摘要
Helium abundance in the air is only 5.3 x 10<sup>-6</sup>. <sup>3</sup>He forms as universe-nuclear is synthesized. <sup>4</sup>He is the result of α-decay of some radioactive isotopes, such as <sup>235</sup>U, <sup>238</sup>U and <sup>252</sup>Th. There are three kinds of helium sources in the earth, that is, meteoric helium, mantle source helium and crust source helium. Their distinguishing sign depends on <sup>3</sup>He/<sup>4</sup>He value.