Expressed as Radium. Also known as "radium equivalent." The radium equivalent concept allows a single index or number to describe the gamma output from different mixtures of uranium (i.e., radium), thorium, and 40K in a material.
Adapted from the CUAHSI HIS SpeciationCV. See: http://his.cuahsi.org/mastercvreg/edit_cv11.aspx?tbl=SpeciationCV.