Electron impact source, development and applications
Creators: name: Al-Atabi, Saleh
Publisher: name: Imperial College London
Year: 2016

description: Many modern x-ray instruments are based at synchrotrons, with corresponding limited accessibility and high costs. This has driven the development of imaging/microscopes using table-top x-ray sources. This thesis presents experimental measurements for both electron impact sources and laser plasma source, also it details the developments for the third generation microfocus x-ray source MKIII and associated optical systems for various applications. For non-pulsed radiation, the commissioning and set up of the new third generation microfocus x-ray source MK III is in progress. The Microfocus x-ray source MKI had been characterised and used for imaging biological samples. Absorption and phase contrast images were obtained for biological samples. In addition, for the first time, the MKI device was used to perform experiments to study the phase contrast x-ray image sensing techniques (PCXI), using a sand paper analyser. Images were successfully recorded and analysed using laboratory electron impact x-ray source (MKI) and laser plasma source (Astra Gemini laser at RAL). The PCXI technique can be developed with the new MKIII x-ray source by using two crystals, one a monochromatic and second as a reflection analyser. Additionally, the MKI and MKIII sources can be coupled with a silica mirror or multilayer monochromator and zone plates to provide high spatial resolution images (hundreds of nanometers beam spot sizes) allowing use for various applications. New interchangeable targets (chromium, titanium, and copper) were designed for MKIII and these can provide a range of different x-ray energies, and may allow a considerably smaller x-ray focal spot size ≈ 1 µm, increasing the range of applicability of the source. Furthermore, having different melting points for target materials, a compensation between electron beam size and energy should be taken to avoid exceeding the target melting point. The heat generated at the surface of different targets was calculated and the survival of the present targets (aluminium and carbon) verified. In this MKI regime, an electron beam of 10 keV is focused to a few tens of micrometres diameters on the solid target and x-rays are emitted. The emission spectrum was characterised using mica and KAP crystal spectrometers. The K-edge intensity lines were recorded and analysed for both aluminium and carbon targets using imaging plate and an x-ray CCD camera. Experimental optimisation of the electron gun electrode operating modes was achieved and verified against the literature. The maximum current measured was 250-300 µA. The x-ray spot size was measured and the minimum spot size recorded was 26.1 × 47.9 µm2. Smaller spot sizes are possible but with a low electron beam current. The design of a focusing magnetic lens suitable for use in the new third generation MKIII x-ray microfocus device was undertaken. The focusing lens was designed and manufactured at Imperial College London. A professional software package was used to design the lens through computational modelling. The experimental maximum magnetic field B measured for the focusing lens was 0.512 T at the lens aperture. The aim is to achieve 1 µm electron beam diameter focused at the target surface, although by this computational model, a satisfactory diameter had been achieved ~20-50 µm, but a smaller electron beam could be achieved. The validity of the solutions generated by computational model needs to be precisely experimentally examined in order to achieve the required beam diameter and suitable beam current incident on the target.