We present the selection method of events originating from the gamma + n -> pi(-) + p reaction obtained by bombarding a liquid deuterium target with a polarised gamma beam of 0.6-1.5 GeV in the framework of the Graal experiment. We show the effect of bi-dimensional cuts (obtained combining measured quantity and reconstructed kinematic variables) and hardware, constraints in order to reduce the contamination coming from the concurrent reaction channels. We describe a new cut, based oil the Fermi momentum reconstruction able to obtain a great increase of the signal/noise ratio. We determine the reaction vertex by using the precise measurement coming from the multi-wire proportional chambers (MWPCs) detectors: in this way we can reconstruct the shape of the spatial distribution of the beam and its position with respect to the target. By the simulation we estimate the contamination degree due to the other reaction channels so we can test the reliability of our method.
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