This approach to the problem has been used for studies on detonations in both the Pacific Proving Ground and at the Nevada Test Site. In general, the data indicate that for close-in, early fall-out, fractionation does occur in many instances and is subject to many variations depending upon conditions, such as height of burst, duration of fireball, etc. As expected from the earlier discussion, strontium exhibits very definite fractionation. On one series of air samples collected at 40,000 feet at Operation CASTLE after the Bravo shot, the R value for strontium-89 was 0.35. For a fall-out sample collected on land at ap- proximately 80 miles from the burst point, the R value for strontium-89 was 0.14. 0.29. The R value for strontium-90 using the same fall-out sample wes If ae 70 is calculated, it indicates that x = 2, or that ~— Sr 9 , Sr 9 strontium-90 is less fractionated close-in than strontium-89 by a factor of about 2. Fractionation of this magnitude of strontium means that the quantity remaining in the atmosphere should be greatly in- creased over the calculated value based on the Hunter-Ballou Tables. The findings of the gum paper experiment of the New York Operations Office, AEC, indicate that this is true and that the discrepancy factor is about 3. A different approach to this problem was followed at Operation UPSHOT-KNOTHOLE.1? It was concluded from biological studies on both vegetation and indigenous herbiverous animals that, percentage-wise, more strontium was available at distance of 150 miles than at 20 miles. The strontium-90 at the more distant station was also about four times as available for biological uptake as close-in fall-out. This fact may have been due either to the particle size being smaller and having more atrontium-90 present percentage-wise, or that the chemical or physical form was such that the plants could more easily assimilate it. From the foregoing discussion, there is very little question that 13 Larson, K.H., et al., Distribution and Characteristics of Fallout at Distances Greater Than Ten Miles from Ground Zero, UPSHOT- KNOTHOLE WI-811, March-April 1953, SECRET Restricted Data. 33