4US Bureau of Land Management.Helium Statistics[R].US Geological Survey,2009.
5National Research Council,The National Academy of Sciences.The Impact of Selling the Federal Helium Reserve[M].Washington D.C.:National Academy Press,2000.
6National Research Council,The National Academy of Sciences.Selling the Nation′s Helium Reserve[M].Washington D.C.:National Academy Press,2010.
7PACHECO N,ALI S F.Helium Resources of the United States-2007,Technical Note 429[R].Bureau of Land Management.Denver,Colorado.BLM/NM/ST-09/001+3745.20 pp,2008.
8BOWE Donald J.Helium Recovery and Recycling Makes Good Business Sense.Industrial Gases/Combustion.Air Products & Chemicals Inc.,Allentown,Pa.IndustrialHeating.com,2004.
9WANG C.Small scale helium liquefaction systems[C]//25th International conference on Low Temperature Physics,Amsterdam:2008.