gasavalanche

Gas avalanche refers to the rapid multiplication of charge in a gas caused when an electron gains sufficient energy between collisions in an electric field to ionize additional gas molecules. The resulting cascade of electrons and positive ions provides a localized signal in gas-filled detectors and is a fundamental form of gas amplification.

Ionization cascade occurs when a primary electron enters a region with electric field E, accelerating and colliding with gas molecules to produce secondary electrons. The average number of ionizations per unit length is described by the first ionization coefficient, alpha, which depends on E and the gas properties. The electron population grows roughly as n(x) = n0 exp(alpha x). If space charge does not distort the field, this exponential growth continues over a short distance, creating an avalanche. If the field is sufficiently strong or the avalanche becomes large, it can evolve into a streamer or lead to breakdown.

Practical gas mixtures and devices rely on this amplification mechanism. Avalanche multiplication is exploited in proportional counters and other gas detectors to enhance signals from ionizing radiation. Gas choices typically include noble gases with small admixtures of molecular quenchers to absorb photons that could trigger secondary avalanches. The voltage and gap geometry control the avalanche gain; Paschen’s law describes how breakdown voltage depends on gas pressure and electrode separation.

History and applications: The phenomenon was described in the early 1900s in the work of Townsend, which gave rise to the concept of the Townsend avalanche. Modern devices such as gas electron multipliers (GEMs) and micro-pattern gas detectors depend on localized gas avalanches in microstructured elements to achieve high gain with position sensitivity.