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The authors have measured the cross sections for single electron loss and single electron capture for 0.6-2.2 MeV He+ colliding with He, Ne and Ar using the growth rate method and a molecular beam target. The results are compared with other available experimental data. For single electron loss theoretical comparisons are also made with the quantal first Born approximation and some semiclassical calculations. For single electron capture, comparisons are made with the modified classical calculations based on the binary encounter model. To date there seems to be no one theory which accurately describes the data. The theory of Kaneko (1985, 1986) is clearly promising for the neon target at energies greater than 0.15 MeV amu-1 but this could be fortuitous since the theory does not similarly reproduce the data for helium and argon targets. The theory of Kumar and Roy (1979) reproduces the energy dependence of the capture cross sections in the range 0.1-1.0 MeV amu-1 but the absolute values of the cross sections are subject to a scaling of the effective charge of the He. Unfortunately no single value of this effective charge applies to all three target gases.

The authors present an analysis of electron loss to the continuum (ELC) for 1.6-2.8 MeV He+ colliding with He, Ne and Ar using the first six terms of the multipole expansion method. They give absolute values for B00 and discuss the way in which the method of analysis influences the values of the dipole and quadrupole parameters. The authors' dipole parameter is slightly positive for the He target and becomes progressively more negative for Ne and Ar. Their quadrupole parameter is slightly positive for He and Ne and nearly zero for Ar. For He++He the magnitude of B00 and its velocity dependence are in reasonable quantitative agreement with a recent theoretical calculation based on the second Born approximation. However their dipole and quadrupole parameters are both more positive than predicted.

A formula is derived for the transmission probability (and hence, gas flow conductance) of a long tube, having a uniform elliptical cross-section, valid in the molecular flow regime (long mean free paths). Comparison with long tubes of circular and rectangular section having the same area showed relatively close agreement in transmission probabilities between elliptical and rectangular section tubes for the same ratio of axes or length of sides respectively. Variations from a circular, square (or triangular) section remained within 10% down to axes or side ratios of 0.3. Existing formulae in the literature, applicable to elliptical sections, show deviations increasing with the ellipticity. It is shown that the frequently used relation involving the ratio of cross-sectional area to perimeter for tubes of various shapes (other than circular) may lead to large errors.

The performance of simple molecular beam sources as targets for ion-atom collision studies is analysed in respect of beam density, angular and spatial distribution in relation to background gas. The directional properties of single tubes with various length/diameter ratios is compared with that of multi-capillary arrays when both are operating in the molecular flow regime. Conditions are derived for obtaining a well localised gas beam when gas flow is limited by the available pumping speed; the corresponding ratio of molecular densities in the directed beam to the random background density is evaluated for a number of possible arrangements, without the complication of differential pumping.

Differential spectra for electron production in the forward direction have been obtained by collision of H2+ and He+ ions with H2, He, Ne and Ar gases. The measurements where made with ions of energy 0.8-2.8 MeV. The target consisted of a jet of gas from a multicapillary array, thereby providing a well localised and directed gas beam. A 30 degrees parallel-plate electrostatic analyser having a FWHM resolution (dE/E)=10-3 was used to study the energy of the ejected electrons. A cusp-shaped peak centred on ve=vi was observed for all the electron spectra, suggesting that these electrons were ejected with only a small energy relative to the parent ions. This process has been called electron loss to the continuum (ELC) since in some ways it mimics the now well established phenomenon of charge exchange to the continuum (CEC). In contrast to data for ELC from heavy ions the cusps observed here were generally asymmetric. They did, however, show a width approximately independent of projectile velocity, a feature which seems to distinguish ELC from CEC. Both the cusp shapes and widths are compared with theoretical predictions and other experimental measurements.

It is shown that the shapes of the peaks in the forward electron velocity distributions, observed when protons and H2+ ions of 0.5-2.5 MeV are transmitted through thin carbon foils, are consistent with 'charge exchange to continuum theories'. Two possible reasons, concerning instrumental resolution and the treatment of the background, are advanced for the inability of others to confirm these theories.

For original paper see S. Cunningham and F. Stern, ibid., vol.14, p.1847-51 (1981). In a recent publication by Cunningham and Stern on the calculation of the integrated sticking coefficient, some fundamental mistakes were made in the derivation of the momentum transfer by the molecular bombardment surfaces. The corrections required are discussed and these are shown to be consistent with the published literature.

