Neutron detectors are used to detect neutron radiation in science, security, and other applications. For example, large-area detectors detect neutrons at science facilities across the world and radiation portal monitors screen vehicles and cargo at ports and border crossings for nuclear material that terrorists could use in a nuclear weapon. Helium-3 is a critical component of such neutron detectors, and in 2008 the U.S. government became aware of a severe shortage of helium-3 gas. While demand for it has increased, helium-3 is currently produced as a byproduct of the radioactive decay of tritium, and the United States ceased tritium production in 1988. The shortage has led science facilities and federal agencies such as the DOD and DHS to identify or develop alternative detector technologies. GAO was asked to review the effectiveness of alternative neutron detector technologies that do not use helium-3. GAO assessed (1) what alternative neutron detectors are currently available and their effectiveness, and (2) the status of research on alternative neutron detector technologies under development for future availability. GAO reviewed agency documents and interviewed agency officials and detector developers. With assistance from the National Academy of Sciences, GAO also assembled a group of experts to review and advise on this study.