Effect of well diameter upon microwell detector performance

MicroWell (MWD) detectors have been produced by laser micromachining of 125 /spl mu/m thick Kapton/sup TM/ polyimide foil. Wells produced with this technique have near-vertical sidewalls with less than 10/spl deg/ of slope. It is an ideal tool to produce MWD prototype arrays with different well diameters for an experimental determination of the optimum ratio of well diameter to substrate thickness. Arrays with four different well diameters (60, 100, 150, and 180 /spl mu/m) were machined into 125 /spl mu/m thick Kapton/sup TM/ polyimide foil. Detectors with well diameters commensurate with the substrate thickness had better performance, with the best design being an array of 150 /spl mu/m diameter wells on a 200 /spl mu/m Cartesian grid. This design achieved a gas gain of 17,000 in a counting gas of 70% argon and 30% carbon dioxide. Other significant advantages of this design included good charge collection from the drift region and increased gas gain with higher drift fields.