GEM-MIGAS Gain Optimisation for High Pressure Operation in CF $_{4}$ and He/CF $_{4}$ Mixtures

In this work, the GEM-MIGAS configuration was investigated to evaluate its gain properties at elevated gas pressures using CF₄ and He/CF₄ mixtures. The experimental work consisted of gain optimization as a function of the induction region thickness (20 to 300 μm) using a standard GEM (50 μm hole diameter) and an investigation on the influence of the GEM hole diameter on gain at various CF₄ pressures. As expected, GEMs with narrower holes achieved higher gains, with the advantage of a lower operational voltage. For the GEM-MIGAS using a standard GEM, with 50 μm hole diameter, the gain decrease rapidly with pressure, from 1.5 ×10⁴ to 400 at 1 and 2.6 bar CF₄, respectively. In contrast, the GEM-MIGAS gain drop with pressure is less steep when using a GEM with 30 μm hole diameter, decreasing from 3.0 × 10⁴ to 6.0 × 10³ for 1 and 2.6 bar CF₄, respectively. A GEM-MIGAS configuration with 50 μm induction gap and a GEM with 30 μm hole diameter was used in He/CF₄ mixtures and yielded maximum gains of 3 × 10³ and 2 × 10³ for a mixture of 2.6 CF₄ with 1 and 2 bar He, respectively.