Werner Brandt

Positron dynamics in solids

The implantation and propagation of positrons in solids are discussed. Several examples for the application of the positron method in the study of solid state properties are presented.

Effective charges of ions and the stopping power of dense media

Projectiles of atomic number Z1 moving at velocity ν1 in dense media with ionic charge Q1 (ν1) by general arguments lose energy at a rate as if they were in an effective charge state Z1∗ > Q1. We derive Z1∗/Q1 in a dielectric-response approximation and test the predictions on new low-velocity precision experiments and high-velocity channeling experiments. The “Z2 oscillations” of stopping powers in targets of atomic number Z2 are shown to be related to the variations of the conduction-electron densities in the periodic system. In a different vein, the electronic stopping power of stars for massive neutrinos, through the interaction with the neutrino magnetic moment, is calculated, and some cosmological inferences are drawn.

Penetration of swift ion clusters through solids

Abstract The propagation of spatially correlated ions through condensed matter is influenced by the wakes of coherent excitations trailing each ion. Experimental consequences of the vicinage effects in ion clusters are discussed.

Capture of polarized electrons by deuterons emerging from a magnetized nickel foil

Abstract The spin polarization observed by Kaminsky in the neutrals of a deuteron beam emerging from a ferromagnetic single-crystal nickel foil is attributed to electrons captured in the tail of the surface electron distribution.

UNIFIED SPUTTERING THEORY.

Abstract A simple unified theory of sputtering describes the ablation of random targets by swift heavy particles in terms of the atomic processes triggered by the energy transfer from the impinging particles to the target atoms. A comprehensive sputtering yield curve is derived in an approximate manner, applicable to a wide range of particle-target combinations at particle energies large compared to the atomic displacement energy in the target. The properties and limitations of this yield curve are examined and compared with recent experimental data.

Positron lifetimes in solid and liquid metals

Abstract The observed changes of positron lifetimes in Na and Ga on melting are at variance with the predictions of the free electron gas model. They appear to be determined by the increase in vacancy concentration on melting.

Binding effects in electronic excitations by heavy charged particles

The ratios of K-shell X-ray production cross sections by He+ and H+ ions of 25 to 50 keV per amu energy in Al and Mg are found to be influenced strongly by additional binding of the target electrons at the time of excitation.

Velocity spectra of convoy electrons emerging with swift ions from solids

Abstract Swift ions emerging from targets carry convoys of electrons. Velocity distributions are calculated for convoy electrons that trailed ions in wakebound states inside solids. They are compared with predictions of convoy formation by charge transfer to the continuum and with experiments.

Diffusion constant and surface states of positrons in metals

Positron lifetime spectra and angular correlation curves for seven fine-grained powders of Fe, Co, Ni, and W are analyzed. From the lifetime data, the positron diffusion constant in metals atT=300°K was found to beD+=(1.0±0.5)×10−2 cm2 sec−1. Evidence is presented that positrons are trapped in metal surface states.

Ion Screening in Solids

In 1950, Hall [1] reported the results of an investigation of charge states in proton beams emerging from metals with energies between 20 and 400 keV, as summarized in Fig. 1.