latter factor, the direct determination of uranium in a sample aliquot may conveniently be used to obtain an approximate bomb fraction value. Such results must be confirmed by determinations that employ the separation procedure. d. Contamination. Since the final amount of uranium determined is on the order of 0.1 to 0.01 “g it is essential to run « blank involving all reagents to provide a correction for the small amounts of uranium they may contein. It is also essential to be particularly on guard against accidental introduction of uranium in all operations. Errors due to the latter factor are quite variable end can be checked by considering the precision of parallel determinstions. e. Present Application. In the determination of the very small quan- tities of uranium collected from the cloud thet are to be used to measure the bomb fraction present, it is pertinent to consider the quantities of uranium involved compared to thet of all other substances which are likely to be present and may interfere in the analysis. The uranium present in the initial bomb is far outweighed by the uranium tracer placed on the barge and can be ignored in the analysis. the barge is iron. ‘The most massive element present in An explosion in the megaton range can be expected to carry large amounts of sea weter into the bomb cloud, presumably well mixed with ell other bomb debris. ‘The fluorescence method of uranium anelysis takes advantege of the extreme sensitivity and specificity of this measurement. -45- a