. "d_{1/2}" . . . . . . . . . . . . "http://www.iso.org/iso/catalogue_detail?csnumber=31895"^^ . . . . . . . . . . . . . . "The \"Half-Value Thickness\" is the thickness of the material at which the intensity of radiation entering it is reduced by one half." . . . . . "Half-Value Thickness"@en . . . . . . . . . . "http://en.wikipedia.org/wiki/Half-value_layer"^^ . . .