Progresses in the production of large-size THGEM boards

The THicK GEM (THGEM) electron multipliers are derived from the GEM design, by scaling the geometrical parameters and changing the production technology. Small-size (a few cm(2)) detectors exhibit superb performance, while larger ones exhibit gain response and uniformity limitations. We have studied with a systematic approach several aspects concerning the material (type and thickness of the fibreglass plates) and the production procedure, in particular the cleaning and polishing stages. The net result is the production of large THGEM multipliers reproducing the performance of the small ones. We report in detail about the studies and the results.