403 ISOTOPIC ANALYSES, HEAVY ELEMENTS A comparison of close-in and LRD uranium data shows that with the exception of Shot 4 Guam LRD values, uranium-to-fission ratios agree as well as the internal. uranium ratios. The low atom-to-fission ratios for Shot 4 Guam LRD values can perhaps be explaineds Since collections wre made at approximately 2,000 feet, the debris collected in this case may have been from the stem portion of the cloud. It is possible that the close proximity of the water resulted in more-rapid cooling of this portion of the cloud. Since molybdenum is emong the first elements to condense, and total fissions are calculeted from this isotope, low atom-to-fission ratios would te expected. Frectionation of the internal uranium isotopic ratios would not be ordinarily expected, as the decay of these isotopes is not significant during the time required for cooling of the fireball. However, the LRD and close-in values for these ratios differ by as much as 15 percent in several instances. Since this difference could not be accounted for in terms of analytical error, it is possible that this apparent fractionation occurs during a carrier-free dissolution of close-in samples, at which time the U259 is determined by analyzing for its decay product, Np**™*. — - _ _— __] These unusual- ly high values are indicative of the large fluxes of high-energy neutrons generated in the explosions. Additional evidence of the thermonuclear nature of these tests was the_presence_inthe debris of such multiple neutron capture products! _=ss:éM ny’ thermonuclear events can supply the lerge neutron flux necessary for multiple n, y reactions of this magnitude. Large amounts of depleted uranium were placed in close proximity to the Castle devices, with the consequence that the uranium mass~spectrometric results cannot be interpreted in terms of bomb-reaction products. In fact, the U?® abundance in the mass-~spectromter samples is less than the normal abundance in natural]. uranium. In comparing mass~spectrometric close-in and LRD data, the U2 to U2® LRD ratios closely approach the natural a- bundance of these isotopes---thus demonstrating the contribution of atmossheric uranium background. For the events for which mass-spectrometric data is available, it is possible to account for all of the observed Pu2% as the decay product of U*%, 4.4 INDUCED ACTIVITIES The isotope Mn®4 produced primarily by the (n,p) reaction on Fe®4, may be indicative of the amount of iron presentinthedevice.r 38