A neutral, ground-state, 4 He atom beam has been focused using Fresnel zone plates. At only moderate demagnification ͑0.403͒, a focused spot diameter of #2.0 mm is achieved at a signal intensity of 500 counts͞s. This is an improvement over previous work by a factor of 10 in resolution, 10 3 in signal intensity, and 10 8 in focused beam density, and allows the full intrinsic intensity profile of a focused atomic Fresnel spot to be measured for the first time. Raster scans of 15 and 25 mm wide slits across the focused spot demonstrate transmission scanning helium de Broglie microscopy of micron resolution.Essentially all focusing microscopy employs either photons or electrons as the means whereby an object is imaged. This is largely due to the ease with which these two species can be focused, either to demagnify an incident beam (raster microscopy) or to magnify a transmitted or scattered beam (imaging microscopy). Apart from this, a helium atom might well be the preferred particle by virtue of attributes which have made 4 He scattering an indispensable surface science tool [1]. Specifically, a ground-state thermal energy helium atom (5-100 meV) is completely nonpenetrating, chemically inert, and produces essentially no mechanical damage. Intense, highly monochromatic beams are easily generated via supersonic free-jet expansion. Helium atoms are well matched in momentum and energy to the reciprocal lattice and vibrations of a single-crystal surface, offering both elastic scattering (specular, diffraction) and inelastic scattering (single phonon excitation/deexcitation) as possible microscopy contrast mechanisms. The de Broglie wavelength of a thermal energy helium atom is small, at most a few angstrom, providing much better diffraction-limited resolution than that of optical microscopy or of electron microscopy at comparable incident energies. Therefore microscopy based on helium atoms-helium atom de Broglie microscopy-potentially offers substantial improvements in surface sensitivity, spatial resolution, contrast mechanisms, and reduced sample damage.Focusing of a neutral atomic beam is difficult, however, and while molecular de Broglie microscopy has been demonstrated [2], this was done through aperturing rather than focusing. The very low resultant intensities make aperturing a doubtful approach for general de Broglie microscopy. Focusing is particularly challenging with 4 He atoms, for which low polarizability and lack of spin preclude standard techniques [3] of electromagnetic focusing. One approach has been to focus 4 He beams via reflection from a perfect, monolithic, bent single-crystal mirror [4], and focal spots as small as 210 mm have been attained in this fashion [5]. Bent-crystal mirrors offer both achromatic focusing and large numerical aperture, but bending a crystal surface accurately on atomic length scales remains problematic, and maintaining the mirror surface atomically clean for long periods is difficult.These complications are avoided with free-standing Fresnel zone plates, which deflect and f...
The transmission x-ray microscope at the BESSY storage ring is being used for investigations in the fields of biology, biophysics, medicine, colloid chemistry, and soil sciences. The system has been instrumented for phase contrast studies with the possibility of quick change between imaging in amplitude and phase contrast mode. Phase contrast has the advantage of considerable higher image contrast and reduced radiation dosage compared to amplitude contrast. A cryogenic object chamber has been implemented on the x-ray microscope. First experiments have shown that at cryogenic temperatures the structural stability of biological specimens is increased by three orders of magnitude in comparison with unfixed wet specimens at room temperature.
SummaryMatter-wave microscopy can be dated back to 1932 when Max Knoll and Ernst Ruska published the first image obtained with a beam of focussed electrons. In this paper a new step in the development of matter-wave microscopy is presented. We have created an instrument where a focussed beam of neutral, ground-state atoms (helium) is used to image a sample. We present the first 2D images obtained using this new technique. The imaged sample is a free-standing hexagonal copper grating (with a period of about 36 μm and rod thickness of about 8 μm). The images were obtained in transmission mode by scanning the focussed beam, which had a minimum spot size of about 2.0 μm in diameter (full width at half maximum) across the sample. The smallest focus achieved was 1.9 ± 0.1 μm. The resolution for this experiment was limited by the speed ratio of the atomic beam through the chromatic aberrations of the zone plate that was used to focus. Ultimately the theoretical resolution limit is set by the wavelength of the probing particle. In praxis, the resolution is limited by the source and the focussing optics.
About 70 plant extracts were tested for their ability to repel the attacks of blood-sucking arthropods. It was found that a CO2 extract of the seeds of the Mediterranean plant Vitex agnus castus (monk's pepper) can be used as a spray to keep away especially Ixodes ricinus and Rhipicephalus sanguineus ticks from animals and humans for at least 6 h. In addition mosquitoes, biting flies and fleas are also repelled for about 6 h.
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