Ion back-flow gating in a micromegas device

As several research and development programs on Gaseous Photomultipliers are running on, the main aging factor on the photocathode, in particular for bi-alkali photocathodes, is the ion back-flow. This effect has also a major impact on the behaviour of Time Projecting Chamber in terms of electron drift perturbations. This paper presents a new device that has been tested with a Micromegas in order to reduce the ion back-flow in this type of detector. Instead of using a single mesh, as it is the case in a classical use of this micropattern detector, a double-mesh is installed. When the good conditions of electric field are applied, the device is transparent to electrons but gates the ions. Several measurements of ion back-flow ratio have been performed using a high power Xenon lamp to create photo-electrons on a Ni photocathode. The currents on the different electrodes are then measured with a pico-ammeter and the ion back-flow ratio is given by the ratio Ic/Ia (where Ic and Ia are respectively the cathode and the anode currents). For a classical mode of working (single mesh mode) the Micromegas ion back-flow is about few percent and is proportional to the field ratio. We have demonstrated that it is possible to reach a back-flow ratio close to few 10−6 in a double mesh mode, for a gain of several thousands, depending on the gas mixture and the electric fields ratio between the different electrodes.