Throughout the ALPHA experiment, charged particles such as electrons, positrons and antiprotons are stored inside devices known as Penning traps. Micro-channel plates (MCPs) are used to image clouds of particles that are extracted from these traps, allowing us to measure and optimise their properties for antihydrogen production.
Each MCP is formed from a thin layer of electrically resistive material, with a dense ‘honeycomb’ pattern of holes (each only a few micrometres in diameter) cut through it from one side to the other. High voltages are applied to each side of the MCP so that incoming particles are pulled into these channels, releasing an ‘avalanche’ of secondary electrons as they repeatedly collide with the walls of the structure. These electrons can be accelerated towards a phosphor screen, which releases a flash of fluorescence light wherever it is hit by a particle. By capturing the visible light that is emitted by the phosphor screen, we are able to produce an image of the charged particles that originally collided with the MCP.