For pt.I see ibid., vol.19, p.401 (1986). Absolute cross sections for electron loss to the continuum (ELC) have been obtained for 0.8-2.8 MeV H2+ and He+ ions incident on H2, He, Ne and Ar gases. The target consisted of a jet of gas from a multicapillary array, thereby providing a well localised and directed gas beam. A 30 degrees parallel-plate analyser having a FWHM resolution of (dE/E)=10-3 was used to study the energy spectra of the ejected electrons as detected in a cone of solid angle 1.56*10-3 sr about the forward direction. The cross sections were obtained by numerically integrating the ejected-electron velocity spectra over a width of 0.25 au on either side of the centre of the cusp-shaped peak. When defined in this way the cross sections are found to decrease slowly with increasing projectile velocity, in agreement with an available theoretical calculation and other experimental results.

Describes the design and performance of a uniform electric field analyser used in the second-order focusing mode. Such an analyser is particularly suitable for the study of charge exchange to the continuum (CEC) and other ionisation processes where it is necessary to separate an electron signal from an ion beam moving with similar velocity. As an example of the use of the analyser a study of the characteristic cusp obtained in CEC is discussed.

Following a brief historical introduction an overview is given relating the most recent studies of rarefied gas flow to the early work of Knudsen. The first paper submitted in October 1908 (published in 1909) initiated a period of intense activity by Knudsen, Smoluchowski (1910) and, a little later, by Gaede (1913) and Langmuir (1912). This also covered the transition to the already well established hydrodynamic flow expressed in terms of the ratio of mean free path to critical apparatus dimension: which is now referred to as the Knudsen number. The desorption, evaporation and scattering of molecules from surfaces was described in terms of the Knudsen cosine law of scattering. The Knudsen effusion method for determining vapour pressure, also introduced in 1909, has become the main tool for studies of the related problem of the dissociation, chemical bonding and the vaporisation process itself. Clausing (1926) developed, as an alternative to conductances, the concept of transmission probability, still referred to as the Clausing factor, and provided a procedure for their more accurate evaluation in long and short tubes. A number of misconceptions of these early efforts have found their way into the literature and current books on vacuum science and technology. However, detailed studies have clarified the problem of gas-surface interactions; the gas flow in tubes has been tackled with Clausing-type integral equations and by statistical computation techniques based on Monte Carlo analysis procedures adaptable to more complex systems. Results have been confirmed experimentally using molecular-impact pressure probe measuring techniques.

For pt.I see ibid., vol.11, p.2907, 1978. Experiments are described using a cylindrical sector energy analyser to measure the stopping power of 5-10 mu g cm-2 carbon foils for atomic and molecular hydrogen ions. The incident energies of the ions varied between 100 and 300 keV amu-1 and precautions were taken to avoid foil thickening due to contamination effects. A ratio of the stopping for H2+ ions to that for protons of 1.06+or-0.03 was obtained rather than values up to 1.3 suggested in the literature. Stopping powers for protons at the lower energies were observed to be higher than those given in tabulated data.

The authors discuss the experimental problems associated with measurement of molecular impact rates inside cylindrical tubes under rarefied gas flow conditions. These are reviewed with particular reference to the recent paper by Davies and Lucas (1983) who found a discrepancy between their calculations and earlier measurements of Kurepa and Lucas (1981). The authors show that the impact probe sampling orientation in these earlier experiments could well be the cause of the discrepancy and would qualitatively explain their results. They suggest improvements in the sampling probe arrangement which should give better agreement with the theory removing the need for a modification of the usual well founded assumptions regarding surface interactions under free molecular flow (e.g. cosine distribution).

As the title suggests, though most people would know how to measure low pressures, there exists some doubt about what they really do measure quite apart from the question of accuracy. While the pressure is undoubtedly the most widely quoted parameter in vacuum technology, because of difficulties in accurate low-pressure measurement the term is generally used rather loosely to express the degree of vacuum or the cleanliness of the vacuum environment. It may serve as a means of roughly comparing different systems, but when used to express related physical parameters the need for accurate measurements becomes apparent.

