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From left to right: Mechanical Engineering Associate Professor Steve Biegalski, Postdoctoral Associate Andrew Osborne and Assistant Professor Mark Deinert.

From left to right: Mechanical Engineering Associate Professor Steve Biegalski, Postdoctoral Associate Andrew Osborne and Assistant Professor Mark Deinert.

Justin Lowery, first author.

Justin Lowery, first author.

A research team from the Department of Mechanical Engineering at The University of Texas at Austin recently had a paper profiled in the prestigious journal on January 18, 2013. The first author on the study is Ph.D. student Justin Lowrey, along with Associate Professor Steve Biegalski, Postdoctoral Associate Andrew Osborne, and Assistant Professor Mark Deinert. The paper, which appeared in , 40, 1-5, is entitled "Subsurface mass transport affect the radioxenon signatures that are used to identify clandestine nuclear tests." The Editor's Choice section of (pdf) profiles articles from other publications thought to be of interest and significance to the readers of Science.

The acquisition of nuclear weapons by unstable nations, or subnational groups, is considered by many to be the single biggest security risk faced by the United States and other western nations. Since 1945 more than 2000 nuclear tests have taken place, the last in North Korea. While now rare, they are considered so important that an international monitoring network exists that is constantly on the lookout for evidence that one has taken place. Detection of secret weapons tests is central to efforts intended to limit this proliferation of nuclear weapons and it is achieved through the use of different kinds of sensors, including those to detect radioactive materials that do not occur naturally. Detection of radioactive Xenon is particularly important because it is non-reactive which allows it to migrate to the surface after even well contained underground detonations.

—Dr. Mark Deinert

If a nuclear test is suspected, the international community will compare the ratios of specific isotopes 135Xe/133Xe and 133mXe/131mXe, with those expected for surface-level and subsurface detonations, (Fig. 1). It is hard to overstate the importance of correctly interpreting xenon ratio signals: fail to verify that a suspected test has taken place and a nuclear weapons program might go unchecked; falsely accuse a country of conducting a test and political ramifications could follow. Right now range of ratios that is assumed to indicate a nuclear weapons test is typically determined under the assumption that the ratios are only a function of how and when the xenon was produced. The purpose of the UT study was to establish if this was true.

Xenon isotopes are produced both directly in a nuclear test, but also by the subsequent decay of radioactive iodine (which decays to xenon). The UT group has developed a way to model the production and decay of Xenon below ground, as well as its movement. Simulations have determined that subsurface mass transport (Xenon movement to the surface) can, in fact, affect the Xenon signals that are registered above ground. Variable surface pressure is the key factor, and it was found that low-pressure events can vacuum Xenon out of the ground. What this means is that any Xenon produced directly by the explosion would be removed, and only Xenon coming from the decay of Iodine would be left behind. The result can be a very different Xenon signal than what is typically expected, (Fig. 2).

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