Ion back flow reduction in a THGEM based detector2012 IEEE Nuclear Science Symposium and Medical Imaging Conference Record (NSS/MIC)

Cherenkov imaging counters requiring large photosensitive areas, the capability to stand high rates and to operate in magnetic field environments could benefit from the use of micropattern gas detectors based on THick Gaseous Electron Multiplier (THGEM) coupled to a solid state CsI photo-cathode. Nevertheless, the ions produced in the charge multiplication processes, which end up in the CsI photocathode Ion Back Flow (IBF) compromise the detector performance: fast ageing of the CsI photocathode, as well as electron extraction resulting in spurious signals and eventual discharges can occur. To avoid and limit these undesired events, several configurations of THGEM based detectors were considered. Some changing the whole detector geometry by changing the relative position of individual THGEMs to take advantage of the microscopic diffusion properties of electrons and ions. One other configuration changing directly the geometry of the THGEM itself to better create an ion trap, the THCOBRA, is also studied. In this work, experimental and simulation studies of these configurations is performed, particularly concerning IBF and gain. Finite element method calculations and Monte-Carlo simulations are performed for a better understanding of the results.