[en] The physics programmes have changed in 2002 for the secondary school examination qualifying for entry to university, the radioactivity is become a more important component of the final scientific class. The I.R.S.N. requested for this subject, proposed to choose the subject of natural radioactivity and more specially the radon, omnipresent at the Earth surface. It is at concentrations always measurable in air whatever the region. The hypothesis is made that it exists in the school, a piece of lawn. The idea is to measure the radon. (N.C.)