Recent advances in the measurement of total pressures and their equivalent both by direct and indirect methods over the whole vacuum range are reviewed from an industrial viewpoint. Mechanical manometers, together with suitable transducers, measure total gas pressure in a more fundamental way than other vacuum gauges and hence the development of micromanometers has been given some attention. The review includes brief discussions on viscosity gauges, especially of the resonance types, recent work on thermal conductivity gauges, the possible application of radiometer and Knudsen type gauges, and improvements in cold-cathode ionization gauges using radioactive materials. Cold-cathode ionization gauges operating in a magnetic field are used widely in industrial vacuum systems and have been subject to a number of investigations. The consolidation of the Bayard-Alpert type ionization gauges has led to many investigations into the physical and chemical interactions between gauges and the gases in vacuum systems leading to developments in gauge construction which are discussed. Finally, some consideration is given to developments of thermionic ionization gauges to extend their range to lower pressures.

The conference at the National Physical Laboratory covered the subject under the general headings: current requirements for accuracy, new developments in vacuum gauges, the measurement of partial pressures, recent developments in measurement below and adjacent to atmospheric pressure, modern trends in gauge calibration and, finally, physical applications (to sorption phenomena, vapour pressure measurements, space research, etc.). A number of invited papers, some reviewing selected topics, as well as submitted contributions were presented and discussed. This report considers the various papers and their discussion, broadly grouped, and indicates some of the main points made during the conference. For convenience, references are also given to related published work on the various topics mentioned, and a complete list of all the 38 papers given at the conference is appended.

A review is given covering the retarding field and electrostatic deflection energy analyser types, since these are the most suitable for use with both ions and electrons. Deflection analysers are considered for various field configurations; the main lines of development are discussed, including a guide to the numerous recent publications dealing with their design and performance.

A new mass-spectrometer leak detector based on the use of an iron- shrouded axially symmetrical magnetic lens for focusing ions in the low mass range was investigated. By taking precautions to keep the hydrogen background at a low and stable level (by the use of a mercury-vapor pump, liquid-air traps, and clean construction) it was possible to use hydrogen as a search gas. Leaks of 5 x 10/sup -10/ torr liter/sec correspending to an indication of a change in ion current reading of about 3 x 10/sup -13/ amp (for hydrogen) were consistently detected with an almost instantaneous response when testing equipment of a few liters volume using either hydrogen or helium as search gas. Some measurements of the mass spectrum obtained with this instrument are given. A very reliable and convenient version of a compressedpowder leak is also described. (auth)

Following an earlier paper on the measurement of the speed of pumps, in which the intrinsic pump speed was determined by choosing a certain position of a tubulated ionization gauge on a test dome, a similar calculation has been carried out for an orifice type pressure transducer. The results indicate that with a test dome of effective length equal to that of the pump diameter the correct gauge position is approximately the same as when using a tubulated gauge, i.e. at a position along the test dome wall from the pump inlet of 1.055 R for an orifice gauge compared with the 1.022 R previously found for a tubulated gauge, where R is the pump radius. For many purposes it is sufficiently accurate to place the gauge at R (i.e. half the diameter). As the test dome length is increased the gauge needs to be positioned further from the pump mouth but to a lesser extent for an orifice as compared with a tubulated gauge. An expression is derived for determining the error in speed measurement with gauge positioning indicating its dependence on the ratio of test header to pump diameter and the capture probability. It is shown that it is possible to arrange the gauge at a position such that measurements are accurate to within ±3% for all test header lengths likely to arise. Finally, the effect of variations in capture probability with the angle of molecular incidence at the pumping surface is briefly considered, although no experimental results are available to indicate the importance of these effects.

A new type of metal gasket vacuum seal is described which depends on a surface-friction effect instead of compression, which is the basis of most seals described so far. Tests have proved the effectiveness of the seal for repeated cycling between - 188 and 800° C on flanges of 3 and 5 in. diameter and on a ½ in. vacuum union. The materials used include mild steel or stainless-steel flanges with flat gasket sheets of soft copper, iron, stainless steel or nickel. Other metal gasket vacuum seals are briefly reviewed and the mechanism of sealing is discussed. A table of recommended dimensions for flanges from 3 to 12 in. diameter is also presented.

Report on Vacuum Standards and Traceability of Vacuum Measurement, 30 October 1975, National Physical Laboratory

N A Krall and A W Trivelpiece London: McGraw-Hill 1973 pp xiii + 674 price £12.25 Starting from a statistical description of the many body system, the macroscopic fluid theory is used to discuss a range of plasma phenomena including magnetohydrodynamics. Problems involving equilibrium waves, instabilities applied to research in controlled thermonuclear fusion, transpoit phenomena in plasmas and MHD generation are considered.

