The “wall-less” GEM-type structure is capable of stable operation and high gas gain in humid ambient air and in photosensitive vapours including, TMAE. It operates similarly to classical GEMs, namely avalanche multiplication in the region of a strong electric field, located between aligned holes of the plates. However, since these multiplication regions (in contrast to ordinary GEMs, TGEMS or RETGEMs) do not have any walls, there is no leakage current due to the humidity or other liquid layer (for example TMAE) adsorbed on the hole surface. Spacers are located in special areas and far away from the holes. They have special shapes that fully prevent any leakage current. Some materials commonly used in RETGEM/TGEM construction, for example PCB (or G-10), adsorb water, thus their resistivity may change with time and this may affect the stability of the detector operation. A special protective coating should be applied to such materials to prevent this. An important feature of this structure is that it can operate at gas gains much higher than GEM, TGEM or RETGEM. If necessary, the structure can also be exploited in cascaded mode or be combined with another detector.
G. Charpak et al., IEEE Nucl Sci., 55, 1657, 2008
G. Charpak, JINST 3 P02006, 2008
G. Charpak et al., arXiv:1002.4732