Radiation portal monitor systems based upon polyvinyl toluene scintillator gamma-ray detectors and pressurized 3 He-based neutron detector tubes have been deployed to detect illicit trafficking in radioactive materials at international border crossings. This paper reviews the neutron detection requirements and capabilities of passive, as opposed to active interrogation, detection systems used for screening of high-volume commerce for illicit sources of radiation at international border crossings. Computational results are given for the impact of cargo materials on neutron spectra, for the response of various detector geometries, the effects of backgrounds including ''ship effect'' neutrons, and for simulation of a large neutron detection array.
a b s t r a c tOne of the main uses for 3 He is in gas proportional counters for neutron detection. Such detectors are used at neutron scattering science facilities and in radiation portal monitors deployed for homeland security and non-proliferation applications. Other uses of 3 He are for research detectors, commercial instruments, well logging detectors, dilution refrigerators, lung imaging, for targets in nuclear research, and for basic research in condensed matter physics. The supply of 3 He comes entirely from the decay of tritium produced for nuclear weapons in the U.S. and Russia. Due to the large increase in use of 3 He for science and homeland security (since 2002), the supply could no longer meet the demand. This has led to the development of a number of alternative neutron detection schemes.
Executive SummaryRadiation portal monitors used for interdiction of illicit materials at borders include highly sensitive neutron detection systems. The main reason for having neutron detection capability is to detect fission neutrons from plutonium. Most currently deployed radiation portal monitors (RPMs) use neutron detectors based upon 3 He-filled gas proportional counters, which are the most common large area neutron detector. This type of neutron detector is used in the RPMs installed in international locations made by TSA and others, and in the Ludlum and Science Applications International Corporation RPMs deployed primarily for domestic applications. There is a declining supply of 3 He in the world and, thus, methods to reduce the use of this gas in RPMs with minimal changes to the current system designs and sensitivity to cargo-borne neutrons are being investigated.Four technologies have been identified as being currently commercially available and potential alternative neutron detectors to replace the use of 3 He in RPMs. These technologies are: 1) Boron trifluoride-filled proportional counters, 2) Boron-lined proportional counters, 3) Lithium-loaded glass fibers, and 4) Coated wavelength-shifting plastic fibers.Reported here is a summary of the testing carried out at Pacific Northwest National Laboratory on these technologies to date, as well as measurements on 3 He tubes at various pressures. Details on these measurements are available in the referenced reports. Sponsors of these tests include the Department of Energy (DOE), Department of Homeland Security (DHS), the Department of Defense (DoD), and internal Pacific Northwest National Laboratory funds.The purpose of this testing was to measure the efficiency and gamma sensitivity of the various neutron detection systems and configurations to determine which of these technologies could meet the neutron detection requirements while not exceeding the current footprint of the 3 He-based neutron module in the RPMs. The measurements made as part of this testing included the response of each system to moderated neutrons and to a high gamma-ray exposure rate. As part of this testing, various configurations of 3 Hebased detectors were also measured. The results reported here are from a limited set of tests to measure the capability of each technology listed above to meet the basic requirements. Additional requirements, such as robustness to different environmental conditions, would need to be tested prior to implementation.The requirements used in this testing are from the specification for RPMs developed for the domestic deployments under the Radiation Portal Monitor Project (RPMP). These requirements allow for testing of individual modules with 252 Cf, a common industrial neutron source. Simulations were performed that indicate the TSA system has comparable efficiency per unit surface area, and therefore if the technology meets the RPMP specification, it will likely meet the requirements of the Second Line of Defense (SLD) program for the TSA RPM in the sam...
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