Information obtained from the analysis of the spectra of the Townsend avalanche amplification provides the basis for the study of average microscopic behaviour of the electrons. The electrical field E is uniform, and the avalanches are triggered off at the cathode by a single primary electron. Experimental conditions are such that no secondary effects or space charge can take place. The results of these experiments (methane, 50<E/p<300 V cm−1 Torr−1) prove that the multiplication process is a non-markovian one, and also that it rapidly attains asymptotic behaviour. When interpreted in the light of the age-dependent branching processes, the results enable one to determine some distribution functions: the distribution g(θ) of the time intervals θ between two ionizing collisions, the multiplication coefficient a(τ) characterizing the ionizing capacity of the electron as a function of its `age' τ. a(τ) describes the phenomenon more precisely than does the first Townsend coefficient. The application of this method when E/p and pd are constant (p represents gas pressure and d gap distance) shows that these distributions comply with the similarity principle.

Nous étudions les fluctuations de l'amplification de l'avalanche de Townsend dans le méthane soumis à un champ réduit E/p uniforme de valeur élevée et les décrivons par un processus non-markovien de Bellman-Harris. L'évolution des spectres quand varie la distance interélectrodes à E et p constants montre que l'avalanche atteint très rapidement un état asymptotique. Nous proposons alors un modèle microscopique directement issu de l'expérience

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