Set namespace to XElement

I get an empty xmlns="" attribute in XElemenet in an XML tree. When I set its namespace to the document namespace, like this:

string xmlns="FreeForm/SchemaDescription";//Tili
XNamespace ana = xmlns;//Tili
XElement interactiveRootTag = new XElement(ana + "InteractiveRootTag");//Tili

the empty xmlns="" is not exist any more, but all of the children of this XElement, get the empty xmlns="".

Any ideas?