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It is well known that for a 2 dimensional hydrogenated amorphous silicon thin film position sensitive detector (a-Si:H TFPSD), the calculated coordinates of the position, from the currents measured on the PSD, do not represent the real spot position. The errors result from the generally used position-current expression, and also from device fabrication defects such as the inhomogeneity of TCO thickness and the electrode contact resistance. The best way to correct the errors is to calibrate the device. In this paper, we present a calibration method developed earlier for a smart pressure sensor to calibrate a 2D tetralateral a-Si:H TFPSD. The calibration algorithm is presented. Experimental results are shown and the calibration error is discussed.

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.