FEEDBACK AND BREAKDOWN IN PARALLEL-PLATE CHAMBERS

The existence of a strong connection between the photon feedback and the breakdown phenomena in parallel plate avalanche chambers (PPACs) is generally accepted. In this work we made a detailed study of these processes in mixtures of Ar and He with CH4, C2H6 and triethylamine (TEA) vapors. Two breakdown modes (fast and slow) were identified, active for different levels of photon feedback. The breakdown limit (as against the photon feedback) is found to be quite independent of the nature of the filling gas, depending mainly on the total charge in the avalanche. Analytical expressions, based on our measurements, are proposed for the first Townsend coefficient and for the VUV light production coefficient as a function of the electric field and quencher partial pressure. These expressions allow to justify the constant fast breakdown limit on the basis of a space-charge induced electric field distortion. Some results of other authors, concerning a counting rate dependence of the breakdown limit, can also be explained in the framework of this model.