GEM-MIGAS Electron Multiplier Operation in Argon-Methane Mixtures
A Gas Electron Multiplier with a Micro-Induction Gap Amplifying Structure (GEM-MIGAS) is formed when a conventional GEM is operated with a short induction gap, typically 50 mum. A three-dimensional electric field simulation of a GEM-MIGAS detector was performed in order to evaluate the electric field profile in the different regions of the structure. Different electrostatic conditions were undertaken to forecast the detector performance in gas medium. An experimental study of GEM-MIGAS operating in an Ar/CH4 (90/10%) gas mixture, in pure CH4 and in pure Ar was carried out. Performance characteristics including charge gain and energy resolution for 5.9 keV X-rays are presented.
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 CF<sub>4</sub> and He/CF<sub>4</sub> 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 CF<sub>4</sub> 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 ×10<sup>4</sup> to 400 at 1 and 2.6 bar CF<sub>4</sub>, 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 × 10<sup>4</sup> to 6.0 × 10<sup>3</sup> for 1 and 2.6 bar CF<sub>4</sub>, respectively. A GEM-MIGAS configuration with 50 μm induction gap and a GEM with 30 μm hole diameter was used in He/CF<sub>4</sub> mixtures and yielded maximum gains of 3 × 10<sup>3</sup> and 2 × 10<sup>3</sup> for a mixture of 2.6 CF<sub>4</sub> with 1 and 2 bar He, respectively.
A new method of near field analysis
The problem of the accurate determination of the total electromagnetic field produced by a current distribution on a circular disc type aperture is examined. For comparison of the near and Fresnel region solutions, an exact Maxwellian integral is used as the starting point. Aperture and space coordinates are separated by the use of Bessel function addition theorems, allowing the field to be expressed as a sum of terms containing radial and angular integrals. The radial integrals involve only the disc (aperture) distribution and coordinates while the angular integrals involve only the elevation angle. The distance from origin appears implicitly in the series coefficients. These integrals are suitable for high speed computations and have been evaluated for a number of cases. The results are compared with the small angle and general Fresnel formula values for various observation angles, distances from the aperture, and aperture distributions.