Investigation of discharge limits in diamond coated microstrip gas chambers

Microstrip gas chambers are highly performing in terms of stable operation at high rates and fluxes of minimum ionizing particles. They are manufactured on thin glass ( D-263) coated with a thin diamond-like layer by Chemical Vapor Deposition ( CVD) before photolithogrpahy ( undercoated) or after ( overcoated), thus rendering the substrate within the desired range of surface resistivities 10^14-10^16 W/q. Several overcoated and undercoated chambers have been operational for detecting minimum ionizing particles. The highly ionizing fragments due to copious rate of thermal neutrons and low energy hadrons however, are a serious concern for the discharge limits of these devices in a severe environment like that of the CMS tracker at LHC. In this work, we have performed a systematic investigation of such limits exposing overcoated and undercoated chambers to 5 MeV alphas, operating with gas mixtu res of argon or neon with DME. The results indicate a definite decrease of maximum voltage and a weak dependence on the gas choice. The dependence on the artwork and the metalization used for the strips remains to be investigated.