I just stumbled on this by accident
http://news.yahoo.com/s/ap/20060328...L4ZYEWs0NUE;_ylu=X3oDMTA3b2NibDltBHNlYwM3MTY-
The system of detecting the material seems to work, but false papers are used to overcome this step.
Quote
"To test security at U.S. borders with Mexico and Canada, GAO investigators last year represented themselves as employees of a fake company and obtained cesium-137.
They attempted to cross into the United States with the substance — enough to possibly create two crude radiological bombs that could spread radiation if spread by the blast of a conventional explosive.
When stopped, the investigators presented counterfeit shipping papers and NRC documents that allegedly permitted them to receive, acquire, possess and transfer radioactive substances."
And further on in the article : "False alarms can be caused by ceramics, fertilizers, bananas and even patients who have recently undergone some types of medical procedures. "