We have addressed the problem of threshold voltage (VTH) variation in Flash memory cells after heavy ion irradiation by using specially designed array structures and test instruments. After irradiation, low VTH tails appear in VTH distributions, growing with ion LET and fluence. In particular, high LET ions, such as Iodine used in this work, can produce a bit flip. Since the existing models cannot account for large charge losses from the Floating Gate, we propose a new mechanism, based on the excess of positive charge produced by a single ion, temporarily lowering the tunnel oxide barrier (Positive charge Assisted Leakage Current, PALC) and enhancing the tunneling current. This mechanism fully explains experimental data we present