When a solid is bombarded with atomic particles, some of the particles will be backscattered from the surface, while other will penetrate the solid. In the solid the particles lose energy due to collisions with target atoms (elastic energy loss) and the electrons (inelastic energy loss). The collisions with target atoms also cause changes in the direction of the flight path, so that a fraction of the penetrating particles is directed back through the surface and reflected, whereas the remaining particle fraction comes to rest in the solid. These fractions will depend on the energy of the incident particles and their angle of incidence. The energy of the reflected particles will also depend on their path length in the solid.
Particle reflection from solid surfaces is an important parameter in the modelling of fusion energy devices with particular relevance to the recycling of fuel (H, D and T), ash (He) and impurities from machine walls, limiters and divertor plates. For fusion related applications, impact energies will range from a few times 10 eV, characteristic of first wall recycling to several hundreds of keV, that might be appropriate to neutral beams used for plasma heating.
The particle reflection is characterized by two parameters. First the number reflection coefficient, RN, defined as the ratio of all particles backscattered N from the surface to the number of particles incident N0. Secondly, the energy reflection coefficient, RE defined as the energy carried away by the reflected particles divided by the energy of the incident particles. Both coefficients are taken as integrated over the half space outside the surface. It is of interest to know how RN and RE vary with incident projectile energy and seek systematic behaviours as a function of projectile-target combination. There is also interest in the energy and angular distribution of back scattered particles, and in the dependence of reflected particle fluxes on the angle of incidence of the bombarding ion. However, most information is available for normal incidence onto the surface.
A significant effort has already been devoted to the collection of data and attempts to describe the reflection coefficients analytically. Eckstein and Verbeek tabulated the results of their own extensive measurements and calculations through 1979; the work of that group continues to be the dominant source of information. Itoh et al  in 1985 collected all available information and developed certain useful scaling relations. There are also review papers dealing specifically with the relevance of such information to fusion,
Generally speaking, the particle reflection coefficients are high at low energies and decrease monotonically as projectile energy increases. It has been shown that at sufficiently high energies in the MeV region, as inelastic processes begin to dominate the stopping power, the reflection coefficients eventually become constant with energy. There is some uncertainty as to how one should describe reflection at vanishingly low energies. If a projectile can be retained in the target then reflection should tend to zero at some finite threshold. This might be the case for hydrogen on carbon or a metallic ion incident on a solid of the same type (self-ion). For a projectile that cannot be retained, for example He on Fe, the reflection coefficient should presumably tend to unity. There is very little information for these low energy, near threshold, conditions although computer simulations are available for metallic ions on the surface of the parent material where retention is demonstrated .
Evaluated Particle Reflection Data Base
A set of best available particle reflection data (as of 1991) for a variety of projectile-target combinations relevant to fusion is evaluated in the reference. Particle and Energy Reflection Data for H+, D+, T+ and 4He+ colliding with Be, Be, C, Al, Si, Ti, Fe, Ni, Cu, Mo, W and Au surfaces at normal incidence are stored at IAEA A+M Data Unit Database ALADDIN.
- ↑ 1.0 1.1 R. W. Thomas, R. K. Janev and J.J. Smith, “Particle Reflection from Surfaces - a Recommended Data Base “, INDC(NDS)-249 report (1991)
- ↑ W. Eckstein, H. Verbeek, "Data on Light Ion Reflection", Max-Planck-Institut fur Plasmaphysik, Garching, Report IPP 9/32, August (1979)
- ↑ R. Ito, T. Tabata, N. Itoh, K. Morita, T. Kato and H. Tawara, "Data on the Backscattering Coefficients of Light Ions from Solids", Institute of Plasma Physics, Nagoya, Report IPPJ-AM-41 (1985)
- ↑ W. Eckstein and H. Verbeek, Nuclear Fusion Special Issue 1984, p. 12 (1984)
- ↑ W. Eckstein, "Reflection", Nucl. Fus. Supl. Vol. I (1991)]
- ↑ W. Eckstein and J.P. Biersack, Z. Phys. A. - Atom and Nuclei 310 1 (1983)
- ↑ W. Eckstein and J.B. Biersack, Z. Phys. B - Condensed Matter 63 109 (1986)