Characterization of a neutral helium beam for helium microscopy
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Neutral helium beam microscopy is a new development in scientific instrumentation where neutral helium atoms are utilized to image a sample. The main advantage of using neutral helium is the significantly smaller energy the atoms have for a given wavelength compared to electrons and photons. For example, the energy for a helium atom with a 0.1 nm wavelength is 0.02 eV. The energies for an electron and a photon with similar wavelength are approximately 150 eV and 12 keV, respectively. Other advantages include the facts that the atoms are both inert and neutral. This means that the samples remain undamaged, and therefore the direct imaging of fragile, insulating samples is possible. Due to the low polarization coefficient and neutrality of the helium beam, only classical optical elements such as mirrors or lenses can be used to manipulate the beam. The neutral helium beams are created in a supersonic expansion through a small nozzle. A skimmer is utilized in front of the beam to select the central part of the beam. This beam skimmer is a crucial part in the microscope because it defines the "object" that is focussed by the optical element (mirror or lens) onto the probe to be investigated. Given the geometrical limitations of such a microscope instrument, the size of the skimmer effectively determines the focal spot size through a demagnification factor, and thus the obtainable resolution of the microscope. The main focus of this thesis has been to characterize neutral helium beams created using skimmers of three varying shapes and six different diameters. Skimmers with orifice diameters of 4 µm, 5 µm, 18 µm, 100 µm, 120 µm and 390 µm have been utilized to determine the center line intensity beam characteristics, and the 4 µm and 390 µm skimmers have been utilized in addition for the determination of the virtual source size characteristics and the determination of the beam's monochromaticity. Microskimmers have been used very little until now and so far there are only two papers in the literature investigating their properties. The work presented in this thesis can thus really be considered ground breaking. The results obtained for this thesis show some surprising effects. Most importantly, the results show that the center line intensities for the small skimmers do not follow the theoretical models. It was observed that the reduction of the skimmer orifice causes a dramatic drop in the center line intensity. However, the time of flight measurements show that the monochromaticity of the beam is not influenced by the size of the skimmer orifice. The virtual source measurements show that we obtain a smaller virtual source size utilizing a small skimmer orifice diameter, which is really puzzling. The results presented here show that the ideal situation of utilizing a small skimmer opening diameter to gain a small focal spot size in a helium microscope proves to be challenging. While the monochromaticity of the beam remains unchanged, the reduced center line intensity affects the microscopes ability of actually generating an image. Overall the results presented show that a design change is probably required for the microscope.