These more distant times and reach the ground some distance downwind. particles tend to have more cesium (daughter of gaseous xenon) and stroatium (daughter of gaseous krypton) relative to other fission product activities than do the heavier, and thus nearby, particles. Thus, tended through the rather few surface explosions at Eniwetok Atoll fractionation to result in the amounts of cesium-137 and strontium-90 deposited on the test island and nearby islands being less relative to other fission products than the amounts produced by fission in the nuclear explosion. An example of this fractionation is provided by analyses of the particles in the cloud from the La Crosse test (Nathans, 1970). La Crosse was a coral surface burst of about 40 kilotons on Runit Island. The samnte af tha radioactive clad was 2.6 hours after the event. colleeted at f500 meters at The specific activity (the radioactive with a smaller 9-micron diameter particle. (Little variation was found in specific activities between the smaller particle sizes ranging between 0.5 and 13 microns.) Specific Activity for 9 microns Radionuclide Specific Activity for 50 microns strontium-90 296 promethium- 147 229 uranium . 0.47 5 - 2 inane 0 Ctl Ba OTK MT ly EVE UO ef Ie! factors when the largest particle of 50-microns diameter is compared we Maeve, was found to increase with decreasing particle size by the following ope a YRT. * disentegrations per unit time and per unit weight of the particle)