Single electron Response and Energy Resolution of a Micromegas Detector

In gaseous detectors, spatial and energy resolutions are limited by the gain fluctuations. We have measured the single electron response of a Micromegas detector and also its energy resolution as a function of the number of electrons by means of a point-like electron source. A pulsed nitrogen UV laser is focused on the drift electrode made of a thin metal layer deposited on a quartz lamina, creating with an accuracy of 1 micron a spot of an adjustable and well known number of photoelectrons, with a transverse size smaller than 100 microns. With a drift distance of 3 mm, and a Ne-isobutane (90-10%) gas mixture the transverse diffusion is 200 microns rms. The avalanche in the multiplication gap induces charge on the anode made of 128 strips with a 338 microns pitch. For the single electron measurements, the total number of electrons is about 150, in such a way that the corresponding number of electrons contributing to a signal on a strip located at a transverse distance of 4-5 standard deviations from the spot is never greater than one.