Smith, Andrew Duncan; (1992) A study of microchannel plate photon detectors employing wedge and strip readouts. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

There is a growing demand in astronomy, as there is also in many other branches of science, for detectors capable of recording an image. Microchannel plates (MCPs) are excellent detectors for the extreme ultra-violet (EUV) and soft X-ray (200eV-10KeV) regions of the spectrum. Coupled to wedge and strip anode (VVSA) readouts they form very robust and versatile imaging systems. This thesis examines the performance of this combination in detail. The operating characteristics of the MCPs are studied to optimise the gain and pulse height distributions obtainable. This is found to occur with a small accelerating voltage across a narrow interplate gap between the MCPs in a chevron pair assembly. The limit of position resolution obtainable is investigated. Due to a novel construction method of wedge and strip anodes recently developed, it is now possible in principle to image at a much reduced MCP gain, therefore a comparison is made between a normal chevron pair of MCPs and a curved channel C-plate. A position resolution of 25μm FWHM is achieved using the chevron pair. For the C-plate device the resolution is found to be limited by the occurrence of partition noise, giving the first experimental confirmation of its existence. Using a special anode of small dimension to reduce this noise component the individual pores of the C-plate are imaged. The position resolution of better than 20μm obtained demonstrates the fine scale to which a wedge and strip anode is capable of imaging. A comparison between the chevron pair and the C-plate reveal that the emitted electron distribution is very different between the two. This indicates a need for a different drift field geometry between the MCP and anode in the two cases. A study is also made to determine the interaction between the electron cloud emitted from the MCP and the geometry of the wedge and strip anode itself. Any mismatch between these two will give rise to image distortions. It is found that the distribution of electrons at the anode surface is sensitive to the MCP gain and voltage, as well as the drift field and voltage. This thesis concludes with a summary of the work done and some suggestions for future work that could be usefully performed in this field.

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