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Abstract: Magnetic microspheres (MMS) are useful tools for a variety of medical and pharmaceutical applications. Typically, commercially manufactured MMS exhibit broad size distributions. This polydispersity is problematic for many applications. Since the direct synthesis of monodisperse MMS is often fraught with technical challenges, there is considerable interest in and need associated with the development of techniques for size-dependent fractionation of MMS. In this study we demonstrated continuous size-dependent fractionation of sub-micron scale particles driven by secondary (Dean effect) flows in curved microfluidic channels. Our goal was to demonstrate that such techniques can be applied to MMS containing superparamagnetic nanoparticles. To achieve this goal, we developed and tested a microfluidic chip for continuous MMS fractionation. Our data address two key areas. First, the densities of MMS are typically in the range 1.5–2.5g/cm3, and thus they tend be non-neutrally buoyant. Our data demonstrate that efficient size-dependent fractionation of MMS entrained in water (density 1g/cm3) is possible and is not significantly influenced by the density mismatch. In this context we show that a mixture comprising two different monodisperse MMS components can be separated into its constituent parts with 100% and 88% success for the larger and smaller particles, respectively. Similarly, we show that a suspension of polydisperse MMS can be separated into streams containing particles with different mean diameters. Second, our data demonstrate that efficient size-dependent fractionation of MMS is not impeded by magnetic interactions between particles, even under application of homogeneous magnetic fields as large as 35kA/m. The chip is thus suitable for the separation of different particle fractions in a continuous process and the size fractions can be chosen simply by adjusting the flow velocity of the carrier fluid. These facts open the door to size dependent fractionation of MMS. [Copyright &y& Elsevier]

Describes a cavity perturbation technique for accurate measurement of radio frequency magnetic filing factors, which is especially relevant to cryogenic magnetic resonance experiments. Type I perturbations; Type II perturbations; Practical measurements.

• Signal phase in magnetic particle (MP) imaging encodes information about particle dynamics. • Particle dynamics are in turn influenced by the local strength of the non-uniform selection field. • Phase-sensitized MP images yield much higher spatial resolution than magnitude-only MP images. • Phase-sensitized MP images with resolutions of order a few hundred micrometers are presented. Spatial resolution is a key metric for characterizing any imaging modality. Here we describe magnetic particle imaging (MPI) experiments and complementary magnetic particle spectroscopy (MPS) experiments that reveal and probe significant variations in MPI spatial resolution that are intrinsically coupled to the magnetization dynamics of superparamagnetic nanoparticles. In particular, employing a phase-sensitive MPI detection scheme, we observe a marked spatial sharpening effect as the phase of the demodulation reference is advanced. This increase in spatial resolution is accompanied by degradation of image contrast, to the point where reconstructed images effectively vanish when the demodulation phase is advanced by approximately 90 degrees relative to the phase that yields images with the highest signal-to-noise ratio (SNR). Relaxation- or phase-weighted magnetic particle images with apparent spatial resolutions of order a few hundred microns or better, and with SNRs of order 10 dB or more, are presented, and a qualitative framework for interpreting the role of phase as it pertains to spatial resolution in MPI is introduced. [ABSTRACT FROM AUTHOR]

We describe an actively shielded cylindrical RF transmit coil producing a highly uniform internal field ( ± 0.5%) over an extended volume and a strongly suppressed ( ÷ 20) external field. Direct field mapping and experimental checks using in-situ NMR and MRI of polarised 3 He at low temperature demonstrate performance consistent with numerical field computations. [ABSTRACT FROM AUTHOR]

Abstract: We provide an exact expression for the magnetic field produced by cylindrical saddle-shaped coils and their ideal shield currents in the low-frequency limit. The stream function associated with the shield surface current is also determined. The results of the analysis are useful for the design of actively shielded radio-frequency (RF) coils. Examples pertinent to very low field nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) are presented and discussed. [Copyright &y& Elsevier]

Abstract: The ALPHA experiment has succeeded in trapping antihydrogen, a major milestone on the road to spectroscopic comparisons of antihydrogen with hydrogen. An annihilation vertex detector, which determines the time and position of antiproton annihilations, has been central to this achievement. This detector, an array of double-sided silicon microstrip detector modules arranged in three concentric cylindrical tiers, is sensitive to the passage of charged particles resulting from antiproton annihilation. This article describes the method used to reconstruct the annihilation location and to distinguish the annihilation signal from the cosmic ray background. Recent experimental results using this detector are outlined. [Copyright &y& Elsevier]

