Thoron gas (220Rn, T1/2 : 55 sec) decay by emiting an alpha particle to 216Po (T1/2 : 0,14 sec, Eα =6,28 MeV). This nucleus of Po216 is unstable and decay immediately into 212Pb giving a second alpha particle. The 212Pb decay by beta- but it's half life is too long to see the electron (T1/2 : 10,6 hrs, β-). We observe "V" shapes (2 alpha particles) which come from the radioactive decay of 220Rn into 216 Po and 216 Po into 212 Pb