3D-reconstruction of gamma-ray showers with a stereoscopic system

We report on a new 3D-reconstruction of $\gamma$-ray showers which takes full advantage of the assets of a stereoscopic system of atmospheric Cherenkov telescopes and of the fine-grain imaging. The rich information collected by the cameras allows us to discriminate $\gamma$-ray showers and hadronic showers on the basis of two simple properties of electromagnetic showers : their rotational symmetry with respect to the axis and their relatively small lateral spread. The performance of the method is presented in terms of $\gamma$-ray efficiency, angular resolution and spectral resolution.