Gas electron multiplier beam profile monitor

Multi wire proportional chambers (MWPC's) are used in many transfer lines and experimental areas at CERN. These chambers are mostly used for beam steering but can also provide reliable profiles measurements. MWPC's, however, also present intrinsic limitations such as limited spatial resolution and gain drop at high rate. Experience shows that MWPCs are fragile and expensive both to produce and repair. Profile measurements from MWPC's can not always be trusted. This is the case with the chambers installed in CERN's low energy antiproton decelerator (AD) which are not only destructive for the beam, but also strongly perturb the profile measurement due to the low energy of the AD beam. For these reasons, we have developed a new, robust, low cost and high precision profile monitor based on gas electron multiplication (GEM). Our GEM detector is designed to be backward compatible as a direct replacement for our old MWPCs. The GEM detector performs simultaneous profile measurements in the both X and Y plane. This paper presents the two first prototypes of the GEM profile monitor and report on measurements from both low energy and high energy beams. Finally highlights and drawbacks of both detectors are discussed with a view to replace MWPC's with GEM's for many applications at CERN.