Observation of diffuse scattering in scanning helium microscopy

By studying well defined geometries (microspheres) in scanning helium microscopy (SHeM) the default scattering distribution for technological surfaces in SHeM is found to be diffuse and approximately cosine.

Low-energy electron ionization mass spectrometer for efficient detection of low mass species

The design of a high-efficiency mass spectrometer is described, aimed at residual gas detection of low mass species using low-energy electron impact, with particular applications in helium atom microscopy and atomic or molecular scattering. The instrument consists of an extended ionization volume, where electrons emitted from a hot filament are confined using a solenoidal magnetic field to give a high ionization probability. Electron space charge is used to confine and extract the gas ions formed, which are then passed through a magnetic sector mass filter before reaching an ion counter. The design and implementation of each of the major components are described in turn, followed by the overall performance of the detector in terms of mass separation, detection efficiency, time response, and background count rates. The linearity of response with emission current and magnetic field is discussed. The detection efficiency for helium is very high, reaching as much as 0.5%, with a time constant of (198 ± 6) ms and a background signal equivalent to an incoming helium flux of (8.7 ± 0.2) × 10⁶ s^-1.

A ray tracing method for predicting contrast in neutral atom beam imaging

A ray tracing method for predicting contrast in atom beam imaging is presented. Bespoke computational tools have been developed to simulate the classical trajectories of atoms through the key elements of an atom beam microscope, as described using a triangulated surface mesh, using a combination of MATLAB and C code. These tools enable simulated images to be constructed that are directly analogous to the experimental images formed in a real microscope. It is then possible to understand which mechanisms contribute to contrast in images, with only a small number of base assumptions about the physics of the instrument. In particular, a key benefit of ray tracing is that multiple scattering effects can be included, which cannot be incorporated easily in analytic integral models. The approach has been applied to model the sample environment of the Cambridge scanning helium microscope (SHeM), a recently developed neutral atom pinhole microscope. We describe two applications; (i) understanding contrast and shadowing in images; and (ii) investigation of changes in image formation with pinhole-to-sample working distance. More generally the method has a broad range of potential applications with similar instruments, including understanding imaging from different sample topographies, refinement of a particular microscope geometry to enhance specific forms of contrast, and relating scattered intensity distributions to experimental measurements.