The study of thermal vacancy formation in pure metals by positron annihilation and the determination of vacancy formation enthalpies is reviewed with particular emphasis to the potential of positron lifetime spectroscopy. The investigation of high-temperature vacancy migration by positron annihilation in pulse-heating experiments is outlined and first results are reported.



Es wird ein Uberblick gegeben uber die Untersuchung von Leerstellen im thermischen Gleichgewicht in reinen Metallen mit Hilfe der Positronenzerstrahlung und uber die dabei gewonnenen Resultate der Leerstellenbildungsenthalpie. Hierbei wird insbesondere auf die spezifischen Moglichkeiten der Positronlebensdauerspektroskopie hingewiesen. Es wird zudem uber die Ergebnisse von Messungen der Positronenzerstrahlung in schnellen Aufheizexperimenten berichtet, mit denen die Leerstellenwanderung bei hohen Temperaturen untersucht werden kann.

Investigation of Thermal Equilibrium Vacancies in Metals by Positron Annihilation

This article summarizes theoretical work on sputtering during the past decade, with the emphasis on elemental sputtering in the linear cascade and spike regimes as well as alloy sputtering. The sputtering of molecules and clusters, and electronic sputtering of insulators and biomolecular materials are discussed more briefly, and topics like charge and excitation states of sputtered particles as well as surface topography are left out, in view of contemporary summary papers in this issue and elsewhere. The paper is nontutorial and assumes some general knowledge of the field on the part of the reader, based e.g. on ref. [1] or ref. [2]. The discussion emphasizes principles and methods as well as open problems rather than quantitative results. A few general recommendations for efficient utilization of computer simulation conclude the paper.

Mechanisms and theory of physical sputtering by particle impact

Sputtering yields for different ions and materials at low ion energies have a similar energy dependence. Due to this similarity, yield data can be characterized by a normalized energy function and two parameters for each ion target combination. One of these parameters is the threshold energy. An energy scaling can be based on this parameter. The other parameter is a multiplication factor. Both parameters depend mainly on the ion and target mass M1 and M2 and on the surface binding energy EB. An analytic expression for the normalized functions and both the parameters is given. This empirical relation also allows an estimate of unknown sputtering data, if M1, M2, and EB are noted. A physical interpretation of the empirical relation is given for the case M1≪M2, as in this case special collision processes which dominate the sputtering can be identified.

An Analytical Formula and Important Parameters for Low-Energy Ion Sputtering: Erratum of the Publication in Journal of Applied Physics Vol. 51, S. 2861 ff, (1980)

Man's (or, more probably, Woman's) first cereal crops were sown from seed gathered from wild stands, and it was in the course of cultivation that domestication occurred Experiments in the measurement of domestication rates indicate that in wild-type crops of einkorn, emmer, and barley under primitive systems of husbandry: (a) domestication will occur only if they are harvested when partially or nearly ripe, using specific harvesting methods; (b) exposure to shifting cultivation may sometimes have been required; and (c) under these conditions, the crops could become completely domesticated within 200 years, and perhaps only 20–30 years, without any conscious selection This paper (a) considers possible delays in the start of domestication due to early crops of wild-type cereals lacking domestic-types mutants; (b) examines the husbandry practices necessary for these mutants to enjoy any selective advantage; (c) considers the state of ripeness at harvest necessary for the crops to respond to these selective pressures; (d) outlines field measurements of the selective intensities arising from analogous husbandry practices applied experimentally to living wild-type crops; (e) summarizes a mathematical model which incorporates the measured selective intensities and other key variables and which describes the rate of increase in domestic-type mutants in early populations of wild-type cereals under specific combinations of primitive husbandry practices; (f) considers why very early cultivators should have used those husbandry methods which, we suggest, led unconsciously to the domestication of wild wheats and barley; and (g) considers whether these events are likely to leave archaeologically recognizable traces

Measured domestication rates in wild wheats and barley under primitive cultivation, and their archaeological implications

Energy and angular distributions of sputtered particles

Measurements of angular distributions of material sputtered from polycrystalline copper and platinum and amorphous germanium targets by irradiation with argon ions are reported. The beam energy was varied from 1.25 keV to 320 keV (for Cu and Pt upwards from 10 keV only). All targets yielded an angular distribution more outward peaked than the cosine predicted by collision-cascade theory. The germanium result were well-fitted by cos n distributions with n varying from 1.3 at the lowest to 1.6 at higher energies. The copper and platinum targets yielded distributions of the same general shape superimposed with distinct shoulders indicating preferential ejection along nonrandomly distributed close packed directions typical for a polycrystalline textured target. The measured distributions for germanium are in excellent agreement with the results from recent computer simulations of sputtering.

Angular distribution of particles sputtered from Cu, Pt and Ge targets by keV Ar+ ion bombardment

Computer simulations of atomic collisions in solids are traditionally divided into fully interacting or molecular dynamics (MD) simulations on the one side and simulations based on the binary collision approximation (BCA) on the other. The historical development of both branches is followed and other dichotomies, viz. between static and dynamic target models, and between models using crystalline and amorphous targets, are introduced. The influence of the main input parameters, viz. interatomic potentials, surface-and bulk-binding energies and inelasticity, is discussed before selected results are treated. Here, results for nonlinear effects, clusters, fluctuations and for angular distributions are presented. The review is concluded with a discussion of the influence of computer developments on future simulations.

Computer simulations of atomic collisions in solids with special emphasis on sputtering

Microstructural characterization of materials by non-microscopical techniques

Yield data are reviewed for metal, semiconductor and insulator targets and found to be in good agreement with the predictions of transport-theory-based analytical results provided the non-linear (spike) regime and electronic sputtering are excluded. The surface-binding energies to be used in connection with yield calculations are discussed. Angular distributions from elemental targets, isotopic mixtures and binary alloys are analysed and the “missing-plane” surface-scattering model discussed and found to give a possible explanation of the data.

Hans Henrik Andersen

Papers

Angular distribution of particles sputtered from Cu, Pt and Ge targets by keV Ar+ ion bombardment

Computer simulations of atomic collisions in solids with special emphasis on sputtering

Microstructural characterization of materials by non-microscopical techniques

Sputtering from atomic-collision cascades

Wear traces and patination on Danish flint artefacts