The main drift chamber (MDC) is the first inner detector layer around the beam pipe and collision point. The MDC's main purpose is to measure the momentum and energy loss per unit distance (dE/dx) from charged particles. The chamber is 2.4 meters long and contains 6796 gold coated 25-micron tungsten signal wires that are arranged in 44 cylindrical layers. The half width of the inner 8 layers is 6 mm and the half width of the outer layers are 8.1 mm. Between the wires, a mixture of Helium and Propane gas are mixed together at a ratio of 60/40, designed to minimize multiple scattering and maintain a high dE/dx. Aluminum 110-micron wires are strung across the chamber for field shaping. As a charged particle passes through the chamber the gas becomes ionized along the path of the particle and the ions drift to the nearest wires. The particles path will be curved because of the magnetic field the solenoid creates. The amount of curvature allows for the momentum of the particle to be calculated.
