- Title
- Scanning helium microscopy
- Creator
- Myles, Thomas Andrew
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2023
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Neutral helium gas first demonstrated its imaging potential some 14 years ago, though the field is distinctively still in its infancy. This thesis presents an in-depth look at how and why a pinhole scanning helium microscope (SHeM) – such as those found at the University of Newcastle and the University of Cambridge – behaves as it does. Chapter 1 presents a history of neutral helium microscopy, and the motivation behind its development as a field. Chapter 2 catalogues our current best understanding of the physical principles behind what has been, and what could be, seen using neutral helium as a probe. Additionally, key technical aspects of the Newcastle SHeM on which this research project was undertaken are detailed. Theoretical and experimental research begins in chapter 3, which is dedicated to answering the basic question 'how can we best measure a signal with a SHeM?' By investigating such a fundamental point we formalise the language around fixed dwell imaging, which have been ubiquitous through neutral helium microscopy. Additionally, we devise a new approach, variable dwell imaging, which provides on-the-fly high dynamic-range imaging. For either case optimisation routines are developed to determine the best compromise between detector response time and signal level. Where chapter 3 is dedicated to the back end of the microscope (i.e., the detector), chapter 4 is to the front end. Specifically, the size of the virtual source – apparent spatial width – of a free-jet expansion. First a meta-analysis of previous studies was undertaken, with the intent to link the physical properties of the stagnant gas to the virtual source size. A new measurement procedure for the virtual source, which involved scanning the nozzle across the optical system, was then developed and applied to the Newcastle SHeM source. In doing so, we present the first virtual source measurements for a supersonic (converging-diverging) nozzle. The virtual source size and brightness is of primary interest as it is theoretically linked to the final strength of the supersonic beam. In chapter 5 we attempt to experimentally verify this link by comparing virtual source measurements collected using the procedure developed in chapter 4 against the observed strength of the supersonic beam. Through this investigation we also present the first virtual source measurements above 320 K. A natural consequence of the supersonic beam formation process is the generation of a secondary effusive beam. In chapter 6 we turn our attention to understanding the secondary beam and other sources of background signal found within a pinhole SHeM. After characterising the background signal, a new pinhole plate design was devised and implemented using stereolithography based 3D printing. The performance of this plate in reducing the background was experimentally verified, providing feedback where future designs can be further improved. The final results chapter is dedicated to defining the resolution limit of a SHeM. First, a model of the beam shape of a pinhole-SHeM was developed, based on the premise of the virtual source being macroscopically large compared to the optical system. This model was used to derive the resolution of the Newcastle SHeM when using a range of different skimmer sizes. To end, the beam-shape model was combined with our understanding of the supersonic beam strength and the time required for a fixed-dwell measurement to find the optimum skimmer and pinhole sizes. To conclude, the final chapter utilises the frameworks developed through this thesis to provide a basic design for a high-resolution pinhole SHeM. We also comment on the relative performance of different detector designs based on the idea of the 'normalised gain', originally proposed in chapter 3. The chapter closes with a proposal for a new measurement procedure which should increase the amount of signal collected substantially over present techniques.
- Subject
- microscopy; scanning helium microscopy; condensed matter imaging; helium
- Identifier
- http://hdl.handle.net/1959.13/1477559
- Identifier
- uon:50001
- Rights
- Copyright 2023 Thomas Ansrew Myles
- Language
- eng
- Full Text
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| Thumbnail | File | Description | Size | Format | |||
|---|---|---|---|---|---|---|---|
| View Details Download | ATTACHMENT01 | Thesis | 51 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 178 KB | Adobe Acrobat PDF | View Details Download |