strontium-89 or strontium-90 for incorporation into the large particles because of the short half-lives of the parent isotopes. If the fire- ball is of brief duration, as for low and medium yield weapons, the large particles are cooled when relatively little strontiwm-89 would be found, with more strontium-90 being available. The smaller particles which stay up a longer period of time and travel long distances can absorb krypton and thus are likely to include more radiostrontium than those particles which fall out immediately. On the other hand, a large yield weapon with a longer lasting fireball may have an appreciable amount of strontium-89 present and a larger proportion of the precursors for strontium-90 would have decayed. Thus, fractionation of strontium due to the decay chain process is apt to be less in large yield detona- tions than in those of smaller yield. From the date available, fractionation appears to be more extreme for surface shots and underground bursts than for air bursts of the same yield weapons. Some of the formed radioactive elements that have half-lives of several hours are metals and are found in particles of various sizes in a rather constant perzentage of the fission yield, showing little or no tendency to fractionate 22/ Molybdenum-99 and cerium-l144 are examples of this phenomenon. These elements are frequently used as a reference to determine fractionation of other elements. To illustrate this, an equation of the general form Ay/Ag ” TA7aZ Je is used, in which Ay/Ay is the measured activity for two fission products, the activities having been corrected for decay between time of explosion and time of measurement, and (A,A, Je is the activity ratio of the same two fission products measured under the same laboratory condition, but produced by slow neutron irradiation of uranium-235. An R value of unity for the=e analyzed for these two isotopes would 12 t Renurk Cee.araalFao ‘Phenomen 30 GREENHOUSE 1951, Wt 32 Scientific Director's Restricted Date.