Abstract: We present the results of an experiment to search for trapped antihydrogen atoms with the ALPHA antihydrogen trap at the CERN Antiproton Decelerator. Sensitive diagnostics of the temperatures, sizes, and densities of the trapped antiproton and positron plasmas have been developed, which in turn permitted development of techniques to precisely and reproducibly control the initial experimental parameters. The use of a position-sensitive annihilation vertex detector, together with the capability of controllably quenching the superconducting magnetic minimum trap, enabled us to carry out a high-sensitivity and low-background search for trapped synthesised antihydrogen atoms. We aim to identify the annihilations of antihydrogen atoms held for at least 130 ms in the trap before being released over ∼30 ms. After a three-week experimental run in 2009 involving mixing of 107 antiprotons with positrons to produce antihydrogen atoms, we have identified six antiproton annihilation events that are consistent with the release of trapped antihydrogen. The cosmic ray background, estimated to contribute 0.14 counts, is incompatible with this observation at a significance of 5.6 sigma. Extensive simulations predict that an alternative source of annihilations, the escape of mirror-trapped antiprotons, is highly unlikely, though this possibility has not yet been ruled out experimentally. [Copyright &y& Elsevier]

Abstract: Antihydrogen production in a neutral atom trap formed by an octupole-based magnetic field minimum is demonstrated using field-ionization of weakly bound anti-atoms. Using our unique annihilation imaging detector, we correlate antihydrogen detection by imaging and by field-ionization for the first time. We further establish how field-ionization causes radial redistribution of the antiprotons during antihydrogen formation and use this effect for the first simultaneous measurements of strongly and weakly bound antihydrogen atoms. Distinguishing between these provides critical information needed in the process of optimizing for trappable antihydrogen. These observations are of crucial importance to the ultimate goal of performing CPT tests involving antihydrogen, which likely depends upon trapping the anti-atom. [Copyright &y& Elsevier]

Abstract: The diffusion-limited release of radioactive 120Xe from several thin metal foils abruptly heated to temperatures of order 1000°C while in a vacuum is characterized and interpreted in terms of impurity atom diffusion coefficients. The 120Xe impurities were produced in situ from the decay of 120Cs implanted into Ta, Pt, and Zr foils from a 30keV mass-separated ion beam. The temperature dependence of the diffusion coefficients extracted from our data is well described by Arrhenius relations, for which best-fit parameters are presented. The fastest release of 120Xe for a given set of implantation conditions and temperature is observed for Zr, although its reactivity may prove problematic for its use as a catcher foil. A number of convenient analytic formulae for the diffusion-limited release of impurity atoms are presented. [Copyright &y& Elsevier]

Abstract: Techniques have been developed for on-line collection and transfer to a measurement cell of rare noble gas isotopes. A low-energy isotope separator beam is stopped in a foil. After collection, the foil is heated, and the noble gas atoms are cryopumped to a cold finger and then transferred with a buffer gas to a measurement cell. This has been tested with 120Xe produced from a 120Cs beam at the TRIUMF ISAC facility, and greater than 40% efficiency for transfer from the foil to the cell has been demonstrated. The technique will be used for electric dipole moment measurements with radon isotopes and may also be useful for other measurements with gaseous targets including laser spectroscopy. [Copyright &y& Elsevier]

A novel low-temperature bolometric method has been devised and implemented for high-precision measurements of the microwave surface resistance of small single-crystal platelet samples having very low absorption, as a continuous function of frequency. The key to the success of this nonresonant method is the in situ use of a normal metal reference sample that calibrates the absolute rf field strength. The sample temperature can be controlled independently of the 1.2 K liquid-helium bath, allowing for measurements of the temperature evolution of the absorption. However, the sensitivity of the instrument decreases at higher temperatures, placing a limit on the useful temperature range. Using this method, the minimum detectable power at 1.3 K is 1.5 pW, corresponding to a surface resistance sensitivity of ≈1 μΩ for a typical 1 mm×1 mm platelet sample. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]