"The surface called \"surface\" means the lower boundary of the atmosphere. \"Content\" indicates a quantity per unit area. \"Radioactivity\" means the number of radioactive decays of a material per second. \"Ra\" means the element \"radium\" and \"228Ra\" is the isotope \"radium-228\" with a half-life of 2.45e+03 days."@en . "2018-02-12 16:53:18.0" . "SDN:P07::7ZC0ORUD" . "" . . "false" . "surface_radioactivity_content_of_228Ra"@en . "2018-02-12 16:53:18.0" . . . "accepted"@en . . "SDN:P07::7ZC0ORUD" . . "1" . "1" . "SDN:P07::7ZC0ORUD" . .