Time dependence of the helium flux measured by PAMELA

The last solar cycle has presented a peculiarly long quiet phase with consequent minimum modulation conditions for cosmic rays. The proton and electron spectra were measured from July 2006 to December 2009 by PAMELA experiment, providing fundamental information about the transport and modulation of cosmic rays inside the heliosphere. These studies allow to obtain a more complete description of the cosmic radiation. In this picture the time dependence of the helium spectrum become very important to constrain parameters of the actual solar modulation model. The crucial point for this analysis is the selection of a dataset of helium events which ensure high statistics with a very low contamination. In this paper the definition of the selection criteria for helium events with data taken from July 2006 to June 2014 by PAMELA experiment is reported.