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The development of buffer-gas trapping and magnetized beam formation has provided positron beams with significantly improved energy resolution (⌬Eр0.025 eV) compared to those available previously. Analysis techniques have been developed to take advantage of the fact that the beam is in a magnetic field of ϳ0.1 T. This has enabled scattering experiments at lower energies and with significantly improved state selectivity for excitation experiments than had been possible previously. A detailed description of these techniques is presented. Data are presented for a variety of cross sections for scattering of positrons from atomic and molecular targets.

Absolute measurements are presented for the positron-impact cross sections for direct ionization and positronium formation of noble gas atoms in the range of energies from threshold to 90 eV. The experiment uses a cold, trap-based positron beam and the technique of studying positron scattering in a strong magnetic field. The current data show generally good, quantitative agreement with previous measurements taken using a qualitatively different method. However, significant differences in the cross sections for both direct ionization and positronium formation are also observed. An analysis is presented that yields another, independent measurement of the direct ionization and positronium formation cross sections that is in agreement with the present, direct measurements to within ±10% for argon, krypton, and xenon. Comparison with available theoretical predictions yields good quantitative agreement for direct ionization cross sections, and qualitative agreement in the case of positronium formation.

Absolute measurements are presented for the positron-impact cross sections for positronium formation, direct ionization, and total ionization of the diatomic molecules N 2 , CO, and O 2 , in the range of energies from threshold to 90 eV. Cross sections for the electronic excitation of the a 1 ⌸ and aЈ 1 ⌺ state in N 2 and the A 1 ⌸ state in CO near threshold are also presented. The experiment uses a cold, trap-based positron beam and the technique of studying positron scattering in a strong magnetic field. In O 2 , a feature previously seen in the total ionization cross section is observed in both the positronium formation and total ionization cross sections. The possible origin of this feature and its relationship to positron-induced dissociation is discussed. In N 2 , the near-threshold electronic excitation cross section is larger than that for positronium formation. This likely explains the relatively high efficiency of this molecule when used for buffer-gas positron trapping.

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An evaluation of anti-rubella virus immunoglobulin G (IgG) immunoassays that report in international units per milliliter (IU/ml) was performed to determine their analytical performance and the degree of correlation of the test results. A total of 321 samples were characterized based on results from a hemagglutination inhibition assay. The 48 negative and 273 positive samples were used to determine the sensitivity and specificity of the assays. When equivocal results were interpreted as reactive, the sensitivity of the immunoassays ranged from 98.9 to 99.9% and the specificity ranged from 77.1 to 95.8%. All assays had positive and negative delta values of less than 2. A significant difference between the mean results of all assays was demonstrated by analysis of variance. However, post hoc analysis showed there was good correlation in the mean results expressed in IU/ml between some of the assays. Our results show the level of standardization between anti-rubella virus IgG immunoassays reporting results expressed as IU/ml has improved since a previous study in 1992, but further improvement is required.Rubella virus causes a relatively benign childhood rash and fever. However, primary maternal infection during the first trimester is associated with a 80 to 90% risk of congenital rubella syndrome (2,3,25). In developed countries, the risk of congenital rubella syndrome has been minimized through vaccination programs (22-24) and by testing pregnant women for evidence of rubella virus immunoglobulin G (IgG) at their first antenatal visit (10, 11). Since the isolation of rubella virus in 1962, rubella testing has developed continuously, with the hemagglutination inhibition (HAI) assay often being considered the reference method (4,15,29).Since the 1980s, rubella virus IgG assays have been calibrated against the same World Health Organization (WHO) international standard rubella virus serum (second standard preparation) and test results have been reported in international units per milliliter (IU/ml). The introduction of quantitative measurement of rubella virus IgG had the potential to increase standardization and facilitate the comparison between the results of different tests.In 1992, we published a multicenter evaluation comparing commercial immunoassays used to measure rubella virus IgG antibodies (9). The conclusion was that, although there was a moderate degree of correlation, reporting anti-rubella virus IgG levels in IU/ml had insufficient practical use. At that time, we concluded that the results of rubella virus antibody testing be confined to a statement concerning immunity rather than a numerical value. More than 15 years later, the assays compared in the 1992 study are no longer in common usage in Australia and have generally been replaced with random-access analyzers that perform a range of immunoassays of multiple disciplines. A comparison of six random-access and two microtiter plate (MTP) immunoassays that report anti-rubella virus IgG levels in IU/ml was undertaken to review analytical performance a...

We demonstrate a general technique to achieve a precise radial displacement of the nodal line of the radiofrequency (rf) field in a linear Paul trap. The technique relies on selective adjustment of the load capacitance of the trap electrodes, achieved through the addition of capacitors to the basic resonant rf-circuit used to drive the trap. Displacements of up to ∼ 100 µm with micrometer precision are measured using a combination of fluorescence images of ion Coulomb crystals and coherent coupling of such crystals to a mode of an optical cavity. The displacements are made without measurable distortion of the shape or structure of the Coulomb crystals, as well as without introducing excess heating commonly associated with the radial displacement of crystals by adjustment through static potentials. We expect this technique to be of importance for future developments of microtrap architectures and ion-based cavity QED.