Manchester Physics Spark Chamber

1. Light detector (Avalanche photodiode) Manchester Physics Spark Chamber Neutrally charged Atom Positively charged Ion Electron Electric Fields Muon (µ±) Discriminator • As the muon passes through the spark chamber it collides with the neutrally charged atoms and ionises them by knocking off electrons, leaving free electrons and positively charged ions. High Voltage Switch Neon/Helium gas mixture Scintillator Cosmic rays Muon detectors • The high voltage on the plates in the spark chamber creates a strong electric field which accelerates the electrons and ions towards the plates. As the charged particles move towards the plate they collide with more neutrally charged atoms creating more charged particles. This continues in an avalanche process Coincidence High Voltage power supply (10kV) Ground (0 Volts) Scintillator • When the number of electrons in the avalanche reaches 100 million, an electric field forms due to the charged ions and electrons. This causes the electrons and ions to recombine, in doing so they release photons which ionises more particles, causing further avalanches. Muon (µ±) Discriminator Steel plates Light detector (Avalanche photodiode) • The light from the scintillator passes in to the wavelength shifting bars, which convert the blue light to red. The thin rectangular shape of the wavelength shifters concentrates the light at the end of the bars, making it easier to detect. • These scintillators are blocks of plastic containing fluorescent molecules. When a muon passes through the scintillator it gives off some of its energy which the scintillator converts in to blue light. • The different avalanches merge together until there is a continuous plasma between the high voltage plates. The plasma has a low electrical resistance which allows a spark to flow between the two plates. How a spark chamber works How sparks form