Detailed measurements of the total ionization cross-sections for Ne, Ar, Kr and Xe by positron impact are presented in the energy range from threshold up to ~1 keV. From these and other available data, the cross-sections for positronium formation are extracted. Comparison is made with other results where available. Structure in the positronium formation cross-section has been found which appears to be associated with positronium formation in excited states. Upper and lower limits on the contribution from these are estimated.

An array of miniaturized cylindrical quadrupole ion traps, with a radius of 20 microm, is fabricated using silicon micromachining using phosphorus doped polysilicon and silicon dioxide for the purpose of creating a mass spectrometer on a chip. We have operated the array for mass-selective ion ejection and mass analysis using Xe ions at a pressure of 10(-4). The scaling rules for the ion trap in relation to operating pressure, voltage, and frequency are examined.

A critical requirement for high gain and low noise avalanche photodiodes is the single-carrier avalanche multiplication. We propose that the single-carrier avalanche multiplication can be achieved in materials with a limited width of the conduction or valence band resulting in a restriction of kinetic energy for one of the charge carriers. This feature is not common to the majority of technologically relevant semiconductors, but it is observed in chalcogenides, such as Selenium and compound I 2-II-IV-VI 4 alloys.

Measurements of the single and the ratio of double to single ionization cross sections for positron impact on neon, krypton and xenon are presented from near threshold to 1000 eV. By multiplying the values of each target by the appropriate , new data for the corresponding double ionization cross sections have also been obtained. The ratio between positron and electron maxima is generally seen to be lower for higher target atomic number, as expected from the static interaction between the projectile and undistorted target. Comparison with available data suggests that Ps formation accompanied by the production of singly or doubly charged ions is a significant channel in positron - atom scattering.

Detailed measurements of the total ionization cross-section by positron impact on He from threshold up to 850 eV are reported. Both the present total ionization cross-section and the direct ionization cross-section of Moxom et al (1996) and Ashley et al (1996) have been normalized to the latest electron-impact ionization cross-sections. The positronium formation cross-section for this target is thus extracted and compared with the existing experimental and theoretical data.