Isotopic Characterization of Radioiodine and Radioxenon in Releases from Underground Nuclear Explosions with Various Degrees of Fractionation MARTIN B. KALINOWSKI 1 and YEN-YO LIAO 1 Abstract—Both radioxenon and radioiodine are possible indi- cators for a nuclear explosion. Therefore, they will be, together with other relevant radionuclides, globally monitored by the International Monitoring System in order to verify compliance with the Com- prehensive Nuclear-Test-Ban Treaty once the treaty has entered into force. This paper studies the temporal development of radioxenon and radioiodine activities with two different assumptions on frac- tionation during the release from an underground test. In the first case, only the noble gases are released, in the second case, radioio- dine is released as well while the precursors remain underground. For the second case, the simulated curves of activity ratios are compared to prompt and delayed atmospheric radioactivity releases from underground nuclear tests at Nevada as a function of the time of atmospheric air sampling for concentration measurements of 135 I, 133 I and 131 I. In addition, the effect of both fractionation cases on the isotopic activity ratios is shown in the four-isotope-plot (with 135 Xe, 133m Xe, 133 Xe and 131m Xe) that can be utilized for distinguishing nuclear explosion sources from civilian releases. Key words: Nuclear explosion, test ban, CTBT, isotope activity ratios, radioiodine, radioxenon, fractionation, radioactivity monitoring, source discrimination. 1. Introduction Due to their half-lives, fission yields and decay radiation the iodine radionuclides 135 I, 133 I and 131 I as well as the xenon radionuclides 135 Xe, 133m Xe, 133 Xe and 131m Xe are relevant for detecting a nuclear explosion (DE GEER, 2001). Though two of the latter are metastable isomers, for convenience, this paper refers to the entities of this quartet as the four relevant xenon isotopes. Xenon isotopes are the most likely observable radioactive signatures of underground nuclear explosions because xenon is a gas and chemically inert. Radioiodine is volatile and can also be released from underground nuclear explosions. Since it is less likely to escape from deep underground than xenon, however. it is still much more likely to be released than its particle bound precursors in the decay chains, and some degree of fractionation can be expected. This fractionation influences the temporal develop- ment of the radioxenon isotopes released from underground into an atmospheric plume, because it depends on the amount of its radioiodine precursor isotopes which are present in the same plume. This paper studies empirically the degree of fractionation and theoretically the impact of various fractionation scenarios on the isotopic activity ratios of radioxenon. Atmospheric radioiodine and radioxenon are monitored on a daily basis at the radionuclide stations (see, e.g., KALINOWSKI and SCHULZE, 2002) of the International Monitoring System (IMS) that is cur- rently being established (see, e.g., HOFFMANN et al., 1999;KALINOWSKI 2006). Though it may be released in gaseous form, during its transport through the atmosphere iodine tends to undergo to some degree chemical reactions and subsequently to attach itself to aerosol particles. It will be collected on particle filters at the 80 radionuclide stations of the IMS network. For radioxenon, suitable sensors will be installed at 40 of these stations with the option of a later expansion to all 80 sites of the radionuclide particle monitoring network. A previous paper described the temporal devel- opment of prompt and delayed atmospheric radioactivity releases from underground nuclear tests at the Nevada Test Site in the United States (KALINOWSKI, 2011). A subset of this data is used in this paper to investigate the degree of fraction- ation that occurred with the reported releases of 1 Carl Friedrich von Weizsa ¨cker Center for Science and Peace Research (ZNF), Beim Schlump 83, 20144 Hamburg, Ger- many. E-mail: martin.kalinowski@uni-hamburg.de Pure Appl. Geophys. Ó 2012 Springer Basel AG DOI 10.1007/s00024-012-0580-7 Pure and Applied Geophysics