This paper proposes a layered container inspection system for detecting illicit nuclear materials using radiography information. We argue that the current inspection system, relying heavily on the Automated Targeting System (ATS) and passive radiation detectors, is inherently incapable of reliably detecting shielded radioactive materials, especially highly enriched uranium (HEU). This motivates the development of a new inspection system, which is designed to address a fundamental flaw of the ATS-based system, allowing for improved defense against sophisticated adversaries. In the proposed inspection system, all cargo containers go through x-ray imaging equipment first. From the x-ray image, a hardness measure of the container is computed. This hardness measure characterizes how likely it is that shielded HEU, if it does exist in the container, will not be detected in a subsequent passive detection step. Depending on the value of the hardness, the lower-hardness containers are sent to passive detection and the high-hardness containers are sent directly to active detection. This paper explores the trade-off between the detection probability of the new inspection system and the expected sojourn time a container spends in the system. The solution details and decision-making tools for using such a system are provided. Comparisons are made between the proposed system and the current ATS-based nuclear inspection system.

In recent years, the United States, along with many other countries, has significantly increased its detection and defense mechanisms against terrorist attacks. A potential attack with a nuclear weapon, using nuclear materials smuggled into the country, has been identified as a particularly grave threat. The system for detecting illicit nuclear materials that is currently in place at U.S. ports of entry relies heavily on passive radiation detectors and a risk-scoring approach using the automated targeting system (ATS). In this article we analyze this existing inspection system and demonstrate its performance for several smuggling scenarios. We provide evidence that the current inspection system is inherently incapable of reliably detecting sophisticated smuggling attempts that use small quantities of well-shielded nuclear material. To counter the weaknesses of the current ATS-based inspection system, we propose two new inspection systems: the hardness control system (HCS) and the hybrid inspection system (HYB). The HCS uses radiography information to classify incoming containers based on their cargo content into "hard" or "soft" containers, which then go through different inspection treatment. The HYB combines the radiography information with the intelligence information from the ATS. We compare and contrast the relative performance of these two new inspection systems with the existing ATS-based system. Our studies indicate that the HCS and HYB policies outperform the ATS-based policy for a wide range of realistic smuggling scenarios. We also examine the impact of changes in adversary behavior on the new inspection systems and find that they effectively preclude strategic gaming behavior of the adversary.

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Radioiodine thyroid ablation therapy is a common method for treatment of felines exhibiting hyperthyroidism. Due to the high gamma-ray emission rate of radioiodine (I), patients following treatment must be held in isolation for several days before release to prevent unnecessary dose to owners and members of the public. Dose rate measurement on the external surface of the patient of ≤ 20 μSv h is maintained as the patient release criterion without regard to residual activity. However, the Texas Department of State Health Services regulatory guide recommends a release limit of 3.7 MBq to households with non-pregnant women and children over the age of 18 y, and a limit of 925 kBq to households of pregnant women and children who can be supervised. In this paper, Monte Carlo computational radiation transport techniques are employed to predict and standardize the patient isolation time at the clinic by correlating the thyroid burden and surface dose rates of felines. Measurements of patient dose rate as a function of time are used to determine the patient-specific effective half-life experimentally and to validate the model results. Results show that an average holding time of 8 to 9 d is sufficient to reduce the residual activity to 3.7 MBq levels. Additionally, contact dose rate measurements of 20 μSv h or less correlate to residual activity levels of approximately 925 kBq. Based on the model and measurements, a protocol was developed for clinical use at Texas A&M University Veterinary Medical Teaching Hospital to allow estimation of residual activity following injection. This in turn confirms that the surface dose rates used as the release criteria follow the release limits recommended in the regulatory guide.