Cosmic Rays (CR) are high energy particles which come from the universe. When one of those particles enters to the earth's atmosphere it produces an air shower, conformed by secondary particles in which the initial energy is distributed. The Pierre Auger Observatory, located in Argentina, is dedicated to the study of those events. One of the main goals is to find out where those CR are coming from and which kind of chemical composition do they have. In this work we show the status of a study of the risetime (t1/2) as a function of the distance to the shower core (near to the air shower's axis) for different zenith angles and energies, obtaining a new variable that will be compared with other variables used by the Observatory. The main objective of this study is to better understand risetime as a mass composition sensitive parameter of CR.