United States. Government Accountability Office. Combating Nuclear Smuggling: Recent Testing Raises Issues About the Potential Effectiveness of Advanced Radiation Detection Portal Monitors, text, November 17, 2009; (https://digital.library.unt.edu/ark:/67531/metadc290925/m1/1/: accessed September 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; .