A relatively high critical temperature, T c , approaching 40 K, places the recently-discovered [1] superconductor magnesium diboride (MgB 2 ) intermediate between the families of low-and copper-oxide-based high-temperature superconductors (HTS). Supercurrent flow in MgB 2 is unhindered by grain boundaries [2, 3], unlike the HTS materials. Thus, long polycrystalline MgB 2 conductors may be easier to fabricate, and so could fill a potentially important niche of applications in the 20 to 30 K temperature range. However, one disadvantage of MgB 2 is that in bulk material the critical current density, J c , appears to drop more rapidly with increasing magnetic field than it does in the HTS phases [4]. The magnitude and field dependence of J c are related to the presence of structural defects that can "pin" the quantised magnetic vortices that permeate the material, and prevent them from moving under the action of the Lorentz force. Vortex studies [3] suggest that it is the paucity of suitable defects in MgB 2 that causes the rapid decay of J c with field. Here we show that modest levels of atomic disorder, induced by proton irradiation, enhance the pinning, and so increase J c significantly at high fields. We anticipate that chemical doping or mechanical processing should be capable of generating similar levels of disorder, and so achieve technologically-attractive performance in MgB 2 by economically-viable routes.A Type II superconductor undergoes the transition to the normal state at the upper critical field, H c2 . However, the ability to carry dissipation-free current ceases at a lower field, the irreversibility field, H irr . Above H irr the Lorentz force on the vortices is large enough for them to become detached from pinning defects, and virtually free to move. In MgB 2 powder (and also wires and tapes), H irr is approximately half of H c2 [5], so that there is an extended field domain where there might be a useful J c if only the pinning could be strengthened.Ion irradiation is a reproducible means of inducing crystalline disorder. Energetic ions displace atoms from their equilibrium lattice sites, creating a variety of defects, including vacancies and interstitials. Such defects tend to depress the superconducting order parameter locally, and thereby create pinning sites for the vortices. We chose protons for this initial study because at the maximum beam energy available to us of 2 MeV, they can penetrate 40 to 50µm into MgB 2 . A series of irradiations were performed in order to create significant defect densities. The probability of displacement per atomic site (dpa) can be simulated using commercial software [6], and we aimed to generate as uniform a profile as possible through the sample depth (Fig.1).As in our previous study [3], we selected fragments of about 100 µm size from commercial MgB 2 powder (Alfa Aesar Co., 98% purity). Several samples were prepared, each of 20 fragments embedded in silver-loaded epoxy and then polished, so that a defined geometry was obtained, with an estimated thickness of...

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Measurements have been made of the distribution of natural ultraviolet radiation (UVR) at 41 sites on the face of a large fibreglass model of a head. The dosemeter used was the polymer film polysulphone. The results have been correlated with published values of the distribution of basal cell carcinomas on the face. Although the comparison has highlighted the problems of trying to relate frequency of skin tumour occurrence to sunlight exposure, the present results are compatible with the hypotheses that human skin cancer incidence increases with environmental UVR exposure, and that sunlight is not the only factor in the aetiology of basal cell carcinomas of the face.

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Abstract-The design of a miniature quadrupole lens based on metallized glass electrodes mounted in two anisotropically etched silicon mounts that are separated by precision spacers is described. The analysis of conventional quadrupole lenses is first reviewed. The fabrication process is then described, and the analysis techniques are applied to the new geometry. The potential distribution near the axis is first calculated using a finite difference method. Expansion into solutions of Laplace's equation in cylindrical coordinates is then used to determine the potential distortion introduced by the microengineered mount. Design rules are given for a mount and an electrode radius that achieve minimal distortion.Index Terms-Mass spectrometer, microengineering, quadrupole lens, residual gas analysis.