Performance of a gas avalanche pixel detector of 50 μm pitch

We describe the performance of a square micro-dot (MDOT) detector of 50 μm pitch, in terms of the defocussing effect and gas gain. Both the count rate variation measurement and the computer simulation showed that the defocussing effect disappeared in the 50 μm pitch MDOT. Initial charge build-up on the detector surface was also minimized when the small lateral dimension was combined with a surface coating. The maximum gas gain was limited to ~1200 due to the small distance available for avalanche multiplication. We also developed a two-level microdot, namely a closed-end gas electron multiplication (CEGEM) chamber. A CEGEM detector having a pitch of 200 μm, holes of 40 μm width and a spacer layer of 18.5 μm thickness showed a maximum gain of ~400. This gain is expected to increase by increasing the spacer layer thickness and adjusting the hole size