A circuit is described which automatically controls the emission of thermionic devices for large changes of emission characteristics, and is also independent of large mains voltage fluctuations. In this circuit the power required to heat the emissive surface is controlled by two valves which have the two primary windings of the filament transformer as their load. Part of the electron current to be stabilized is used to control the two valves via a pre-amplifier, so that an increase in emission will reduce the current passed by the valves, and thus the filament current. A phase-shift network is used to obtain high stabilization ratios without setting up oscillations in the feedback network when controlling massive filaments. A circuit is given applying these principles to the emission control of an ionization gauge in which the grid current is kept constant to within 0.5% under all conditions.

Experiments are described which were performed with a resistance heated evaporation source while the mass of the deposit was continuously monitored using a quartz crystal monitor. With the evaporation source supplied from an unstabilized voltage, large fluctuations of both source current and evaporation rate were observed. A source current stabilizer was designed and tests with this indicated that in many instances greatly improved stability in evaporation rate was obtained.

In an earlier paper by the authors, molecular wall flux for a cylindrical test dome was assumed to be linear. It is shown that if deviations from linearity are taken into account a correction can be applied to the recommended gauge position for test domes with a ratio of length to radius larger than 4.

The problem of rarefied gas flow in vacuum systems and particularly the effects of non-uniform gas distributions likely to occur in practice have been the subject of many recent detailed investigations. The results of these studies have in turn influenced the techniques used in obtaining meaningful measurements. A brief review of recent activities is followed by a discussion of dynamic gas flow conditions as they affect the various recommendations for standardization of system parameters, measurements and calibration techniques. Methods have been agreed for the determination of volume rate offlow for pumps. Some of the remaining problems are concerned with measurements involving non uniform and possibly non-isothermal gas flow in systems containing localised or distributed sources of gas and cryogenic, sorbing or gettering surfaces as pumps.

Herbert B Sachse Chichester: J Wiley 1975 pp x + 380 price £13.65 The properties and structure of the three basic types: negative temperature coefficient (NTC or thermistor), positive temperature coefficient, and materials showing semiconductor–metal transitions are reviewed with an emphasis on their use, performance and manufacture into temperature sensing devices. The last third of the book covers all kinds of instrument applications of these materials, both as ambient temperature sensors, and in heated devices.

Paul Schulz Karlsruhe, Germany: G Braun 1974 pp 384 price DM 54 Serving as an introduction to the fundamentals of physical electronics, the book relates to a course of lectures given at the Technical University of Karlsruhe. A concise account of the basic concepts of the kinetic theory of gases and quantum physics is followed by a discussion on electrical discharges in gases and (with greater emphasis in this second edition) an analysis of electrical phenomena in the solid state.

The authors investigate the n-3 scaling law for electron capture into high Rydberg states of fast protons. By extrapolating this electron capture process and the charge transfer to continuum states' process to the ionisation limit, a relationship is established that allows one to determine the exponent of the scaling law from the shape of the 'charge transfer to continuum states' peak. The asymmetry of this peak is also discussed. Data from proton-carbon-foil collisions give an average scaling law exponent of 2.95 for protons in the energy range 0.5-2.5 MeV.

Crasemann (ed) London and New York: Academic 1975 Vol 1 pp xii + 468 price £22.80 Vol 2 pp x + 220 price £13.20 A timely publication in view of the recent renewed interest in the physics of atomic inner shells. The two volumes give a review of the theoretical and experimental aspects of the atomic inner shell excitation processes resulting from interaction with electrons, heavy charged particles and photons, and the decay of the excited atoms.

L Marton (ed) New York: Academic Press 1972 pp x + 315 price: vol 31 £9.20, vol 32 £11.30 Another two volumes in this by now well established series have appeared in quick succession. Volume 31 covers perhaps a wider range of subject than in previous volumes.

S R Seshadri Amsterdam: Elsevier 1973 pp xiii + 545 price Dfl72, S25.30 The subject is developed from a discussion of the essentials of macroscopic kinetic theory based on the Boltzmann equation and leads up to cold and warm plasma models and a number of basic plasma phenomena. The interaction of charged particles with electromagnetic fields is discussed together with the classical dynamics of collisions and several applications.