Here, to the left, we now have an alpha decay. It is the decay of the element Polonium discovered by Marie Curie, with mass number 210. The isotope Po is the penultimate member of the uranium-radium- decay series ; it decays into a stable lead -isotope with a half-life of 138 days. In almost all cases, the decay is via the emission of an alpha particle of 5.305 MeV. Only in one case of 100000, an alpha particle of lower energy appears; in this case, the decay leads to an excited level of Pb, which then decays to the ground state via gamma radiation.