With the very rapid development of high intensity lasers, very high protons fluxes (at least 1013 protons are expected in a few picoseconds bunch) will be available which cannot be characterized by usual detection techniques. For that purpose, we have developed a method in which the particles induce nuclear reactions in a stack of copper foils leading to β+ emitters of short periods which can be detected with a very good signal to noise ratio. The initial kinetic energy distribution of the incident particles is reconstructed from the number of reactions induced in each foil. This technique has been validated at the 12 MV Tandem of CEA/DAM Bruyères-le-Châtel and used at the 100TW laser of LULI to characterize laser-accelerated proton beams. In the latter case, comparison is made with the results obtained with radiochromic films
