ions of energies form 1.9 keV/u to 11.3 keV/u have been measured. We find that the forward and the backward electron emission yields increase with the projectile kinetic energy. Further studies showed that the forward and the backward electron emission yields are approximately proportional to the electronic stopping power at the exit and entrance surfaces, respectively.

Charge state effect on Si K X-ray emission induced by Iq+ ions impacting

K X-ray emission of Si induced by Iq+ (q=20, 22, 25) ion impact has been investigated. The results show a much higher intensity of X-ray emission for I25+ ions bombardment compared to I20+ and I22+ ions. The experimental data are explained within the framework of 3dπ, δ-3dσ rotational coupling.

Energy dependence of highly charged ions guided through nanocapillaries in polycarbonate

The transmission of Xe10+ ions, with incident kinetic energies of 200?500?keV, through polycarbonate capillaries, with a diameter of 150?nm and a length of 30??m, has been studied using a one-dimensional position sensitive detector. The transmitted ion yields were measured as a function of the tilt angle. The results are used to evaluate the guiding angle, which is a measure of the guiding power specifying the capability of insulating capillaries to guide ions at equilibrium. Following the semi-empirical scaling law, the potential in the entrance region in this work is nearly an order of magnitude larger than that proposed by low energy data.