Micro-Pattern Gaseous Detectors (MPGDs), such as the GEM and the Micromegas, are currently used in almost all particle physics experiments and are part of the detection upgrades of all LHC experiments. In modern nuclear physics, they are considered as amplification devices in new generation of active target detectors where the gas is both the target to induce nuclear reactions and the detection medium. For instance, a set-up measured successfully a rare mode of radioactivity [1] and other projects, such as the AT-TPC [2] at MSU and the ACTAR [3] detector at GANIL, are currently under development.
The relative gain variance, which quantifies avalanche fluctuations, limits the resolutions that can be reached by gaseous detectors. Although the knowledge of this quantity is critical to assess detector performances, experimental data remain rather scarce in tables and literature. The single-electron response of the detector is the most direct, but most challenging measure to deduce the relative gain variance since it requires an experimental set-up with an optimized signal-to-noise ratio. A dedicated test-bench, based on a UV laser, has thus been developed and continuously improved at the IPN Orsay for this purpose. The single-electron response of Micromegas detectors were measured at several amplification field values and for several binary gas mixtures made of a rare gas and isobutane at 5%.
Analytic models succeed to reproduce some experimental relative gain variances but suffer of their simplified description of the avalanche development and of a gas-dependent parameterisation. Consequently, the Simulation Working Group of the RD51 Collaboration is constantly developing Monte-Carlo models, based on Magboltz [4], where all microscopic processes involved in the avalanche are taken into account, according to their cross-sections. Recent improvements include excitation-ionisation transfer mechanism (Penning effect) in these calculations. Their predictions on gain and relative gain variance are compared and discussed with the experimental results obtained with the Orsay test-bench to extract information relevant for an optimized operation of MPGDs.
[1] B. Blank et al., Nuclear Instruments and Methods in Physics Research Section A 613 (2010) 65.
[2] D. Suzuki et al., Nuclear Instruments and Methods in Physics Research Section A 691 (2012) 39.
[3] J. Pancin et al., Nuclear Instruments and Methods in Physics Research Section A 735 (2014) 532.
[4] S.F. Biagi, Nuclear Instruments and Methods in Physics Research Section A 421 (1999) 234.
IPNO