The U.S. Government Accountability Office was asked to report on the sustainability of the radiation portal monitors (RPM) fleet. This report provides information on: (a) Department of Homeland Security’s (DHS) assessment of the condition of its RPM fleet and how, if at all, that assessment has changed over time; and (b) DHS’s plans for meeting detection requirements in the future.
http://www.gao.gov/assets/690/680706.pdf